’03 Chevy Blazer — where’s my 4 wheel drive? (UPDATED)

Well, today I fixed yet another problem with my 2003 Chevrolet Blazer — Friday past, my in-dash 4×4 controls stopped working and hence I could not engage 4 wheel drive. Of course, these things will always happen at the most inconvenient time.2003_chevrolet_blazer_ls

I had to drive to Ottawa, ON (about 5 hours from me) last Friday and, as luck would have it, encountered an ice storm that made driving very treachurous. So, I thought it would be a good idea to engage 4 wheel drive… I pushed the 4 wheel high button and… nothing. No lights on the dash and no 4 wheel drive! What?

I pulled over and checked the fuse — everything was good there, but I decided to change it just in case. Still nothing. After trying everything I could think of, I just had to deal with it and continue on in 2 wheel drive. Every car I passed in the ditch, I just kept praying my 4 wheel drive would come on… but no such luck. The real pisser here is, I tried it that morning and all was fine… now, when I need it, nothing!

Anyway, skip forward… all went well last weekend, but I still had a broken 4×4 to deal with. And, as if Murphy hasn’t already done enough, it’s the holiday season and most places are only open a few odd days. That made things difficult when I found out what I needed to look for!

The problem, as I discovered, was corroded wires in the 4×4 module. The module is located behind the kick panel on the passengers side (see picture below). How did I know this? Every time I would engage my 4 wheel drive, I would hear a “click – click” coming from that area… never thought anything of it ’til now! When it stopped working, I went searching… first behind the glove box, but I could clearly see that the only relay (the things that make that “click – click” sound) there was for the signals (turning on the hazard lights confirmed this as you will be able to clearly hear the relay, not to mention feel the click if you put your hand on it).


A shot with the kick panel in place

A shot with the kick panel removed

What we’re after — the TCCM
It only took a few minutes to locate the module and the problem was clearly visible after removing the module from the frame to have a closer look. The wires/plug connector were badly corroded on a few pins — which led to two broken wires (pin #1, grey w/ black and pin #6, orange).


A shot of the wire harness

OK, so how the heck am I going to fix this? I can only imaging what GM would want to do (read as “harness replacement”… $$$$$$$$$$$$$$). And from asking around about obtaining a replacement plug, my only option would be a scrap yard. Upon talking to a couple yards, they apparently take these modules out ($$$$$$$) and simply cut the connector off and sell it with the module. So, that really only leaves me to a scrap yard that lets you walk around… and haven’t removed the module… don’t wanna go there. ;)

Plan B… I’ll try my local electronics store and see if I can get a few of the female ends (metal clips on the end of each individual wire… there are about 30 wires, I didn’t actually count). Again I hit some obstacles. I was told that the clips could probably be found, but they would need a sample to send off. I’ll keep looking.

At the last place I tried, I did find a female connector that would do the trick… it just didn’t have the tab to hold it in the main connector (each individual wire clips into a spot in the main connector). I thought I’d give it a try and see what I could do (the plastic bag you see in the third image contains a couple more spare connectors in case it ever happens again… I’ll know right where they are!).

After cleaning up the corrosion on all remaining wires, and cleaning out the main connector, I stripped and attached new clips to the two broken wires and stuck them in the main connector. OK, now I just need to make sure they don’t get pushed out when the connection is made… how? Hmmm… how ’bout hot glue? :) I actually plugged in the main connector to the module (after sealing it all up with dielectric gel… which would have prevented this in the first place) and then inserted the wire into pin #1 and made the connection, and the same for pin #6.

I turned the key in the ignition… Houston, we have ignition! Yahoo! I pushed the 4 wheel high button and heard the familiar sound of the 4×4 module relay and the transfer case motor… wonderful sounds!

So, I have everything working… time for the hot glue. I put a dab of glue down the top of pins #1 and #6 to hold them in place, let it set and tried to pull them out. I was quite pleased when they held, that’s for sure! I mounted the module back on the frame, reinstalled the kick panel and my 4×4 was back in business.

Total cost — twenty cents (a little dielectric gel and about a half hour labour). I’d love to see an invoice for a comparable fix by the Chevrolet dealer!

When I get more time, I’ll tell you about all the other things that have gone wrong with my Blazer since the day I drove it off the lot, with 12 KM on it. It is, as so many other blogs I have read from Blazer owners on the web, the story of a lemon.

UPDATE (finally – 07/03/07) — added images

UPDATE (12/04/07) — added schematics

133 thoughts on “’03 Chevy Blazer — where’s my 4 wheel drive? (UPDATED)”

  1. Hey Kyle,
    Same thing as Pablo above. Your blog helped me so much I cant thank you enough. Just a little tip, I couldn’t find any terminals at the electronics stores that would fit this connector, so I rigged some computer terminal wires and they fit just fine, no glue needed either. You saved me a “bunch of money” and I didn’t even have to switch to Geico.
    Thanks.

    Kyle’s reply;

    Awesome! I’m glad I was able to help solve your problem. :)

    Kyle

    1. Hi Kyle. I can’t thank you enough for this site! It saved me tons of time and money trying to figure out why my 4wd wasn’t working. I checked the TCCM, repaired the wires…still nothing. Checked all the fuses and….problem solved. Thanks for helping me out! I’d like to return the favour. If you have a real estate question or one about renovations, just let me know.

      Thanks again!
      Blair

      Kyle’s reply;

      Hi Blair — great news — thanks for the update!

      As for real estate and renovations — well, I’m also pretty well versed in both subjects! I like to buy old houses and rebuild them from the ground up. I just finished the house I am now living in — a double brick home built in 1912 — one of the first homes in my area. I’ve always meant to get some pictures up of the process — I’ll have to do that soon.

      Thanks for the offer, and thanks for your comments, they are appreciated!

      Regards;
      Kyle

  2. Kyle….any idea where i could get my hands on a schematic for the vacuum system for the 2003 Blazer? I have a heater issue that i believe is tied to the vacuum system. After the truck is warmed up, under 40mph, the heater blows cool air, then at highway speeds it’s hotter than blazes! any thoughts?

    Kyle’s reply;

    Hey Jay;

    No, I don’t have anything AFAIK — but I do have a buddy that has the All-Data system and may have it (he printed out some great schematics for my TCCM yesterday) — I’ll ask him today and if so I will post it here for you.

    As for your problem — I have noticed that as well — not to the extreme that you state, but I have noticed that the Blazer runs cool when at idle and/or low speeds — on the highway you’re melting your eyebrows!

    I’ve meant to look at the heating system since like last winter — just never seem to get around to it (maybe ’cause I don’t use it in the summer… WHEN I feel like fixing it!). I have a definite “vacuum” noise coming from under the dash, near the heater core. At low speeds it “whistles” (like a leaking vacuum would sound like), and goes away at higher speeds.

    So, I will try to find a schematic for you, and hopefully you can help ME solve the same problem! :)

    Kyle

    1. HEY GUYS GREAT BLOG! i have a 2000 4×4 chvy s-10 blazer. (if its even worthy of being called a 4×4) same problems pretty much as everyone else…dash lites blink, i can hear the transfer case motor clicking and whining, but no 4 hi…4 lo works fine…sometimes both high and lo indicater lights will glow and the trans will seem like its slipping….then after i reverse and drive again it will finally slip into 4 low…already replaced the push button, and the differential motor…still same problem… my mechanics dang busted on what to do….i read something above about the mode forks being made out of plastic and wearing out allowing the gears to float…and something else about GMs design flaw… how do i go about seeing if there was a recall or something so i can get my pos fixed. us mountain boys up here in the do need all 4 weels turning at once. specially during this time of year.

      Kyle’s reply

      Howdee!

      You know, other than what I’ve posted above about the forks/gears, I don’t really know much about them — I had the problem explained to me while I was driving home on the highway in rush hour traffic. :) I never really asked any questions further than that — ’cause my 4WD worked!

      Mine would go into 4WD, both Lo and Hi, but would sometimes slip out of 4WD (mostly in Hi) causing a loud grinding/banging noise. When testing it in 4Lo, it would sometimes not go anywhere, then all of a sudden drop into gear like and snap your head back (with a hell of a bang as well). It stumped my mechanics for some time — everything else was checked (they even ordered up parts for the front diff, and then sent them back) and it all looked good. The transfer case was the last thing they looked at (don’t ask me why — I guess the symptoms don’t point to it).

      First, I’d try resetting the system by disconnecting the battery for a 1/2 hour and then cycle the key from on to off 3 times. I know it sounds fruitless, but it has solved a few problems before now — what’s it take, a 1/2 hour of your time (and even then you can go watch TV!). Then, I’d maybe mention the forks/gears/plastic issue to your mechanic and he may be able to research it more. Don’t hold your breath on a recall though — if it’s not a safety issue, and it lasts more than your warranty period, it’ll never get recalled! ;)

      Regards;
      Kyle

    2. Igot a 1999 s10 blazer wont go in 4hi 4lo works and you can hear actutor shift.I have replaced all vacuum lines The vacuum on mine comes off intake to check value then to vacuum switch on transfer case i have 15lbs vacuum at switch. push 4hi button and 4hi&4lo lights flash then it goes back in 2hi Can not hear actutor shift when you push 4hi Thank you Alan

      Kyle’s reply;

      Hi Alan;

      Could be a TCCM problem, encoder motor problem or a problem in your transfer case — all of which has been covered here.

      Kyle

  3. I have a 1999 Blazer LT that I am trying to get to engage in auto 4X4 or 4hi. When I press either button the lights blink as though it wants to go into 4 wheel drive but then it goes right back to 2hi. Can you tell me what my problem is and if it is an easy repair? I hear no noise while I am pushing the button, just the lights blink and then return to 2hi. Can anyone help me?

    Kyle’s reply;

    Hi Alice;

    What you have is a breakdown in the system. Yeah, I know that sounds pretty obvious — what you have in a 4WD system is multiple components that talk to each other (electronically) — if one on the components doesn’t respond the system doesn’t know what to do and, basically, will do nothing. To put it into terms that may be easier to understand…

    You have a cordless phone — you pick it up and press the talk button to get a dial tone, but you don’t get a dial tone — instead, you get a series of beeps after a few seconds. What is happening is, you press the button on the phone (buttons on your dash) — a signal is sent from the phone to the base unit (Transfer Case Control Module) — the base unit is not plugged in, so it does not respond to the signal from the phone — the phone doesn’t know what to do, so it beeps at you to say there is an error. This is, basically, how your 4WD system works.

    Anyway, it could be a few things;

    1) let’s start off with the easy things — have you checked your fuses? Don’t assume that just because the fuses are labeled 4WD that they are the only ones for your 4WD system! Fuse 10 (on my 2003 Blazer) is for the courtesy lights — but guess what? It also controls the logic side of the TCCM (+5VDC). Pin 6 (thinner orange wire) should produce +5VDC and should be HOT AT ALL TIMES (meaning you should be able to read +5VDC on the line anytime, key on or off). The two large orange wires are your +12VDC lines and are controlled from the 10A fuse located in spot #15. I would check your LARGE orange wires as you are getting +5VDC logic as the lights are coming on on the dash. The +12VDC is used for the encoder motor — check to make sure you have +12VDC at both the large orange wires (they are right beside each other… and there are two large black wires beside them that are ground).

    Notes: All black wires are ground — all orange wires are power (hot). There is a 10A fuse (#15) for the 4WD system, located in the fuse panel. There is a 20A fuse, located under the hood, that is also for the 4WD system (I’ll have to look up the fuse number). The 4WD system also pulls from the 10A courtesy lamp fuse (#10).

    2) the vacuum switch (located on the firewall — see one of my posts above for a picture) or vacuum diaphram (located under the battery) could be malfunctioning. The job of these components is to pull on a cable which engages the front, right 4WD hub. I don’t think this is likely to be your problem as you should probably hear the encoder motor running (changing the drive mode from 2Hi to 4Hi/4Lo) — but there is a sensor on the hub to tell the TCCM when it is engaged, so it could be problem.

    3) the encoder motor/module is bad. This is the module/motor that is mounted directly to the transfer case, who’s job it is to physically move the gears of the transfer case to put the TC into the various modes (2Hi, 4Hi, 4Lo). You also want to check the wires at the encoder motor as they are exposed to the elements and *could* be damaged by something flying up under your vehicle (not likely, but possible).

    4) the TCCM (which is what started this thread — located under the passengers kick panel) has wires that are corroded / broken. Poor connections anywhere along the route will cause problems — most times intermittently, which are a real PITA to find. (See my above posts for resolving this issue.) The TCCM could be bad as well.

    If I had to guess — either your encoder motor is shot, or bad wires on the TCCM. If you’re really into it, I’ll post the schematics for the wiring of the system — this will allow you to trace and test points along the system to pinpoint the problem. (I have to scan them, so it may not be today!)

    Well, I hope I have given you some direction to be able to start troubleshooting your 4WD system.

    Regards;
    Kyle

    (UPDATED! 12/04/07 – Schematics have been added to the main post above.)

    1. Im posting this because recently i have worked on two s10 blazers with the same problem and everyone kept telling me to check vacuum and and the acuator. First off both of them the 4wd light would light like it was working but the front axle was not engaging i checked the acuator and it was working so i jacked the vehicle up and tried to manualy engage the axle with no luck. So i decided to pull the axle tube off the passenger side and check the locking hub. Pieces of metal had fallen out. Too my amzement both blazers had the same problem it was a thrust washer that that goes behind the axle courier connector works like a spacer. You can only buy these from a dealer the come in whats called a washer kit $10.40. Im posting this too help anyone out who has exhausted themselves looking at what everyone tells them too because the problem could be internal in the locking hub. I have a diagram if anyone has a similar problem. Ill send it to you if you email me at wildman510@yahoo.com

      Kyle’s reply

      Hey Josh;

      $10 to fix your 4WD — awesome! Thanks for sharing that info with myself and my readers — I’m sure it will help a few people save a ton of $$$!

      I’m going to email you, as I would like to get a hold of your diagram — hopefully you’ll let me post it here to save people some time (and you from having to email it out to individual people all the time).

      Thanks for your comments!

      Regards;
      Kyle

      UPDATE — Josh emailed me back with the following information — thanks, Josh!


      Click for a full size view

      1. shaft 2.deflector 3.seal 4. bearing 5. tube 6.bolt 7. thrustwasher 8. retaining ring 9.carrier connector 10. shiftcable 11.shiftcable housing 12.gasket 13. indicator switch 14. Spring 15. seal 16. spring 17. shift shaft fork 18.bolt 19.tab washer 20. thrust washer 21. differential pilot bearing 22. sleeve 23. output shaft 24. washer 25. plug 26. washer 27. pin 28. bolt 29. carrier case 30. bearing 31. insert 32. sleeve 33. slide bearing 34. differential case 36.screw 37. pinion 38. shim 39. bearing 40. spacer 41. bearing 42. seal 43. deflector 44. flange 45. washer 46. nut 47. plug 48. bushing 49. vent hose 50. vent 51. fitting 52. shaft 53. thrust washer 54. side gear 55. thrust washer 56. differential pinion gear 57. cover 58. bolt 59. shaft

      Number 7 has been the culpret in 2 of the blazers i have worked on its hardned steel so do not try and make one out of any washer it will fail. GM sells this washer in what they call a washer kit it includes 7, 20, and 24. If anyone has this problem its a simple fix and you dont even have to remove the diff, just the axle tube, but youll have to remove the halfshaft 1st this fix took me just over an hour to complete. I hope this info helps anyone with a similar problem. Also i have breakdowns of everything on these vehicles feel free to email me for something else i love your sight and believe info like this should be shared to anyone.

    2. i have replaced the gear and spring and fork in my blazer the gear was ground up about a yaer ago now i have the same problem could this be a bad transfer case vaccume switch no leting the gaer fully engauge ????thanks scott

      Kyle’s reply;

      Hello Scott;

      Unfortunately, I have no idea — I am not a mechanic.

      Regards;
      Kyle

  4. Thanks for the great pointers. I was able to figure out some of your more basic tips and verify that my front drive shaft is turning while in 4wd but not in 2wd. So far so good (it seems to me), so I’m investigating using my newfound knowledge re. vacuum lines etc. I was wondering if you could provide a bit more info with respect to the diaphragm that pulls the cable to the differential. I’m totally new to this and probably won’t dive too far, but it’s snowing here and the 4WD isn’t working, so I’ll hold the darn cable tight with vicegrips for a few months if I have to!!

    Kyle’s reply;

    Hey Greg;

    I’m not sure why you would want to pull the cable on the front diff — I thought you said the front axle turns when in 4WD but not 2WD? If that is the case, the axle is engaged (meaning the cable is being pulled).

    Now, there is a sensor on the front, right hub to detect when it is engaged — I have not tried this, so I don’t know 100% for sure, but if the front, right hub is not engaging, the system should detect this and NOT enter 4WD.

    What you have to do is remove the battery and then the battery holder — and you will then see the vacuum diaphram for the 4WD system. The battery cables are fairly short, so I removed the battery completely to work on the system, then put it back in on it’s side to test it (but don’t leave it like this for long — it will leak!).

    When you start the vehicle, and engage the 4WD system, vacuum will pull this diaphram closed and engage the front, right hub. So, start the vehicle and try to put it in 4WD — watch the diaphram and see what happens. If it does not move, or only moves partially, you have either a leaking vacuum line, your vacuum switch is shot (see pic in a post above) or your diaphram is leaking.

    Using a pair of ViseGrips will allow you to test the front, right hub, but I wouldn’t recommend leaving it like that — for one, the area under the battery is extremely small — I couldn’t get the ViseGrips on and put the battery back in place (which I wouldn’t want to do anyway… just testing). Besides, the ViseGrips are going to tear up that cable and now you have a cable to replace as well.

    Well, I hope some of this info helps you solve your problem — good luck!

    Regards;
    Kyle

  5. Kyle,
    I have an ’02 Chev 2500 with a manuel 4X4 shifter. My problem is that 4hi won’t engage. 4lo seems to work just fine, axle engages, lights on the dash come on, I can hear front axle actuator engaging, but when I try 4hi nothing seems to happen. It feels like the gears are engaging in the transfer case but I don’t hear the actuator engaging and the lights on the dash don’t come on. Any suggestions?

    Kyle’s reply;

    Hi J.P.,

    Hmmm… my first thought is that it is a wiring problem. Why? Simply because you can prove that the system works by putting it into 4Lo. If you can get the system into 4Lo, that tells me the TCCM is working (or at least the 4Lo part is), the encoder motor is working, and the vacuum is working (the front hub is engaging — the vacuum switch and diaphram are working). I’d check the wires are the TCCM first, then possibly the encoder motor next.

    I’ve still got to get some schematics posted for another guest poster above — hopefully I will remember to do so today. ;)

    Regards;
    Kyle

    (UPDATED! 12/04/07 – Schematics have been added to the main post above.)

    1. Hey all!! First off, I cannot lock into 4×4 while driving. My 2002 S-10 Blazer has the 3-button 2-hi, 4-hi, 2-lo. I, when playing around trying to get it to work, found that in order to engage the 4wheel drive I must first put it into Neutral (when completely stopped), then push the 4-lo, then go back to Neutral and push 4-hi. I cannot directly push 4-hi. It will only go into 4-hi after first being put into 4-lo. And sometimes, after getting it into 4-hi it takes a few tries before it wants to kick back into 2-hi. (very odd situation overall)

      I checked the wiring harness behing the passenger kick plate and it is fine and in tact. The wires are all making contact.

      If I push the 4-hi or 4-lo when in park or drive, I get nothing. No clicking, and the light will not light up. But If I do it from Neutral like I just explained, it works like a charm and the lights work as well.

      Any Ideas? No one else seems to have had a problem like this from what I have read. I appreciate any feedback.. Does it sound like a vacuum problem somehweres?

      Kyle’s reply;

      Hi Amy;

      Actually, it sounds like the system is checking the neutral switch and will only actuate in neutral. 4Lo is supposed to operate this way, but 4Hi should switch on the fly.

      Take a look at the schematic “front axel switch & pcm.jpg” — and then have another look at the wires on the TCCM (Transfer Case Control Module), located under the passengers kick panel. Wire #1 (grey with a black stripe) is the PCM (Power Control Module) 4WD Lo signal line — and is one that commonly corrodes.

      The other wire I would check would be the A/T (Automatic Transmission) shift lock switch — this is wire #31 and is light green with a black stripe (there are more pages of the schematics above… the first is a hand -written page of the TCCM connector pinout). This wire should have voltage applied to it at certain times — unfortunately, I have never had to diagnose that part of the system, so I don’t know if it should be +5VDC or +12VDC (but it will be one of them!) and I don’t know if it should be hot when the transmission is in neutral or the other way around — you’ll have to figure that part out. What I would do is start the vehicle and test wire 31 for power — then (obviously, with the assistance of a friend), put the vehicle in gear and test it again — finally, put the vehicle in neutral and test it again. I would *think* neutral and park should be the same (I’d have to test mine to see) and the voltage would be the compliment when the vehicle is in any gear.

      I could be wrong with this, but basic logics (and the schematics) tells me to start looking at the wire that prevents the TC from shifting if the A/T is in gear.

      Good luck and let me know how you make out!

      Regards;
      Kyle

    2. Hey Kyle (and readers)

      I did recheck the TCCM and the wires were all fine and intact.

      However, after playing around some more, I found that there was a bad vacuum switch on top of the transmission behind the transfer case. (I have been trying to post pictures.. but it isn’t letting me.) — see below!

      What it is is a nut no larger than my thumb with 3 vacuum lines into one end and what seems to be a spring loaded ball on the opposing end. The one that I took off my blazer I can easily press this ball in and out of the casing. The new one I put in it next to impossible to press in. The end with the ball gets screwed into the transmission and it looks like the vacuum system controls the movement of that. (from my guess)

      I paid $39.95 for the part and $45 labor.. total of $90.05!!

      I probably saved hundreds if I would have gone to a dealer. I would have done the work myself, but I do not have a lift and that little part was a pain to access, or even get it out for that matter.

      Thanks for your input, it was a great place to start. I hope this might be able to help any others with similar situations.

      Kyle’s reply;

      Hmmm… thought I published this one — sorry for the delay, and thanks for the update!

      $90 — what a nice feeling! ;)

      Kyle

      PS – I have changed the settings to allow registered members to upload images/files. If you wish to upload, you must register. Sorry, but the spam and other crap is just ridiculous anymore.

  6. Hi I just bought a 2001 blazer with 34000 miles on it when I got it home I tried to get in 4-wheel drive but all I can get it to do is blink the lights on the dash, when I press the 4lo the trans goes in to 4lo but the front axle does not engage so whats do you think is going on. thanks Jon

    Kyle’s reply;

    Hi Jon;

    If the transmission engages, but the front hub does not, you have a vacuum problem. Now, this does not always mean there is a problem with the physical vacuum lines, or vacuum itself — it *could* mean the vacuum switch is not getting power, which would not allow it to turn on, which would produce the effect of no vacuum at the diaphram and, well, if you have no vacuum at the diaphram, your front, right axle will no engage.

    That, or you have major problems in the transfer case.

    Regards;
    Kyle

    1. what is it with this number 1 and 6 pin lol awesome information kyle i read your stuff went out pulled it apart and sure enough that was it my 4wd is up and running now im getting a for sale sign because im tired of working on this thing lol thanks jay

      Kyle’s reply;

      Awesome work, Jay! Glad I could help!

      Kyle

  7. I have a 2000 Blazer also, and what a suprise have no 4 wheel drive haven’t scinse last winter, anytime it was really slushy, or wet snow, you know when you need it, it would never work the next day it would, first it was the wires in the dash, they were cleaned put back together it worked..for a day or two, then the module, took it apart the whole thing…that worked for a while, now it’s something else…not the module it’s been checked, anywhere else on this thing that could cause problems with 4 wheel drive…used a test light there’s power there, although the lights don’t come on…please help the snow will be falling soon, and I hope this winter I might have the 4×4 I payed for…

    Kyle’s reply;

    Hey E;

    There’s not much more that I can offer you, other that to say I will post the schematics later today. From there, at least you will know what wires go where and you can test them.

    The first thing to check, though, are your fuses. If there are no lights on the dash, then you may not have “logic” power to the TCCM (logic power is +5VDC). The logic power is supplied through the courtesy lights (do your interior dome lights come on when you open the door?) — check that fuse! (Fuse #8, I believe — it’s NOT listed as having anything to do with the 4WD system — I found that through the schematics.) There are three fuses that I am aware of having to do with the 4WD system — on under the hood (20A), one in the dash fuse paned (10A) and the other is the courtesy lights (15A, I believe).

    I will get the schematics posted here as soon as I can.

    Regards;
    Kyle

    (UPDATED! 12/04/07 – Schematics have been added to the main post above.)

    1. I have a 1998 S10 Blazer with no 4wd.I checked The tccm,diaphram,and encoder motor which all seem to be working properly.The switch on the dash doesn’t flash ,it lights up like it’s supposed to do as if everything is working properly. The only thing i couldn’t find was the vaccuum switch on the firewall. Any suggestions?P.S. great forum!

      Kyle’s reply;

      Hey Raymond;

      If I’m not mistaken 1999 and back, the vacuum switch is on top of the transfer case. 2000 and up it’s on the firewall.

      If it’s on top of the TC, it looks like a nut with 3 hoses sprouting from it.

      Regards;
      Kyle

  8. I had someone help me and found out that the blazer had two problems. First, the hose from the vacuum actuator that goes to the firewall had a hole in it. Secondly, the switch on the encoder motor was bad.
    Thanks for your help.
    Although, I would like to get the wiring schematic from you, if your able to get one.

    Thanks again

    Kyle’s reply;

    Hey, that’s good news — glad I could help! :)

    As for the schematics — yes, but now it sounds like I am making excuses (which I am not!) — I spent all day yesterday redoing my laptop and one (Windows) PC, and do not have everything back in yet (like my all-in-one center for my scanner). I actually sat down to do a little work on the site yesterday, along with updating my backup drive — that’s when the problems started. Anyway, before I turn this post into a rant… I have the schematics sitting right in front of me, ready to be scanned as soon as I am able to get this machine restored.

    Thanks for your comments!

    Regards;
    Kyle

    (UPDATED! 12/04/07 – Schematics have been added to the main post above.)

  9. Hello again got my blazer out of the shop at one time the battery leaked out on that switch under the battery there for it wasnt telling the transfer case to engage got it repaired labor and parts 140.00 thanks Jon

    Kyle’s reply;

    Hey $140 is pretty darn good compared to some of the quotes I have seen people get to fix their 4WD! :)

    Regards;
    Kyle

    1. My service 4wd light is on my knob will not light up when turned I have nothing.. What do you think it may be? I really liked mt tb till right now.

      Kyle’s reply;

      This is the last post like this you will see here — I am getting so tired of the crap that is posted — if you haven’t read the last two pages of posts, or cannot articulate your question in am intelligent manner, your post will simply be deleted.

      Please note that I get 5 times the crap in my inbox than I get posted here — the same applies. If you don’t get a response, you’ll know why.

      KyleStubbins.com is now a “moron free” website — if you are one, don’t bother posting.

  10. I just wanted to say that your site gave me the most insight to my problem and I also want to say thank you! My transfer case wasn’t completing the shift into 4WD Hi/Lo and the buttons were blinking. Last summer I performed a tune up on the truck and noticed that a vacuum hose was worn. I didn’t think anything of it except that my floor heat was probably going to blow in my face. The fix that I performed in the parking lot of the local auto parts store took 5 minutes and the vacuum coupler tube only cost about 75 cents. Thanks again for saving me a butt load of money!

    Kyle’s reply;

    Hi Nick;

    That’s awesome!

    Well, I think you’ll take the cake — I don’t think we’re going to see a repair for any less than 75 cents! Good work!

    Regards;
    Kyle

  11. 4wd will not engage? The lights on the dash dont blink, they are lit as if everything is normal, but no 4 wd. I checked all the fuses and they are fine. Could it be the vacume are something else?

    Kyle’s reply;

    Hello Mike;

    You aren’t getting any flashing of the lights at all (when you try to engage 4WD)? There should only be one light on at a time (indicating which range you are in) — multiple lights should flash when you try to engage 4WD (either Hi or Lo). If you try to engage 4WD and the lights do not change (ie. no flashing), I would suspect a problem with either the wiring to the TCCM or the TCCM itself is bad. I don’t want to give you bad news, but I would suspect the latter as the lights will usually flash while the system is trying to engage 4WD, even if it doesn’t engage, the lights flash while it’s trying. If the lights don’t flash, that tells me that the system isn’t even trying to engage 4WD — and that’s controlled by the TCCM.

    Regards;
    Kyle

    1. Thanks for the response to my post earlier, and i had pep boys do a vacuum test on the system and they said every thing was seemed to be ok, so i bought that four wheel drive swith from napa and installed it,(it went into the front axle right where the cable from the actuator attaches), still no luck. so i could see some kind of sensor on top of the transfer case with 3 vacuum lines attached to it, so i replaced i, and once again still nothing, so i have a couple more questions…their is another sensor on the tail pice of the transfer case, i was told it is the speed control sensor, could that be the problem? and do you know if the transfer case would not engage due to low fluid level? i dont want to replace the fluid unless i have to. and would you know how to determine if the switch itself is bad? And do you know how would i tell if the control module under the dash is bad.

      Thanks for your time,
      Ryan

      Kyle’s reply;

      Hello, Ryan. Again, I am not a mechanic, so I really don’t know the answers to most of your questions. I can only offer what I, personally, have had experience with. I will try to address what I can.

      Your question regarding the speed control sensor — this could cause a problem, but I guess it depends on if it’s a transmission speed control sensor, or a vehicle speed control sensor. 4Lo will only engage under 8KM (from memory, I believe it to be correct), but 4Hi is shift-on-the-fly and should engage at, pretty much, any speed (from memory, I think it may not shift above 100KM, but I’m not sure on that).

      Could low fluid cause a problem? Yes. You don’t have to replace your fluid if it’s low — just top it up to the proper amount. However, I’d be asking myself WHY it’s low.

      How to test if the switch is bad? I’m not sure which switch you’re talking about, but no, I do not. I’d simply Google it most of the time to find what I’m looking for. Everything that I ever wanted to test was found through Google.

      How to tell if the TCCM is bad? Well, that’s a little too involved for me to detail here (basically, it involves systematic testing of the various wires). If you can read a schematic, then you’ll know what to do with it — all schematics that you’ll need are in post 1 of this thread. If you look at the schematics and still don’t know what to do (and I’m not being smart), the job is over your head and you should take it to a professional.

      Unfortunately, I don’t really have any advise that is going to help you any further.

      Regards;
      Kyle

  12. ?? 1999 Blazer 4×4. When I try to engage 4×4, fuse under hood, “ATS” blows. Replace fuse and try to engage, blows again. Any suggestions??

    Jim Frohner

    Kyle’s reply;

    Hi Jim;

    Well, that’s one that I haven’t heard yet. The “ATS” fuse (which is marked “ATC” on this schematic) supplies 12VDC to two large orange wires at the TCCM (marked as pins C18 and C19 on the schematic). As I said, I haven’t seen this problem before, so I haven’t traced that part of the system. I can only assume (yeah, I know!) that the because these wires supply 12VDC, they are used to run heavier things such as the encoder motor (5VDC, which is the light guage orange wire, is used for logic — ie. to run the “chips” on the circuit board).

    Hopefully, you can see where I’m going with this. I’m just trying to think about it logically — if the fuse is blowing that supplies 12VDC, what in the 4WD system would use 12VDC? The encoder motor is the first thing that comes to mind.

    Again, I have not tried this, but if it were me, I would try and disconnect the encoder motor and try again (this may not work as the system may detect the unplugged encoder and not try to engage 4WD — the dash lights should flash if it’s trying to engage 4WD). If you don’t blow a fuse with the encoder motor replaced, you have found your problem — well, sort of. From there it could either be the motor itself, a physical obstruction of the motor (the shaft could be frozen (corrosion) to the transfer case preventing the motor from moving) or wiring between the encoder motor and the TCCM (a short from supply to ground).

    Good luck and let me know how it works out.

    Regards;
    Kyle

    1. My 4 wheel drive went out …..is the 92 like the 98’s with the fuse problem or is it something totally different

      Kyle’s reply;

      Don’t know… when you find out, let me know.

      Regards;
      Kyle

  13. I have a 2001 Chevy S-10 unpredictable 4X4. The lights on the dash will blink when pushed, but nothing happens and it goes back into 2 wheel drive – my usual routine is to disconnect the negative battery cable wait for about 15 minutes reconnect the cable turn the truck on and push the 4HI button – this may work to put the truck in 4X4 mode 1 time or the fix may last for a month with no problems with switching from 2 to 4 wheel drive and back, but I never know when its not going to work. Any suggestions as to what may be going wrong? Your site is very informative. Thanks.

    Kyle’s reply;

    Hi Sam;

    Man, I hate “intermittent” problems — they’re, generally, a PITA to track down.

    I really don’t know what to tell you on this one — but I’ll give it some thought.

    My first thought is TCCM problems. Disconnecting the battery is a standard procedure to reset any “OB (On-Board) computer” (although it’s usually recommended to disconnect for 30 minutes — simply so you don’t rush it and it doesn’t reset). So, what you’re doing each time you do this is you are resetting the TCCM. From there, it works, but you never know for how long. Again, that makes me think TCCM. It sounds like there is something going wrong with the logic of the TCCM and it’s “freezing” at a certain spot in the programming. This is, basically, like your computer at home (of course, we’re only talking Windows machines here, not a real OS like Linux!) — when it freezes, you turn it off and start again (reset). Everything works fine, but you know it will happen again, just not when.

    The only thing I can suggest, which may be of no value to you, is to try and swap out the TCCM. You could try a used one from the scrap yard, but you never really know what you’re getting there (you may be in worse shape). There are companies that rebuild them (the last one I heard my buddy at the garage buy was like $150, his cost, for a rebuilt unit), or, hopefully, your mechanic will have a good one around that you can use to see if that is the problem (of course, with the latter method, you will need to have a great relationship with your mechanic!).

    Yes, all of the above “testing” of the TCCM may not be feasible for you. I know it’s a problem that I would not want to tackle as I do not have access to another TCCM, so it makes like rough.

    One of my hobbies is electronics design and programming — so, if it were me, I would take the schematics (all schematics are in the first post of this thread) and start “probing” the TCCM to see what is doing what. I would record that info, and then do the exact same procedure the next time I had the problem to see what has changed. It may not help you actually fix the problem, but it will help you find it — once it’s found it can be fixed.

    I know I’ve probably just rambled on and told you nothing — well, that was simply my thought process when trying to find a problem. I can only hope it helped.

    Let me know how you make out. If I think of anything else, I will update this post.

    Regards;
    Kyle

    1. Wierd problem w/ my 2000 Blazer. Everything is responding perfect in the “electrics” says it shifts to 4H & 4L. worked perfect a week ago… Now it snows, and I’m stuck w/ 2WD. The indicators are working perfect (flash, flash, flash… Solid light!)
      The problem is when I try to go up my hill on a snow covered road the only tires that engage are the rear. I can hear the hardware adjust under the truck between 2H, 4H & 4L – but the front axel(s?) doesn’t/don’t lock in? It did a week ago, when I pulled another car up the street. I’m lucky, because most of my neighbors drive GMs, but it’s going to be embarassing if I need a honda civic to pull me up the street this winter.

      Thoughts?

      Best way to identify/isolate Trans/Case vs. front gearbox?
      I read the blog, Not sure if Vacuume line resolution would apply? Electrics seem all in place. Adjustment needed?

      Please help me, If you get the resolution I’ll pay ya!
      (I’m sure you’re cheaper than the dealer!)

      –Bryce

      Kyle’s reply;

      Hi Bryce;

      Sounds like a front diff problem. I’m not sure of the years, but the tranny shop that I had mine at first checked for this problem as it is, apparently, common. There is a gear and a ring that holds it in (if I remember correctly, because it wasn’t my problem, so I dismissed it)… the ring breaks and the gear moves out of place. Something to that effect, but I’m not 100%.

      If it was the actuator, I’d think the front end would pull to the left… the actuator controls the right front axle only (AFAIK). The left is always engaged with the front diff.

      One thing that you can check quickly is to lay down beside the vehicle and have someone SLOWLY drive forward (while in 4WD)… you’re looking to see if the driveshaft from the TC to the front diff is actually turning. If it is, the problem is forward of that (ie. front diff), if not, the problem is in the TC.

      Regards;
      Kyle

      PS – Nice to have options with the Polaris though, isn’t it! LOL! Just bought myself a 700 x2 Sportsman this year… and glad I put a blade on it! I’m thinking tracks for next year!

  14. I don’t know if you still have this site or not. The posts that I looked at
    were from 2005 I think. I want to thank you for your time in keeping this
    information on line. I had no 4wd in my 1998 s10. Your post said to check
    several things and the 4wd actuator was my problem. Again, many thanks. Saved me $$$ for repairs. THANKS

    Kyle’s reply;

    Hi Ron;

    Yes, the site is still active — actually, I can’t believe how active (for my little blog site) it has been in the last few weeks — and all about the Blazer! (I guess it’s just that time of year when people are trying their 4WD! :) Hey, I’m glad I can help others fix these problems, instead of shelling out big bucks to the dealers.

    If you don’t mind, I would very much like to post your comments — if you wish, I can leave your name out — of course, the choice is yours and I will respect any decision you make, just let me know.

    Regards;
    Kyle

    Kyle, you can post as you like.To add info, my dealer was talking about replacing the 2 computer modules associated with the 4wd, They said same as when they replaced them in ’05 that both would have to be replaced at $1060 plus tax and that was a mininum. Again many thanks for your information. My 4wd is working now thanks to you.$60 for parts and one hour to reinstall. Ron bobbitt in West Virginia

    Kyle’s reply;

    Great! Thanks again for the feedback, and allowing me to post your comments — they are appreciated!

    Best Regards;
    Kyle

    1. I have a 1995 GMC Jimmy, the selector and transfer case work. the front driveline spins but the tires are not engaging. is it the actuator or something else. i have never worked on an electronic 4WD system and so am lost. i have read all of yhe posts but am confused as to what im looking at. any input would be appreciated. thanks

      Kyle’s reply;

      Check the actuator under the battery… the info you need has been posted within this thread.

      Regards;
      Kyle

  15. i have a 2000 chevy blazer sport model. my 4wd has worked fine up until we had a snow storm and i was trying to get back home on a hill. i attempted to engage 4wd hi by pressing the button, but nothing happened. i put it in neutral, pressed 4hi, and the lights blinked but switched back to 2hi. i pressed the button for 4low, and it switched and i was able to get up the hill. any ideas?

    Kyle’s reply;

    Hmmm… could be a few things. First, if you can get into 4Lo, things probably aren’t as bad as they seem — this, basically, tells you that the components in the system are working. The TCCM is working, the encoder motor is working, the front, right axle is engaging, and your vacuum lines are working (there are more, but these are the important ones). The trick here will be to find out what is 4Hi dependent, and why it won’t kick into 4Hi.

    The next thing I noticed was that you said the lights on the dash will flash when you press 4Hi — this tells you that the button for 4Hi is working, and the TCCM is accepting the signal from it. So, it boils down to a problem somewhere between the TCCM and the encoder motor — I would suspect the TCCM — possibly wiring, or possibly a bad TCCM. It’s really hard to tell without being able to put my hands on it. However, you may also want to check the encoder motor signal lines (C26 through C28 on the pinout from the first post) — they should be high (+5VDC) when the are not in use (resistors pull the lines high), and low when they are active (called “active low”). All schematics are posted in the first post of this thread.

    Good luck and let me know how you make out (or if you have any further information / questions).

    Regards;
    Kyle

    1. Hi Kyle,
      I can’t thank you enough for this blog. I got a 95 Jimmy on an impound auction for $320. I got it started, and sure enough no 4 wheel drive. I noticed the dash buttons seemed like they didn’t click, they lit up when ingnition was turned on, then 2 hi stayed lit, thats all I could get them to do. So I grabed onto the 2 wheel button and gave a little tug and it poped right out of the dash. I went to to local junk yard and got one for $10 and low and behold transfer case locks in, in both HI and LOW. still no 4 wheel yet so I did low tech test on the acuater line (blew into hose) air flowed freely so now I get to fight under a truck. If it wern’t for you, I’d still be scratchin’ my head becomming even more bald.
      THANKS AGAIN,
      Rick

      Kyle’s reply;

      Hi Rick;

      That’s great news! And if you’re getting free flowing vacuum, then you know what the problem is… you just have to find it! Good luck with that, and I hope you have a warm garage to do it in! Let me know.

      Regards;
      Kyle

    2. Hi again Kyle,

      sure enough the vacume line was bad by the valve cover. I first changed the vacume switch on the transfer case but it was only $16 and I never have to worry about it again. So for $26, and the vast information on this blog, I spent about 2 hours to get it working. Can’t thank you enough.

      Rick

      Kyle’s reply;

      Hi Rick;

      So, for $356 (and a few hours of your time) you got a whole truck WITH working 4WD… not too shabby! ;)

      Glad that I could help!

      Regards;
      Kyle

  16. now that i have the blazer on flat land, i tried to engage the 4wd “on the fly” and nothing happened. i put it in park, and again nothing. the only way to engage it (no buttons flashing at all when in park or drive) is to put it in neutral, and again only will go into 4low.

    i will check out the TCCM problems you mentioned. thanks!

    Kyle’s reply;

    So, you’re saying that the lights DO NOT flash when trying to engage 4Hi? That’s a little different, with a couple more small things to check as the lights should flash if the TCCM is getting the signal.

    The grey with black stripe wire (GRY/BLK) at pin 5 is the 4Hi signal wire — when pressing the 4Hi button, this wire should go hot (I haven’t test the wire, so I’m not sure if it’s using logic levels (+5VDC) or battery power (+12VDC) — either way, it should be hot when the button is pressed. You will want to check pin 5 of the TCCM connector, for sure — as you can see from the original post in this thread, the wire harness at the TCCM is a problem — specifically, pins 1 and 6 — pin 5 is right next door to the small orange wire (pin 6).

    Secondly, your dash buttons will pop out of the dash with little effort — pop out the module and check the wiring harness on the back of that. I’ve heard of people experience problems with the actual buttons.

    As I said, I have not tested any of the button wires, as I have not had that particular problem (yet) — so, before you do this, you will want to verify what I am saying is true.

    Put a test lead (volt meter) on the output of the 4Lo button… when you press 4 Lo, does the meter indicate that there is power leaving the button? (It should, if what I am reading here is correct… and I’m reading it correctly.) Now, do the same for 4Hi (do both tests in park) — does it output the same as 4Lo? If not, you may wish to check the harness at the buttons a little more carefully first, and then try applying power to the output side of the 4Hi button (the same level as the output from 4Lo, whether it be +12VDC or +5VDC… I’m thinking it should be +12VDC though). Basically, this will emulate the button being pressed and should trigger the TCCM to engage 4Hi. If it does not, the problem is at the TCCM, most-likely. Again, check pin 5 at the TCCM. You may also wish to try applying power to pin 5 directly at the TCCM and see if it engages 4Hi.

    If you are sure that the signal from the button is reaching the TCCM, then either the TCCM is bad, or the encoder signal lines have been damaged. Again, I’d have to test wires to be positive, but it appears (from the schematics) that the encoder signal lines are hot (pull-up resistors are being shown on each signal line) and are pulled low to activate a particular mode. Pins 26 through 28, on the TCCM pinout, are the encoder signal lines. Basically, test each of the lines — if they are all hot while in 2Hi, then when you try to change to 2Lo, one of the lines should go low (0VDC) — the same should happen with 4Hi.

    So, if you try to engage 4Hi and you’re sure there is power at pin 5 (4Hi button signal), and none of the lines change when trying to change to 4Hi, I would suspect the TCCM. If the encoder signal line DOES change, then the transfer case encoder motor module is bad.

    Be logical, and methodical and eventually you will find your problem.

    DISCLAIMER: Please note that I have stated numerous times that I have not tested any of what I have said here — I am simply thinking about the problem logically while looking at the system schematics. TEST ALL WIRES before applying power to them.

    Good luck and let me know how you make out.

    Regards;
    Kyle

    1. My wifes blazer last winter (last time we used 4×4) worked fine this winter cant get it to work at all if I push 4 low the tranny shifts into low but no 4×4 and the light just blinks, I hit auto 4 the light just blinks, but I hit 4 high I can hear a small eletric motor at the transfer case turn for about a second but still the light just blinks and when I hit 2 hi coming from 4 hi again I hear the the small eletric motor but if i hit 2 hi from any other blinking light the light just stops blinking and 2 hi lights up. I’m about to check the tccm but seince I here the eletric motor move i dont think its the prolbem any ideas?

  17. Hi I have a 97 sonoma zr2 the 4hi works but going up hills it wants to slipp (loses power at hi rpms.) The 4lo light justs blinks all the time never goes in just goes back to 4hi. I here the clicking with the TCCM and I also hear the encoding motor working also.

    Kyle’s reply;

    Sounds like your transfer case gears are worn — I just had mine replaced with very few hours on them.

    Regards;
    Kyle

    1. I have a 2002 Chevy K1500 that has a problems with the 4-wheel drive system. Initially it wouldn’t disengage the 4WD in the front differential. With a little advice from the local service garage I replaced the electronic actuator as well as the swith for the dash. It goes into 4WD but wont disengage unless I crawl underneath and unhook things to back it out manually. The lights for the dash switch will not illuminate 2WD,4WD-high or low now. Before replacing the switch in the dash it was getting stuck with 2WD & 4WD indicator lights illuminated after I put it into 4WD high. I still had to disengage the 4WD in the front differential manually. Is there anything else I can check before buying the expensive module for the 4WD?

      Kyle’s reply;

      As I have stated many times within the three pages of this post… everything that I know about the Chevy 4WD system has been posted here. I have no further information to offer. I am NOT a mechanic.

      Regards;
      Kyle

  18. I wish I could find the schematic you are talking about. I found this link in your posts, but it doesn’t work: http://www.kylestubbins.com/node/files/TCCM_power&ground.jpg

    Thank you,
    Sven

    Kyle’s reply;

    Hi Sven;

    Give them a try now (at the bottom of the first post). I have adjusted the settings of uploaded files, but I am not sure why you are seeing “node” in the link. The links should be http://kylestubbins.com/files/{whatever_the_file_name_is}.

    Regards;
    Kyle

  19. Hey I have a 99 s-10 that is stuck in 4wd. It will shift from 4hi to 4low no problem. But will not shift to 2wd. The lights 2wd, 4hi, and 4low lights work properly. What should I look at? thanks, matt

    Kyle’s reply;

    Hi Matt;

    Hmmm… it’s hard to say — it could be as simple as the encoder motor signal lines being corroded/damaged (check the schematics in the first post), or it could be the transfer case itself.

    If the lights work as they should, then this tells me the 4WD system is engaging, doing what it should. I suspect the transfer case is not shifting properly. As I said to a poster above, it may be time to have your transfer case checked by a tranny shop. The transfer case in this vehicle is weak — it has a design flaw in it from the factory. There is a “fork” (for lack of a better term) with a plastic bushing in the TC that wears out and allows the gears to float — mine were just replaced (the forks have been redesigned) with very few hours on them. In fact, mine were worn so bad that the gears had been rubbing the case and would have rubbed through the TC, had I left it as is, in the very near future.

    Regards;
    Kyle

    UPDATED 12/12/07 – I just wanted to add to this post that the TCCM has two relays within that control the actual switching of the encoder motor — you that is what you hear “clicking” when you try to engage the 4WD system. If you hear clicking going into 4Lo, but not coming out (to 2Hi or 4Hi), then you may also have a bad relay in the TCCM. Replacement of the TCCM will be required (unless you want to take it to an electronics guy who may be able to replace the relay for you).

  20. Searched the Internet and found your site. My 4 Wheel Hi Light would flash for a few seconds and switch to 2 Wheel Hi, I followed you advice and found 2 corroded wires at the 4 wire connector on the T.C.C.M. which I cleaned. My lights would now flash and not switch back to 2 wheel hi.I went further and looked at the Vacuum Switch as suggested in your blog. I connected the vacuum lines together (while the 4 wheel light was flashing) and this engaged the 4 Wheel System. I reconnected the vacuum lines, lightly tapped the Vacuum Switch and it began to work like new. My guess is that the switch stuck due to lack of use. Without your blog the car dealer would be spending my money about now. Thanks!

    Kyle’s reply;

    Hi Dan;

    Thanks for the update — that’s what we like to hear!

    I’m glad to have been able to help keep a few of those greenbacks in your wallet — especially at Christmas!

    Best Regards;
    Kyle

  21. i have a 1998 blazer and i have a similar problem to the rest of problems above. My 4 lo will work. and i hear the module click when it moves into 4lo. but when i try to move into 4 hi the lights flash and then goes directly to 2hi. i was wondering if you can plaese help?

    Kyle’s reply;

    Hi Scott;

    It’s hard to diagnose problems like this without being able to test things — but it sounds like either the TC encoder motor signal lines are bad (a bad ground can be a problem too) or the “forks” in the transfer case are worn, allowing the gears to float, in which case the gears for 4Hi may not be moving into place.

    The first problem you can test yourself. Grab the schematics from the first post and test pins 26 through 29 for logic power (+5VDC), using pin 16 as your ground. You may also want to get under the vehicle and test the same wires at the encoder motor. Test the ground wire there as well.

    The second, well, that’s gonna take a trip to the tranny shop.

    Regards;
    Kyle

  22. My service 4WD light came on, and the push buttons on the dash to activate the 4WD will not work. I checked all fuses in both panels, and the wires in the TCCM harness. Everything checked out. What next?

    Kyle’s reply;

    Hi Jay;

    When SERVICE 4WD appears on your Message Center it indicates that there is a problem with your Automatic Transfer Case. The system is designed to catch many problems before they become catastrophic. When SERVICE 4WD appears you should contact your local Chevrolet dealer immediately.

    Now, I don’t know if I rush right out to the dealer as there can be a few small problems that would cause this condition, depending on what that particular model/year has been programmed to detect.

    Are there lights on the push buttons? Either way, the button module simply snaps into place in the dash and is easy to get out. Pull the button module and check for loose wires on the back of the switch.

    I’d also check the vacuum actuator on the firewall.

    After that, I’d pull the ground from the battery and let it sit for about a 1/2 an hour. Come back, attach the ground, cycle the key from on to off 3 times and try the 4WD system again.

    Of course, your TCCM could be bad, as well as having actual problems within the ATC itself. That’s the problem with systems like this — it can be almost anything.

    If you want to try out a few things and get back to me with some more info/questions, I’d be happy to try and help you through your problem.

    Regards;
    Kyle

  23. I have a 98 Blazer, my 4 wheel drive a few months back would engage while I was driving in 2 wheel drive, causing a loud grinding or clicking noise, but when pushing the 4 wheel drive button the clicking would stop. I’m thinking that the 4 wheel was not fully engaged, caught between 2 and 4. Anyway I went to go use my 4 wheel drive the other day after snow and I hear the shafts engage but have no 4X4 at all. Could I have damaged the transfer case or wiped out the splines on the CV joints. How do I fix it and what much do you think it will cost. Any help would be greatly appreciated I should have took care of the problem when I heard the grinding noise

    Kyle’s reply;

    Hey Tom;

    I don’t want to give you any bad news, but it sounds very similar to what I just went through… and it was the Transfer Case that needed service… badly. My Blazer probably only has 10-15 hours on the 4WD system, yet the “forks” inside the TC were badly worn and allowing my gears to float, which produced symptoms like you describe (among others). It seems this is a design flaw from GM and has been “updated” (my “forks”, and their corresponding bushings, have been replaced with the updated parts… so we’ll see).

    The solution is to take it to a tranny shop and have the forks/bushings replaced.

    Of course, I can’t guarantee this is your problem, especially over the internet, and I hope it isn’t — but I would have it checked out professionally.

    Regards;
    Kyle

  24. Hi,
    I just wanted to say thank you for having this information posted on your blog. The info you have was a very big help is solving my problem. I have a 2001 Chevey Trailblazer 4×4. I was trying to use my 4 wheel drive this winter, and all I could get were blinking lights. My 2 wheel drive would engage but not my 4. I read about the tccm and the vaccum switch and to my certain amazement the vaccum line had broken off the switch. I fixed this issue and now my 4 wheel drive is working again. The info you posted saved me a trip to a dealer!!!
    Thank You,
    Greg

    Kyle’s reply;

    That’s what I like to hear! Extra $$$ for Christmas! ;)

    Glad I could help, Greg.

    Regards;
    Kyle

    1. I have a 99′ S10 Pickup and 4wd works fine but the front drive shaft will not disengage from the axles when switched to 2wd. Basically the transfer case is switching from 4wd to 2wd but the front axle is always engaged. I traced the cable cable back to the vacuum actuator under tha batery and found that the cable is not bound up but the actuator alway has vaccuum to it regardless if the vehicle is in 2wd or 4wd. It looks like the vaccuum is coming from a module on the transfer case but I was not sure how it all worked together. Anybody have any suggestions? I don’t have a service manual for the truck, but am not opposed to buying one if it has a good section on trouble shooting the 4wd system.

      Thanks
      Dan

      Kyle’s reply;

      Hey Dan;

      Sorry for the delay in getting your post up.

      It sounds like a problem another reader had, Amy, in this post.

      Let me know if that helps.

      Regards;
      Kyle

  25. I have and 2003 4wd blazer. When I engaged 4wd, the light will come on instantly but the 4wd will not engage. I hear the TCCM click when I engage and disengage. I also hear what sounds like air leaking when the drivers door is open and 4wd is engaged.

    Kyle’s reply;

    Hi John;

    That’s odd that the light will come on instantly, without flashing and without engaging the 4WD. The solid light coming on at 4Hi is usually an indication that the system is functional.

    Well, let’s start with the most obvious problem — the “leaking air” sound. Most likely this is a vacuum line that has either come off, or is broken. There are a couple of places this could happen — however, hearing it with the drivers door open is probably an indicator that it’s coming either from the mainfold, or the 4WD vacuum switch (pictured above, located on the firewall). The other spot for 4WD vacuum is under the battery — but that is quite a distance from the drivers door, although anything is possible.

    In short, start looking for a vacuum leak.

    Regards;
    Kyle

    1. First off let me start off by saying if it wasnt for this blog i probably wouldn’t have any idea where to start in trying to fix my 4×4 issue. Like many other ppl on this blog i am having an issue trying to engage my 4×4. When i push the 4Hi button all the light does is blink. Following ur advise i checked the harness for any corrision and was amazed to find very little at all so i moved on the the vacuum switch. I found there that one of the hoses seemed to have ripped from the switch and it is no where to be found. Its the top hose and i was wondering if you could give me some insight as to where this one hose goes. Any help would be much apprecitated. It would make sense to me that if this hose was missing so would be my 4×4 lol. Thank you sir!

      Kyle’s reply;

      Hi Mark;

      First, I apologize for the delay in responding — I had a crazy few weeks around the holidays and didn’t do much blogging.

      Unfortunately, I’m not really the guy to help you with the vacuum line as I’ve never had any problems with the vacuum and haven’t had the need to trace hoses.

      You might want to check this post to see if it might be of any assistance to you.

      Regards;
      Kyle

  26. Hi it is scott again. You told me earlier that it was the ecoder motor. is there another name for that or what is it and where is it located because i have never heard of it?

    And i cant use test pins 26 and 29 because mine only goes to 16. but there is a two side labeld c and d on the module connector. What am i suppose to do here.

    Kyle’s reply;

    Hi Scott;

    Hmmm, sounds like you have a different system there — accordingly, I cannot help you any further as I can only help with what I am familiar with.

    If there is no connector, like I describe, then you are going to need another set of schematics, and someone familiar with that system.

    Good luck and best regards;
    Kyle

    PS – nope, no other name for the Encoder Motor, that I know of. It’s located on the side of the transfer case and it what does the physical shifting between “modes” (2Hi, 4Hi/4Lo).

  27. Hey Kyle,

    I ment to mention, I had my transmission rebuilt 2 months ago and the 4wd worked then. I put my Blazer on jack stands and began tracing the hissing sound (air leaking?). I’ve discoverd the hissing noise is coming from the transfer case. I can here the encoder motor activating and deactivating. I wonder if the mechanics did something to the transfer case during rebuilding of the transmission. Anyway, what could be making this hissing sound in or near this transfer case.

    Thanks for you help

    Kyle’s reply;

    Hello again, John;

    I, myself, don’t know of any vacuum located at the transfer case, but I will try to find out for you (I have a transmission shop in the family — I just called, but the knowledgable one is not yet in).

    The TC is very close to the front of the vehicle — are you sure the vacuum is not coming from the firewall area? The vacuum for the 4WD system starts on the drivers side, goes to the vacuum actuator in the middle of the firewall (but below the distributor and hard to see — this is what I suspect you hear — there is a picture up about 10 posts or so) and then goes to the diaphram under the battery.

    If I find any further vacuum info, I’ll post it here for your reference.

    Regards;
    Kyle

  28. I have a 2000 Blazer 4wd. When I engage the 4 wd it goes into 4wd hi ok but after about 30 seconds the service 4wd light comes on. and it will not go back to 2w hi. thanks, great blog.

    Kyle’s reply;

    Hi Dave;

    Doesn’t sound good. It sounds like what I just went through — and it turned out to be the gears/forks in the transfer case. I first noticed that I had difficulty coming out of 4WD, then I started noticing 4Hi “slipping” in and out.

    GM has a design flaw in this particular transfer case — the “forks” had plastic bushing that will wear out and allow the gears of the TC float, and will not pull the gears into their proper positions.

    I would recommend having it looked at by a tranny shop. The reason I suggest this is, if it is the problem I describe, the gears are actually rubbing the side of the transfer case — and they will wear through, effectively causing a lot more damage than where you’re at now (ie. complete replacement of the TC).

    Thanks for your comments!

    Kyle

  29. Greetings. Here’s what I got. If i press the 4hi button, the indicator lights up saying it is working. If I put the truck in neutral and press the 4lo it lights up as if it is working. Neither one seems to work as I got stuck in my driveway. Please help.

    Kyle’s reply;

    You need to do some further investigation to provide more information — there’s all kinds of things to try posted above.

    The first thing I would do is disconnect the battery and let it sit for a 1/2 hour or so — then reconnect the battery and cycle the ignition from on to off three times — this should reset the TCCM.

    Regards;
    Kyle

  30. My sisters 2004 Blazer kicked out of 4-Hi one day while driving in the snow. She couldn’t get it to work thereafter. I found that 4-Lo worked while in neutral, but if I selected 4-Hi I’d hear the relay click in the TCCM, the LED on the switch would flash on, but then it would go to 2WD.

    So, I tried what Sam mentioned above, and also read in other message boards… to disconnect the battery for 1/2 hour to reset the computers. Low and behold, it works perfectly now. How long it will last I don’t know… If anyone learns more about this problem, it would be great to learn what is going on.

    Good Luck,
    Sven

    Kyle’s reply;

    Hi Sven — thanks for the update — glad you got it working again!

    Regards;
    Kyle

  31. Hello, your postings seem very informative so i figured i would pick your brain for a sec. my 4wd would not come out of 4hi and i would get dim lights and hear the encoder motor wind up when i would try to engage it. several people told me that it was most deffinetely the encoder motor. so i tried to change it and when i did it put the transfer case into nuetral and the car goes nowhere and on top of that the 4wd error ligth on my dash will not go out.
    should i be unplugging my tccm and resetting the system after i change to encoder motor. i thought it was a transfer case proplem because i thought it was binding up and not letting teh encoder motor start off in 2wd. so i just spent 7 hours on my back yesterday changing the transfer case just to find out that it is suppsed to be that way. i am out of my mind rigth now and that has to be one of the worst mechanically involved jobs i have ever been involved in.
    so now i have a new transfer case and and a brand new encoder motor that i cant put on because it will not come out of nuetral on the trasfer case unless i shift it manually. and because i have to leave the encoder off the front axle wont engage because its not getting the rigth message.
    please help!

    Kyle’s reply;

    Hi George;

    I do apologize, but I am not following you at all — I’ll try to answer some of your questions.

    Yes, after replacing the TC or the encoder, I would reset the system by disconnecting the battery for 1/2 hour and then cycle the ignition from on to off three times.

    If you can hear the encoder motor “wind up” — the odds of it being shot are slim.

    As for the rest of your problem with the TC being in neutral — I don’t know the answer to your question — as far as I know, 2WD is neutral as far as the TC is concerned.

    Regards;
    Kyle

  32. Thanks for the extra pointers. I ended up just taking it in to the shop because I was in over my head once I got beyond seeing the shaft turning and the need was urgent (truck doesn’t drive worth a darn in ANY amount of snow w/o 4wd).

    They ended up deciding my vacuum switch was shot and when I went back the new switch still didn’t do the trick, so they figure it must be the actual pushbutton assembly on the dash. ugh. No idea what that’s gonna cost, but I bet it’s spendy. On the bright side they showed me where the switch was and how to manually enable/disable 4wd by splicing the vacuum lines together with a snippet of brake line :) Works like a charm in hi and lo now so that seemingly rules out the rest of the system. I was tempted to just manually join/unjoin the vacuum lines until Christmas debt is paid off, but I feel obligated to let them finish the repair now that the pushbutton is in.

    Awesome blog thread!

    Kyle’s reply;

    Hey Greg;

    Man, if it’s just a button assembly, don’t worry — there are no electronics in the button assembly — they are just that — buttons (switches). Meaning, it shouldn’t cost all that much. The most expensive part will be the dealers labour rate.

    Heck, if you want, run yourself a length of vacuum line into the passenger compartment and put a ball valve on it! :) (But, like you say, the assembly is in — and it’s much more professional!)

    Thanks for the update!

    Regards;
    Kyle

  33. Sorry it took so long to get back. I repaced the encoder motor and all is well, working like a charm. You were right, if you disconnect the moter and try to engage, the dash lights up “Service 4X4.”

    Thanks again, Jim Frohner

    Kyle’s reply;

    Hi Jim;

    Excellent — I’m glad that my blog was able to help you solve your problem, and hopefully save some money!

    Regards;
    Kyle

  34. I have a similar, but not identical, situation. When I searched the net, I found another similar problem to yours and a reply. I’ll summarize what I found then explain my problem, in the hope that someone might be able to help me. Many thanks to anyone who can.

    I found a forum where a guy had a problem where his 99 Blazer would not shift to 4HI. When he pushed the 2HI button, the light flashed but nothing happened. Except that he heard a clicking noise coming from around the front wheels. The person responding said the problem is common, caused by a faulty actuator, and replacing it should resolve the issue. I hope this helps.

    Like you, I also have a 98 Blazer but my problem is that it no longer switches from 4-wheel hi to 2-wheel drive smoothly. When I push the 2-wheel button, it continues in 4-wheel drive for about 1-2 minutes. Then, there is a jolt and a noise that feels and sounds just like I hit a pothole. In fact, that’s what I thought had happened the first few times. Instead, this jolt immediately precedes the 2-wheel drive being engaged.

    I’ve tried stopping and putting it in park before switching and, although this did work some of the time, not every time. Also, during the time it takes to switch, the truck tugs slightly to the right. Just like it was slightly out of alignment. However, after it switches, the tugging stops.

    There is no problem switching from 2WD to 4WD.

    Mark Biggerman

    Kyle’s reply;

    Hi Mark;

    Have a look at this post –> More 4 wheel drive problems with my ’03 Blazer (UPDATED).

    It sounds like a similar problem I have already dealt with.

    Regards;
    Kyle

  35. Picture of kick panel? What picture? What is a kick panel?

    OK….I just lost my 4WD trying to get over the snow in front of me in my parking space at work this morning. I got stuck bad four times in the last 1/2 mile before home and finally had to walk the last 1/8 mile. It took me two hours to get home (about 5 miles!!!). Maybe they should have plowed the roads??? My one son in law thinks I may have tried it in reverse. He has a Ford that won’t allow 4WD in reverse. Is Chevy the same way? I remember having it in 4HI then trying 4LO but at one point both lights were blinking now they don’t light at all. I just used 4HI a few days ago but never tried 4LO until today. I read all the reviews about American vehicles and like a fool I bought this Blazer because it seemed like a good deal. I’m still trying to figure out some of the electrical problems out. Our great car companies thought they were going to sell junk forever and we were going to keep buying it. WRONG!!!! This is my last American car until something changes. Just this week I junked my Pontiac with the famous Quad4 engine that broke a connecting rod twice.. A couple years ago I scrapped my Dodge Caravan after the trans mission EXPLODED twice. And I do mean explode! NO MORE GM, Ford, Chrysler!!!

    Kyle’s reply: Hi there! Yeah, I did forget about taking some more pics — for whatever reason (probably the lack of light), the pics I took were very fuzzy. I’ll have to try and remember to take some more.

    I can describe it a little more for you though! The “kick panel” is located on the passengers side, front seat, right where the passengers right foot would rest. If you’re sitting in the passengers seat, “swing” your right foot from left to right while keeping your heel on the floor. When your toes hit something on the right… you have found the kick panel! :) This panel is snapped on, but it does hook under a couple of other plastic panels, so be a little careful with it.

    Once you have removed the plastic kick panel, you will see a “silver box”, approximately 4″ x 6″, with a large electrical plug into the side (about 30 wires) — that is your 4×4 control module. What you’re looking for is a whitish-greenish “powder” on the electrical contacts — that is corrosion and causes loss of signal from the 4×4 module. I had two wires (#2 and #6, if I remember correctly) that were actually broken due to the corrosion.

    Like I said, I will try to remember to get some more pics — hopefully this will help you find your module though!

    Regards;
    Kyle

  36. all indicator lites work and indicate 4wd is engaged or disengaged however no front axle engagement. i can hear the relay fire and hear what sounds like normal gear shifting in t-case. i removed battery and looked at actuator below appeared not to be corroded any help would be appreciated thanks josh

    Kyle’s reply;

    Hi Josh;

    Your problem sounds just like 10 others above you — if you haven’t found the answer in what I have posted thus far I, unfortunately, will not be able to help you and suggest taking your vehicle to a mechanic.

    Regards;
    Kyle

  37. I am having some issues with my 4WD and came accross your site which helped me out so when I managed to get hold of these documents I though I would share them with you.
    Thanks,
    Ray.

    Kyle’s reply

    Wow! What can I say — I thank you, my readers thank you. :D

    Regards;
    Kyle

    UPDATE 03/20/08

    The links to the files have been removed as I did not realize they had been uploaded to a file server to post here. I should have backed them up to my server, but did not — I apologize for that.

    Kyle

  38. The entire panel on driverside jsut stopped working. No power windows and no door locks. Also can not operate other windows. Where are the fuses for these? The box under hood does not say anything about door panel or windows.

    Kyle’s reply;

    Apparently, under the rear seat!

    Kyle

  39. [Originally posted by “josh”]

    great info on your site you saved me large amounts of cash$$$ turns out i had a bad actuator and was able to easily swap out the old part for new..never would have known where to start otherwise thank you josh

    Kyle’s reply;

    Awesome! Glad to help, Josh!

    Regards;
    Kyle

    PS – this was originally posted by a guest “Josh”, but it was posted in the wrong thread — and I couldn’t move it — so I reposted it.

  40. Hi Kyle,
    I have a 2003 chevy blazer and have no drive to the front axle. When I push the buttons the tccm clicks, the buttons light and stay lit you can here the motor on transfer case run (for a brief time) and no power to front axle. I removed the battery tray found the diaphram and it is working. could my problem be vacuum? I guess I didn’t look hard enough because I didn’t see the vacuum switch by distributor and it’s pretty tight in there . Any suggestions? Thanks,Tim

    Kyle’s reply;

    Hi Tim;

    It’s hard to say what your problem is. I’d start with looking at the encoder motor — if you can hear it run briefly, then what I would do is take it off the transfer case and try to engage 4WD then — watch the motor and see what it does — try it in all modes (2Hi, 4Hi, 4Lo) and watch the motor. If that seems to be OK, the next thing I would do is to try and manually shift the TC with the “tab” that should now be sticking out of the side of the TC (under the encoder motor that you removed) — it is this tab that the encoder motor turns to engage the different modes. I can’t tell you which way is which, but mark it where it is when you start and go from there (one way will be 4Hi, the other 4Lo).

    Regards;
    Kyle

  41. I also have a 1999 blazer. The 4 wheel high button light is flashing all the time. The 2 high flashes if you push it and then goes off. the 4 wheel drive indicator on my instrument panel isn’t lighting up and I don’t think the 4 wheel drive is engaged.The TCCM clicks and we can hear a motor running if we push the 2 high button but the 2 high light flashes and then goes off. I can’t get in to a mechanic for several days. My husband and I would like to know If we unplug the wire harnesses from the TCCM and leave them unplugged till I can get to the mechanic, will that harm anything? Thanks for the help

    Kyle’s reply;

    Unplugging the TCCM isn’t a great idea as it really doesn’t achieve anything. If you are stuck in 4WD (which it sounds like you are), unplugging the TCCM will not get you back to 2WD.

    However, I don’t want to tell you to continue to drive the vehicle without knowing further information — you may have a problem with the gears/forks in the TC which may not allow the encoder motor to physically switch modes — of course, driving the vehicle in this condition would only cause greater damage.

    The flashing light indicates the system is trying to enter that mode. If it stops flashing before it does, there was a problem getting to that mode.

    Unfortunately, I’d say stop driving it and get it to a mechanic ASAP, if you cannot further diagnose the situation.

    Regards;
    Kyle

  42. I have a 1999 Chevy blazer. a couple of days ago, my 4 wheel high indicator light came on and flashes constantly and the 2 hi light is off. The indicator light on the instrument panel doesn’t come on and we don’t think that the blazer is in 4 wheel drive. However, when I push the 2 high button it flashes and I hear a click click and a whir of a motor, then a click click and it goes back to the 4 high button flashing again. We have checked fuses,(all ok) the TCCM had a little corrosion but all indications are that it is working. One vacuum line was broken and we repaired it. We are not sure about the vacuum switch working correctly or not. I can’t get into a mechanic for several days. What we would like to know is: Can we unplug the TCCM and drive the car safely until I can get it to a mechanic? Thanks for your help

    Kyle’s reply;

    Hi Jenny;

    First, please note that I am NOT a licensed mechanic! I like auto work and do most of my own work, and thus gain the experience that I share within my blog. OK, so enough of the disclaimers already! ;)

    If by unplugging the TCCM you hope to that the vehicle will default to 2WD — it won’t. As long as it’s actually working and driving your axle(s), leave it be and get it to a mechanic ASAP. I wouldn’t recommend driving it for long periods, especially on dry pavement if it IS in 4WD though — it is extremely hard on the drivetrain / 4WD system.

    What it sounds like is you may have an encoder motor problem — this is the motor on the side of the Transfer Case that controls the shifting of the various “modes” (2Hi, 4Hi & 4Lo). It could be leads to the motor, or the motor itself — it’s hard to say without being able to test things. At best, it’s a corroded wire at the motor — let’s hope for that. :)

    So, without physically removing the encoder motor module and manually shifting the Transfer Case into 2Hi, unfortunately, it’s stuck in 4WD until you can get it to a mechanic. :(

    Regards;
    Kyle

  43. Well after alot of time on my 2001 Jimmy 4wd system I am still stumped. Started with a clicking noise from front end so I checked vacuum actuator and it’s doing it job now that I loosened up the cable. Truck will not go into 2hi or 4lo so I pulled the control module and checked wiring. Large orange wire on power connector was covered in corrosion and one wire on 32 pin connector was corroded. Fixed that and still no 2hi. When I attempt to change from 4hi to 2hi I can hear the encoder motor & vacuum solenoid on the firewall do their job and also the control module clicking so I presume it’s doing it’s job. I have 2 lights on dash causing worry, ‘Service 4wd’ & ‘ABS’. Would the abs system fault prevent it from going into 2hi? I know your not a mechanic but have you any info for me?
    Regards,
    Ray.

    Kyle’s reply;

    Hi Ray;

    It does sound like things are doing their job — I really don’t like saying things like this as I am not a mechanic, so I could be wrong (hopefully), but it sounds like a transfer case problem to me. The service 4wd indicator light is “usually” caused by a problem with the TC, from what I have read on the subject (my manual states this as well).

    As I said to a previous poster, if you want to get into it, remove the encoder motor from the TC and then try to engage the various drive modes — and watch the motor to make sure it’s moving in both directions (it should come to a stop in three different positions, for the three drive modes — mark where it is when you start so you can put it back there if all else fails!). After that, you can try to shift the TC manually with the “tab” that will now be sticking out from the TC after removing the encoder motor.

    Good luck!

    Kyle

  44. Kyle: Thanks so much for this web-site. It was key! I had a 1997 Chevy Blazer (4 wheel drive not working) only 2 wheel drive dash light on. I was looking at the actuator, and/or replacing the TCCM, but with the information I read on this web-site, I replaced 2 fuses , reset the TCCM, and bingo! It worked! Thanks for such an informative web-site.

    Kyle’s reply;

    Thanks for the update!

    It’s nice when it turns out to be something easy! :)

    Kyle

  45. Hi Kyle , My pickup has three button set up[2hi 4hi 4lo] It won’t shift into 4 hi on the fly or sitting in neutral.If press the 4hi button in neutral sitting still 4hi&4lo lights both blink then go back to 2hi moving nothing happens. It will go into 4lo stopped in neutral.Occasionally in neutral if I press 4hi and put it in gear, after I start to move it makes a bang and goes in but once I take it out it usually won’t go back in.

    Kyle’s reply;

    Hi Bradley;

    You know, it sounds an awefull lot like what mine was doing — which turned out to be the “plastic” bushings between the forks inside the transfer case. I really don’t know a whole lot about the internal workings of the TC (I have the pros deal with that stuff), so other than that, I can’t help you much — sorry.

    (Mine banged so loudly, I looked back to see if I left any parts on the road!)

    Regards;
    Kyle

  46. I just replaced the Oxygen sensor on my 04 Blazer (which has been an electrical nightmare from the start). I got it swapped out fine but I was wondering if someone could tell me how to get the damn check engine light to go off now? Thanks

    Kyle’s reply;

    Hi Michael;

    If I am not mistaken, you cannot clear codes yourself — you need to have someone do it with a scanner (ie. SnapOn). I could be wrong, but I don’t think so — I have a tranny shop within the family that has one, so I’ve never had the need to investigate it any further.

    If I’m wrong, please let me know.

    Regards;
    Kyle

  47. Thanks for this site. Hey kyle, I have a Chevy blazer LS and my 4WD does not work. when I press any 4WD button I can hear a click sound and the light associated with the button I pressed lights for a second or two and nothing happens. Per your aforementioned advice, I have checked the module to see if there is any visible corrosion but there isnt any. have any idea on how to fix this??

    Kyle’s reply;

    Hey Moses;

    Unfortunately, your questions are pretty vague and sound just like every other problem, as described above.

    As I am not a mechanic, everything that I know about the Blazer 4WD system has, pretty much, been posted here — you just have to read.

    Regards;
    Kyle

  48. Hi Kyle
    I am having a problem with the 4×4 on my 2001 Chev Blazer. I was checking out your blogs and I picked up a few tips on the problem, but I still can’t seem to get it too work. At first it was making a clicking sound so I took it in to the shop and the mechanic managed to get it to go back into 2HI but right after that when I tried to switch from 2Hi to Auto trak it went right back into 4 Hi. So I parked it and pushed on the buttons and all of a sudden the service 4 wheel drive light came on, so I got frustrated and left it like that and today I was checking the net and I found your site and like I said t the begining I tried a few things and the servivce light came off but I still have no lights on the switches. So I am stumped if you have any suggestions I would greatly appreciate the feed back cause this is driving me NUTS

    Thanks
    Lenard Mercredi

    Kyle’s reply;

    Hi Lenard;

    You are going to have to give me some more information — like what, specifically, were you doing when you got the service light to go out? What is the system doing now, if anything? Have you checked ALL your fuses (even the ones under the hood?). Did you check for corrosion on at the TCCM?

    Keep in mind though, that the AutoTrack system varies from the standard 4WD setup. I don’t know a lot about it, but from what I have read on the subject, the AutoTrack system is “set and forget” — which may contain components not found on the standard 4WD system. Unfortunately, I cannot help you there.

    Regards;
    Kyle

  49. Kyle: I have a clunking noise when in auto 4wd, 4hi and 4lo. The transfer is a np 246 is this case chain driven? Or could it be the bushing and fork that you described?

    Kyle’s reply;

    Hey Tom;

    Sorry, can’t help you with that one — the Silverado I don’t know.

    It sounds like you have AutoTrack though. As I said to a previous poster, I don’t know much at all about AutoTrack, other than it’s a “set and forget” type system. It may have other components, that are not on the standard 4WD system, that may be the cause of your problems.

    As for the internal workings of the transfer case, I leave that to the pros so I can’t answer that question either. :(

    Regards;
    Kyle

  50. Im trying to track down my 4×4 problem. I have all lights on dash,TCCM connector is good and when I switch to 4 hi and lo I can hear the transfer case engaging and when I go to 4 lo and put in gear 4 lo works to the back wheels. Now Im trying to figure out my vacuum problem which I assume is the problem. I found the vacuum lines running to center firewall mounted switch. Now right near drivers hood hinge there is a couple of vacuum tee’s and one has a short piece of hose running towards the fender with a screw in it. My question is should there be a resevoir on the drivers fender area where this line should have went to? There is no way that GM put a sheetmetal screw in it. My friend has a 95 blazer and there is a resevoir un the bottom of hood. What am I missing. I can send pictures if needed. Thanks in advance Shawn

    Kyle’s reply;

    Hey Shawn,

    If you sign up for an account here, you should be able to upload images here (I haven’t had anyone try it yet, but I set it up after another guest said she could not upload images). If you want to email them to me — click the contact link, shoot me an email and I will reply to it, which will give you my email address.

    Regards;
    Kyle

  51. First I have to say that this blog rocks!! I think you are doing an awesome service. I have read many of these problems but have not found one that matches mine exactly. A while back had the classic encoder motor problem where the truck would go into 4-lo or 2-hi, but would flash and default to 2-hi when 4-hi switch depressed. Well check vacuum actuator under battery, Tccm, encoder motor plug,etc and all was fine. Because my battery was disconnected the DTCs were reset and lo and behold my 4WD was working again. When 4wd was disengaged i would get a whirring noise out of the front end, like 4wd wasn’t fully disengaging the front diff. I would stop the vehicle for a few minutes and it seemed to go away. The other day, on the way to work the noise wouldn’t go away, so i tried putting it in and out of 4-hi a few times, until it blew the fuse. Replaced the fuse and immediately blew again, I was stuck in 4-hi. Came home that night, tapped on the transfer case and encoder motor with a rubber mallet, replaced the fuse and 4wd worked. Put truck in 2-hi and no noise. Started the truck yesterday and the fuse was blown again. Finally went and bought an encoder motor and replaced it this morning. Now nothing seems to work but TCCM(which seemed to work fine before). Press 4-hi button, and TCCM clicks about 5 times, then defaults back to 2-hi. Same thing in 4-lo. After trying this about 5 times, it won’t even try to go in 4wd, just clicks and stays in 2-hi. Pull battery and it resets but goes back to the same problem. Remeoved the Encoder motor and reinstalled, no luck. Any ideas?? Do you think I got a bad encoder motor from parts store? I don’ty have enough hair as it is, I don’t want to pull more out. Thanks.

    Kyle’s reply;

    Hey Chris, and thanks for your questions!

    What it sounds like, to me, is a wiring problem — most likely between the encoder and the TCCM (under the kick panel). Have you pulled the kick panel and had a look at the TCCM wiring (as pictured int he first post of this blog entry)? How about the encoder motor connector? I would suspect the latter, a) because it is more susceptible to corrosion due to road salt, etc. and b) because you said the motor kicked in when rapping on the TC with a rubber mallet — that probably caused shock in the connector/wiring, enough to engage the motor.

    I’d check the wires that go to the various “channels” of the encoder motor. First, make sure you have “motor power” — pins 6 (+) and 7 (-). According to the schematic (TC_encoder_motor), pin 3 should be signal ground, while pins 1, 2, 4 and 8 are the encoder signal lines. Now, I haven’t done this, so I’m simply working from the schematic — the lines are tied to +5VDC via a “pullup” resistor (as per the schematic, inside the TCCM) — that’s the squiggly looking thing where it says 5V. What I am assuming is happing is, the pullup resistor pulls the line high (give is power, so it should read +5VDC), the TCCM will pull one, or more (may be combinations for various modes, I don’t really know) of the signal lines low to indicate the mode to the encoder motor which, in turn, moves the transfer case accordingly. Again, I’m working on some presumptions here.

    What I would do is make sure you have +5VDC at pins 1, 2, 4 and 8 — I would also use pin 3, the signal ground, as your ground reference when checking these pins — make sure the signal ground is good. Make sure you have +12VDC at pin 6, using pin 7 as the reference ground. Remember, grounds are VERY important that they are GOOD. I might look a little harder at your motor power lines than anything — it would be those lines that the fuse would be blowing (I would think — but you haven’t stated which fuse is blowing, and the 4WD system has a couple of them — I believe I posted them in a reply here somewhere!).

    The, you should try and put it in 4WD and see if any of the wires change (they should lose their power) — you may have to do this multiple times to read the various wires — and an assistant will be very handy!

    If you’re not getting the above results, check the same wires at the TCCM as see if the results change. If the do, the problem is in the wire somewhere between the TCCM and the encoder motor — and I don’t envy you having to find it, if that is the case. Using what I have said above, and some basic logic, you should be able to tell in the TCCM and encoder are communicating properly.

    Other than that, it’s possible that you got a bum encoder from the parts store, but how likely is it really? Try what I have said above and post back here if things still aren’t working as they should and we’ll see what else we can think of then. ;)

    Regards;
    Kyle

    PS – I really hope I’m not way off base here and sending you on a wild Goose chase, but it’s hard when you can’t see the vehicle. It very well could be a mechanical problem in the transfer case causing the motor to draw excessive current and blow the fuse. Does the motor work well, when NOT attached to the transfer case?

  52. Thanks for the reply Kyle. Due to the time and the temperature outside in sunny Buffalo NY, I won’t be trying anything tonight. I am trying to do everything at work on lunch where its warm. To clear up a few things, I have checked the TCCM. I pulled the cover off and checked the insides and all looks good. Rusty on the out but no sign of corrosion inside the TCCM or the plug. I have visually inspected the EM connector multiple times. I have tugged on the wires, and it appears to be in great shape, no corrosion or apparant broken wires. Since the last post, I have exchanged the new EM with another new one and this one is exhibiting same problem so I am 99.99% sure it isn’t the motor. The old EM worked when I tapped it with the mallet, but only for a short time, basically long enough to get me in 2-hi. Both new EMs have not worked. What is really bothering me is the fact that the new EM is installed and no blown fuse. Yesterday when I first put it in, the fuse blew within a minute of starting my truck. Today at work, (warm in our shop), checked the TCCM plug, pulled TCCM apart to check inside, cleaned anywhere the TCCM contacted the body to make sure it was grounded, replaced fuse and plugged it in. Once again it clicked multiple times and default to 2-hi. No blown fuse though. Cut tape and pulled wire loom off EM plug, and no apparant problem there. Had a helper watch 4wd switch as I #*@%ed with each individual wire and no fuse blew. I haven’t tried the motor off the TC because the sheet with the EM said it would damage it. I know the EM moves freely and I checked to make sure the TC is free too. It moves with a crescent wrench with little effort. So I shy away from a mechanical problem. I am lost and aggravated!! I will try your suggestions ASAP, based on weather and time, and let you know. I want to fix it myself to say I did, but I am real close to giving it to my local 4wd pro and saying don’t give it back til its ready for snow or mudding!! Thanks Kyle for your help, and hopefully I can get back to you with good news.

    Kyle’s reply;

    Buffalo, eh? I hear you — I’m just north of you in Ontario, Canada. You guys been getting pelted this week too? We got 8 inches yesterday, which started around 2:00pm — just in time for the drive home. Thank God I had my 4WD yesterday (not trying to make you feel bad, honestly!) — as I had 3,000 pounds worth of safes on the trailer and was in Toronto (about 50 miles from where I needed to take the safes). The drive took just over three hours — fun wow!

    Well, do what you can and let me know — and I’ll do my best to help you through it — I’d like to see you fix it too!

    Regards;
    Kyle

    PS – the flashing switch lights indicate the system is trying to communicate with the rest of the system. When it stops, it means it could not communicate with part of the system. Grab the schematics in the first post and check ALL sensors, etc. It really does sound like it’s a wiring/sensor problem. You really need to make sure the encoder motor is getting the required power on all lines.

  53. Hi. My problem is with my 04 Jimmy. I love the truck by the way. Here in Winnipeg it gets to -35 degrees C. overnight and in the morning my 4 wheel drive returns to 2 Hi. It seems that if I plug it in overnight that seems to help. Could a cold start and a sudden drain on the battery making the voltage drop to the TCCM make it return to 2 Hi? It is not that big of a problem because once I return it to 4 Auto or 4 Hi it seems to stay there for the day. Thanks for your help and love your website. John in frozen Winnipeg, Canada.

    Kyle’s reply;

    Hi John;

    Hmmm… that’s a problem I haven’t heard of before!

    Reading your first thoughts on it, and I’d have to say I agree with you. Does the 4Hi light come on when you first turn the key to the ON position without attempting to start the vehicle? If so, and it returns to 2Hi after you start the vehicle, your assumptions are probably correct. Do you hear the TCCM click at all after starting the vehicle?

    If you find that it IS your problem — a diode and a capacitor should resolve the problem. What you do is get a diode (1N4148) and place it on the logic supply line of the TCCM (the polarity marker, a bad, towards the TCCM), and place a capacitor (100uF 36v should do it) between the diode and the TCCM. What this does is prevents the battery from pulling power FROM the TCCM (via the diode) and the capacitor makes sure you have enough power there to “temporarily” run the TCCM while the vehicle battery is being drawn from.

    You may also wish to have your battery checked over. Winnipeg is hard on batteries, no doubt. My brother, who is a truck driver, had a battery freeze while in Alberta — it was brand new and NOT covered under warranty. I’m in Ontario and I stood beside a guy in Canadian Tire the other night who brought a battery back and they told him the same thing — no warranty, the battery is frozen. If you freeze a battery, you risk cracking the plates — if you crack the plates, it won’t hold a charge worth s**t.

    Regards;
    Kyle

  54. Hi having a big problem with my four wheel drive it goes into 4 low and 4 high but it jumps out shanking the whole truck and then it will click until i put in back in 2 wheel. I had the front diff rplaced and the cv axles checked for damage but it still does it. Wondering if you could help me out? Thanks

    Kyle’s reply;

    Sounds to me like the transfer case gears / forks — if you dig back through some of the posts, I have described the problem you ask of.

    Regards;
    Kyle

  55. Kyle:

    I have Chevy 2001 Blazer. The problem I had is that I can’t switch on the 4Hi. So I went to dealership. The repair costed me $900 to replace the TCCM. The dealer identified the problem as no power to turn on 4Hi. The TCCM model is $748 through dealer. I am kicking myself now. If I had seen your blog first, I could check the wire connection myself!

    By the way, the dealer told me that 4Lo typically can not be turned on unless you put the vehicle in the neutral. If somehow vehicle goes into 4Lo by itself, just put it in the neutral position, and you can switch out 4Lo. I tested it and it works. I encountered the similar thing Spencer did and this is the solution dealership told me. A straight forward solution.

    James

    Kyle’s reply;

    OUCH! That hurts.

    Yeah, this is really the reason that I put this information up in the first place. I had a ridiculous quote originally, and they (the dealership) hadn’t even looked at it — they were just going to start replacing parts.

    As for shifting out of 4Lo — there’s just one more thing — you MUST be going less than 2 MPH (3.2 KPH), with a “rolling engage” being preferred (ie. don’t do this when stopped). This is a common mistake made when people have a “shift on-the-fly” type system.

    Thanks for your comments, and hopefully next time you’ll check the web first! ;)

    Regards;
    Kyle

  56. Hey,
    I hope this thread is still alive. Anyway, I have issue getting out of 4HI. I have read all of this discussion and Tried many things. I replaced the vacuum switch on the transfer case(mine was stuck). Then I checked all vacuum lines(some were cracked and I replaced). Pulled the encoder motor and cleaned it/took apart and re greased/sealed. Now, this is my current condition. If I unplug the TCCM, then plug it back in this is what happens. I start car, 2HI is lit and it is in fact in 2hi. When I press 4HI it goes in perfect. If I then press 2HI the 2HI light flashes (and you can hear the encoder motor moving and clicking below) But the 2HI light stops flashing after a few seconds and it stays in 4HI with that light illuminated.

    I have found I can get it out of 4HI in one of two ways. If I park while stuck in 4HI and leave the car for a while, when started 4HI will flash and it reverts back to 2HI. OR If I put the truck in neutral, press 4LO (it goes into 4LO) then press 2HI, there is clunk and it goes back into 2HI. If it always worked like this I could deal with it, but after doing this a few times, the truck gets stuck in 2HI and doesn’t respond to pushing any of the buttons unless the TCCM is unpluged and plugged back in. At that point the process repeats itself.Please help if you can!
    Thanks,
    Rick

    Kyle’s reply;

    Hi Rick;

    Unfortunately, I have not had experience with your particular problem — but it does sound like it may either be a) the same problem as I was having (ie. internal gears / forks of the transfer case not allowing full shifting from one mode to the other), or b) a front axle switch problem (which is located within the Power Control Module (PCM) — but that’s about all I can help you with the PCM as I don’t even know where it’s located). The system is based on being able to communicate with all modules — if it can’t it will fail. If it doesn’t know where the front axle is, it will do strange things.

    There is a schematic (on page 1) of the front axle switch — it appears to be tied to ground via a “pull-down” resistor. When engaged, it should be pulled high (+5VDC or +12VDC, I don’t know what it’s using for the switch).

    I hope I’ve helped you, but I really am just guessing.

    Regards;
    Kyle

  57. So, my four wheel drive use to work and recently stopped. Now all it does is the lights will blink when shifting into 4HI or 4Lo and blink for about 5 seconds then go back to 2HI. I hear a clicking that comes on. What are my possible problems?

    Kyle’s reply;

    Are you serious? Did you read ANY of the last two pages?

    Kyle

  58. First, Kyle, great page! found it trying to find what is wrong with my Blazer. The 4 wheel drive will not engage…Was looking for the TCCM, you answered that question, Moving on to Amy, I noticed that my owners manual answered Amy’s problem, it says that if you eventually end up having to use neutral to help engage… the transmission indicator switch may require adjustment.
    Now where the heck that thing is…I have not got that far. Hope this helps all of us expand our knowledge on these great high maintance vehicles!

    I may be back with some questions of my own…
    got heat going in garage, will start ripping my vehicle apart! Ohh I forgot we are getting a storm of 12 inches, better start shovelin’ best regards Doug

    Kyle’s reply;

    Hey Doug — thanks for the information! That makes a lot of sense — now, if we can only find the switch! :)

    If you think about it, the switch you talk about could just be the root of the problem for a few people who’s 4WD won’t engage.

    I’ll ask at the tranny shop on Monday to see if I can find out.

    As for questions of your own — fire away — if I can answer them, I’d be more than happy to!

    Regards;
    Kyle

  59. Lost 4 whl drive when switching from 4×4 auto to 4×4 hi..we heard a clunk sound ( i know not good) then only had 2 whl drive from that point on…
    goes into shop this week..just wondering if it is a linkage of some sort? maybe u joint? The lights shows that the change into 4Hi is complete and 4 auto also..but just no traction from front wheels..maybe a transfer case issue?
    thanks
    joe t

    Kyle’s reply;

    Hi Joe;

    Well, seeing as you’re taking it into the shop, and I am not a mechanic — why guess? What’s the point? Do I win a prize if I’m right?

    You’ll find out soon enough from your mechanic. In the meantime, there’s two pages of Q&A above that you can read.

    Regards;
    Kyle

  60. Perhaps you can help me. Today I left for work and my truck was in 2HI. About 5 minutes down the road no lights were on, in my 4×4 switch on the dash. I pushed the 2HI button and it lit right up. My truck does go into 4LO as I can hear and feel it engaging. My truck also appears to go in and out of 4HI and 2HI.I found a ice spot on the pavement and if its in 2HI it will spin , but when I hit 4HI I hear a slight clunk and it will take off like its in 4HI. My gas mileage on the other hand is way down. I am short 120 miles to a tank. I think my truck may be coming out of 2HI and engaging in 4Hi by itself.( when no lights are on the 4X4 switch on the dash I believe its either in 4HI or stuck in between. Is there a part that “holds” the transfer case in 2HI, 4HI, etc? Could a small vaccum leak cause this? Say I push the button, 2HI engages and then as I am driving down the road the vaccum losses its strength and disengages out of 2HI?

    Sorry for the long post, I just don’t know where to begin. Like I said I think my 4×4 works it just won’t “hold” it in 2HI? Thanks for any help.

    Kyle’s reply;

    Hi Jim;

    The first thing I would check is the 4Hi light — (keep it simple) — does the 4Hi light ever come on?

    It’s possible that the system is engaging itself. However, 4WD engages WITH vacuum, not without — accordingly, you may have a bad vacuum switch which is allowing vacuum through when it shouldn’t.

    Now, why the lights are going out, that is odd. I wouldn’t think the system would “indicate” 4WD when it wasn’t physically put there — meaning, if the 4WD system DID happen to kick in due to vacuum, the TCCM would have never received a signal to PUT the system into 4WD and therefor would not move the encoder motor, or turn on the 4WD indicator lights. If the 4WD system is kicking in due to vacuum, the only real thing to be affected would be the front axle, which is what is controlled via vacuum. Hmmm, another thought just came to mind — the actuator under your battery may be bad and the front passenger axle is stuck in the engaged position (the cable may be stuck) — if that is the case, the front axle would be engaged but NOT being driven by the transfer case. That could, very well, be the cause of the excessive gas consumption.

    Another option you may wish to explore is the TCCM — have you looked for corrosion on the wires? Corroded signal lines could cause a condition where the TCCM “thinks” you’re trying to switch to 4WD.

    Well, that’s a start anyway — let me know what you find and we’ll go from there.

    Regards;
    Kyle

    PS – the “encoder motor” is the part that “holds” your TC in the various modes (well, that’s not 100% accurate either — the encoder motor shifts from one mode to the other — parts inside the TC actually “hold” you in that mode — in my case, it was a set of “forks” with plastic bushings that wore out — which does not sound like your problem). For the motor to be bad is one thing, but for it to actually MOVE and shift your TC into another mode, that would be extremely odd if that was the case (if it WAS the case, I would still suspect TCCM signal lines to the encoder motor before I replaced the motor).

  61. hi! to share my blazer story, trying to sell it, and low and behold the “service 4WD ” light came on this weekend, and now unable to activate any 4WD and no lights on. Repair shop said it was the both the transfer case motor and the TCCM….adding up to almost $1000. I really do not want to spend this, and was wondering if you have any ideas. I already took a look at the wires and they seem o.k. the pins looked perfect, and maybe some corrosion noted on the wires, very small amount, and no broken wires…any input would be much appreciated. Also, the repair shop said that the TCCM got corroded and shorted out the transfer case motor….is this possible??
    thanks!!

    Kyle’s reply;

    Hi Tom;

    I hear you on the cost — it can be shocking.

    As for your question of “is this possible?” — of course, anything is possible. Is it likely? Or, would I trust what the dealer was telling me? Absolutely not. I have heard way too many horror stories with guys (and girls) taking their Blazers to the dealer. The TCCM is the fist thing they replace — and hey, at $600-$700 (???), it’s a two bolt and one harness connection money maker for the dealer. Hey, let’s replace the 2 mosst expensive parts in the system and see if it works. Nevermind the fact that if that is NOT the problem, you’re still paying for the new TCCM and encoder. $5 vacuum lines, and the like, are the LAST things to be replaced (how much money do you think they make off of replacing a vacuum line for you?).

    As for the encoder motor being fried because the TCCM “got corroded and shorted it out” — again, it doesn’t seem likely to me. There are (off-the-top of my head) 7 wires between the TCCM and the encoder motor. 2 of them are power (+/-) and are FUSE protected. 4 of them are signal lines and the last one is signal ground. If the ground wire was corroded and shorted (to what?) it would not cause the encoder to fry. If any of the signal lines are shorted, again, it would not cause the encoder to fry as the signal lines are pulled high with a “pullup resistor” — and are DESIGNED to short out to ground to pull the line low. At worst, if the signal lines were shorting out, you’d go into the wrong mode, or none at all — but it would fry the encoder — I’d be willing to be a weeks paycheck on that.

    Regards;
    Kyle

  62. Hope you still look at this every now and then. My blazer dropped into 4 low after backing out of a parking place at work!! Now I can’t get it to go back into 2WD or 4 HI. It’s stuck in Low range. The electrical box in your thread gives me the all good “click click” when the key is turned. Any suggestions?

    Kyle’s reply;

    Sorry, but no — what I know about the Blazer 4WD system has been posted within this thread.

    Regards;
    Kyle

  63. i have a 2001 blazer no 4wheel drive lights flash when you push button i changed tccm,motor on tranfer case,acturator under battery need to no what else to look for thank you craig

    Kyle’s reply;

    Fuses?

    Kyle

  64. I was driving in 2 hi going down the highway and looked down and saw my 4wd-lo flashing it stopped flashing on it’s own and stayed in 2 hi. Yesterday my wife went to wash the car and it did shift into 4wd lo on it’s own. I had to manually change it back to 2hi worked fine then like i said i saw it flashing today. What does my problem sound like and how can i fix it and the cost involved and is there a chance when going down the highway at 60 mph plus all the sudden it will shift into 4 wd low and my tranny could get ruined. This is a used tranny I got put in last November.Thanks

    Kyle’s reply;

    Hello Mike;

    First, your tranny will not shift into 4Lo at 60 MPH — it is designed to shift ONLY when the transmission reaches speeds of 3MPH (5KM/H) or LESS. The flashing indicates that the TCCM is trying to shift from 2/4Hi to 4Lo, but it cannot due to the speed currently being traveled (hence why it went into 4Lo for your wife at the car wash) — it will flash for 30 seconds (I believe it’s 30 seconds) and then revert back to the mode it was last in (ie. 2/4Hi).

    I always start with the simple stuff and work back from there — what it sounds like to me is the button assembly on your dash, a wire between the button assembly and the TCCM, or the TCCM itself is bad. I’d start with the easiest, and cheapest part — either go to the dealer and order a button assembly, or go to your local scrap yard and yank one out yourself. If that doesn’t work, you have two options left with the wires and TCCM.

    Some how the system thinks the 4Lo button is being pressed, and that’s what you need to find. Unfortunately, intermittent problems are the toughest to find. That being said, I wish you the best of luck. Please let me know when you find the problem as I would like to have the info here to help others with the same problem.

    Regards;
    Kyle

  65. i just bought a used 97 blazer 4×4… in Fed. we had an ice storm and i put in it 4×4 hi and the 4lo the lights came on stating it was in 4×4 but i could not tell a diff between it and 2 wheel drive.. this is my 1st 4×4 truck.. how can you tell if 4×4 works or not…. also we got about 5 inches of rain over a night i was already parked in the mud and the next morning i tired 4hi and low again and nothing but spinning my front 2 tires it looked to me as the back 2 were not pulling at all… thats why i am asking this question… i was told that if a truck is already stuck in the mud then 4×4 will not make a diff is this true?

    Kyle’s reply;

    Hello Guest;

    If you say your front wheels were spinning and your back 2 were not, your 4 wheel drive is working (although I’d be concerned as to why the back to wheels were not working — they are the two the drive the vehicle in both 2 and 4 wheel drive. If the front 2 wheels are driving, you are in 4WD.

    Besides that, you’d know if you were in 4WD or not. Like you said, park in the mud — put it in 2Hi and try to get out. Then put it in 4Hi and do the same. There should be quite the differenc in feeling between the two modes in mud. If there is not, a visit to your local mechanic is in order.

    Regards;
    Kyle

  66. Hi Kyle……..you have a great site and have read just about every item you and your readers have issues with but my wifes truck has the Grand Daddy of problems and if you tell her to sell it she will understand. Here goes….I was allowed to drive her around the other day when I heard what I thought was the blinker going and tried repeatedly to cancel the non blinking lites with no success. After a few minutes the service 4 whl drive lite came on, when we parked I asked if this had happened before and she said yes all the time but it goes away when I park. OK I said. we shopped and came back. I started up the truck and when I put it in drive it gave a strange lurch forward. It had shifted into 4 whl low. There are no lites on the push buttons and it will not shift out using any of the methods in other strings above. The kick space module is impeccable, I even took it apart looking for a cold solder joint, no such luck. The vacuum module under the battery works great as does the vacuum switch above the distributer. I suppose at this stage I will have to crawl under the truck. Or do you think I will be wasting my time? Any suggestions will really help. thanks Bob

    Kyle’s reply;

    Hello Bob;

    First, I apologize for the delay in responding to your question. I have no excuse — I’ve been neglecting my site recently.

    As I said to a poster above (two up from this one), it sounds to me like it may be a button assembly, wire between the button assembly and TCCM or a TCCM problem. The system “thinks” a button is being pressed and trying to shift to 4Lo. Now, I’m making suggestions here, but I have not tried this — there may be a little figuring out to do, but the schematics in the first post should help you out.

    What I would do is test the button assembly wires at the TCCM. If I’m not mistaken, I believe that the buttons are pulled high (+5VDC) and ground out on pressing them (I could be wrong, so please double check!). So, I’d test each one of the button assembly wires at the TCCM. The should all be the same with no buttons pressed. I’m thinking it sounds like your 4Lo button is stuck (or a wire shorted). If it’s stuck, pressing other buttons won’t do anything. However, what does have me stumped is the fact that the service light comes on… but that could be caused if the system detects a stuck button (I don’t know how much diagnostics the system does on itself).

    Oh, the clicking you hear is, most-likely, the relays in the TCCM trying to shift into 4Lo. If you come to a stop while you hear that, you’ll probably shift into 4Lo (your speed must be less than 3MPH or 5KM/H to shift into 4Lo and is why it keeps clicking as you drive — it tries to shift for 30 seconds, then gives up).

    Hopefully, this will set you off in the right direction. Please let me know what you find as it may be of use to someone else here in the future.

    Regards;
    Kyle

  67. Thank you all for this excellent forum. After reading it, I checked ALL the fuses and found #3 for “PK LPS” blown. Changing that fuse got the the 4WD working again.

    Kyle’s reply;

    Hey Adam;

    Awesome news! I’m glad we were able to help you solve your problem.

    Regards;
    Kyle

  68. I have a 2000 s10 blazer and am having issues as well, when I push the 4hi button it will light into 4hi, but pulls severely to the right. So I have not been using ny 4 wheel drive. But when I get into slippery conditions the 2hi light will turn off, it eventually, maybe the next day will come back on, but just an odd thing. Any ideas appreciated. I am pretty mechanical so I understand some of this.

    Kyle’s reply;

    Hi Beth;

    Normally, the front drivers axle is the axle that is permanently engaged within the front differential (that doesn’t mean it’s always driving… only when the front diff is engaged will it actually drive). The front passenger side axle engages when put in 4WD, via an actuator under the battery. So, if it’s pulling to the right, it leads me to believe the front left axle is not driving (which would be odd). There’s not much I can suggest to you for this situation. The only thing I can think of requires a hoist — put the vehicle up on a hoist and engage 4WD, then check the front drive wheels/axles.

    Your comment about getting into slippery conditions and the 2Hi light goes out… the only thing I can think of is the TCCM (Transfer Case Control Module) may have corroded wires, which was the original topic of this thread. Does this happen automatically when you get into slippery conditions? I mean, do your wheels spin in slippery conditions and THEN the 2Hi light goes out? Or do you have it in 4WD and under slippery conditions (again, I’ll assume your wheels are spinning) it reverts to 2Hi but does not indicate so via the dash lights until some time later?

    It almost sounds as if you have two, independent problems with your 4WD system.

    Regards;
    Kyle

  69. I am so tired of telling people I AM NOT A MECHANIC, that I am closing the comments section of this post.

    I post what I have encountered during MY OWN REPAIRS — I post this information and state that, but somehow people never read the posts, yet they leave feedback or comments asking me how to fix something that I know nothing about.

    Again, I AM NOT A MECHANIC!!!!!! Got it?

    Do NOT use my contact form to ask questions about your Blazer as they will be ignored.

    If it is not posted above, I CANNOT HELP YOU — GO TO A PROFESSIONAL MECHANIC!!!!!

  70. Today I am reopening the comments section of this post. As you can read from my comments above, it had become really frustrating that people wouldn’t read what I had posted and would ask the exact same question to me in the comments.

    Well, it’s becoming more frustrating getting those same questions over and over and over… but now in random threads! Anyway, the comments for this thread have been reopened so I can, again, reorganize the comments from other threads and place them back here.

    Please, read what has been posted before asking your question. Again, I am not a mechanic. What I have posted here are simply my personal experiences. It’s like fixing your plugged toilet, posting about it and now everyone thinks you’re a plumber.

    We’ll see how it goes.

  71. my 4 wheel low will work in neutral when you select 4 high or 2 hi you could hear the relay come in. when you hit 4 high the 4 wheel low and 4 wheel high comes on at the same time if you hit 2 high the relay comes on put no light I put the tccm module in to another truck and it works fine.the vacumm actuator only come in 4 wheel low i replaced the vacumm module.I check all the voltages at the module the 4 pins with encodor has the 5 volts on it one of the 4 pin the voltage drop to 4 volts then to 3 volts the other 3 pin drop to 0 volts is ok?when you put it in gear the relay wont come in 4 wheel high or 2 high so i cannot find any thing that control this

    Kyle’s reply;

    Did you even read the comment directly above this?

    Take it to a mechanic.

    Kyle

  72. I have checked and fixed everything like above. How do I know if my 4WD is working on dry ground with any snow yet?

    Kyle’s reply;

    On dry ground… you don’t. Take it to a mud hole, or wait for it to snow.

    Regards;
    Kyle

  73. Thats right…

    2 Quick visual inspections could fix your problems with the 4 wheel drive not working, flashing lights, wont go into 4whl low. I learned alot reading this blog and wanted to ad the onfo I gained and the results I got in about 30min. from 2 visual inspections.

    This first Inspection: Start with the pass side kick panel, remove the panel and inspect for corrosion, loose or broken wires, Clean and repair as needed.

    The Second Inspection is in the engine bay high up on the center of the firewall. Inspect for vacuum at the switch and then make sure the hose is’nt damaged causing leaking (Mine was broken off) Excess heat from the engine compartment will damage vacuum hoses and excessive curves or bending will lead to problems with hose decay and failure. I cut off 2 inches and replaced the hose.

    That took me about 30 minutes and corrected my inop 4 whl Drive on a 2001 Blazer.

    Thanks a lot for the advise!!!

    TK

    Kyle’s reply;

    Hey TK!

    Just one note… not all vacuum switches are located on the firewall. If it’s not there, check the top of your transfer case!

    Thanks for your comments, and the info!

    Regards;
    Kyle

  74. You are a hero to many people! Looking at your blog it appears you have saved people hundreds if not thousands of dollars by helping people fix their 4WD problems themselves. I solved my problem after reading about vacuum leaks. Sure enough the vacuum hose had broken off the switch on firewall. It appears the hose was too short to begin with and was kinked causing it to fail prematurely. Armed with 3 inches of hose and a connector it now works like a charm. Since the system has never worked while I’ve owned it, I have one question. When I push the ‘4WD Auto’ button it takes approximately 10 seconds for it to engage. Is this normal or is something else wrong. It seems to work fine. My only concern is it takes a long time for the light to stop flashing and stay lit.

    Regards,
    John Boy

    Kyle’s reply;

    Hi John Boy;

    A hero? Hardly! :) But hey, if I can help out a few people with my ramblings, awesome! Glad to have helped.

    As for the answer to your question — sorry, I don’t have the foggiest as my Blazer doesn’t have the “Auto” feature. Ya gotta own one of them fancy trucks! :) You know what I’d say though? If it ain’t broke, don’t touch it!

    Thanks for your comments!

    Regards;
    Kyle

  75. I have a 2002 trailblazer lt and no 4wd. Last winter my service 4wd kept coming on and i had to have the tccm replaced. Well now this winter i was driving through a field of course in a4wd and as soon as my front tires grabbed it shut off, service 4wd light came on and dash light stayed in A4wd for about 15 seconds and then lights turned off all together. I brought it in and they told me the encoder motor was bad, ok well easy enough ill just have them fix it. Then i was told that when the encoder motor goes out, the tccm also goes out, is that true? I can deal with fixing the encoder motor, but if i have to replace the tccm again ill be very angry!

    Kyle’s reply:

    It sounds more like “mechanic talk” to me — but who am I? :) What exactly do they claim to be the problem with the TCCM? What part of it is bad? Ask them what would cause that. There are schematics at the beginning of this post… print them out and show them to them and ask what is bad and why. That’s really all I can offer other than my opinion.

    If you look at the schematics, you can see the motor is simply “controlled” by the TCCM via signal wires. I guess it’s possible that if one of the motor wires were to be shorted it may cause damage to the TCCM — do I think this is the case? No. If I recall, the schematics show a pull up/down resistor on the DC lines… this should prevent damage at the TCCM due to shorting.

    Again, if I recall, the 12VDC source for the encoder motor is also supplied through the TCCM… that could produce quite a bit of current, if the motor was damaged.

    What I’d do is go out and start testing the TCCM and connections (if you have that capability). I go through quite a bit of it in this post. Check to see if you’re getting a 12VDC source at the TCCM to start with. I believe it’s the big orange wire (from memory…check it out in the schematics!). Look at it logically, and start testing.

    I really don’t have the experience to say definitively yes, or no — but I wish you the best of luck. Please, let us know how you make out.

    Regards;
    Kyle

  76. Hello,
    I have 2001 S-10 Crew Cab. When I push my 4hi or 4lo
    switch nothing happins. Now when I put the truck in neutral nothing in 4hi but it will go to 4lo. Then I can push 4hi and it will go in. Then I can go back to 2 wheel. But the only time I can do any of this is when It is in neutral and at a complete stop. Any Ideas. Thanks

    Kyle’s reply;

    Hi Steve;

    Although I don’t really have any experience with the problem you describe, it sounds like a switch / sensor to me. I do know there is a speed sensor that stops you from putting the system into 4Lo while you are traveling at a speed of greater than 5MPH (8KMH) — if that is the case, the 4Lo light should flash until you slow down to under 5MPH, then it should drop into 4Lo.

    The system *should* drop into 4Hi at any time (OTF – On The Fly), up to speeds of 80KMH (I believe that is the figure… that’s from memory).

    It sounds “electrical” to me… accordingly, the first place I’d start looking is at the TCCM for corrosion. Then, I’d take a look at the schematics posted within this thread and start looking for sensors.

    Good luck and let me know how you make out, or if you have any further questions.

    Regards;
    Kyle

  77. Hi, I don’t know if this is still active, but, You are kind of my only option right now. So, when I have my 98 blazer in gear or park, and I hit 4 high or low, nothing happens, but, when I put it in neutral, the high and low lights flash, and I can hear a clunking sounds from under my truck. it will go into low, but, not high, and when it is in low, both high and low lights are on. Do you think that is my TCCM? I can hear that clicking. i looked at the wires, and they all seem fine, none are broke, and I checked the wires on the back of my actual switch, and they seem fine. i even checked the fuses that you mentioned earlier…Still nothing…

    Kyle’s reply;

    Hi Jason;

    Hmmm… doesn’t sound like a TCCM problem… sounds more like an encoder motor problem, or possibly a transfer case problem.

    If you hear the clicking of the TCCM, that tells you that the relays are working and are trying to put the encoder motor in gear. There are many reasons for this to happen, broken/leaking vacuum lines to start. The encoder motor wires may be bad. There is a “plastic bushing” that keeps a “fork” in the transfer case in the position (well, the fork keeps the gears in the proper position)… if the plastic bushing is worn, it can cause the transfer case to not switch (this happened to me, but the symptoms were that the system would switch to 4Hi/4Lo, with a big clunk, and it was very difficult to get it back out of 4WD.)

    Unfortunately, I can’t give you a definitive answer to your problem, but if I had to say, I’d look elsewhere, other than your TCCM (encoder or transfer case)… but then again, I’m not a mechanic either… so I am just guessing! ;)

    Good luck!

    Regards;
    Kyle

  78. I have a 2003 2dr 4×4 Blazer. 2hi works fine, when I press 4hi, both 4hi & 4lo flash & nothing happens. When I push 4lo works fine, front axel engages but when in 4lo & press 4hi it goes back to 2hi.

    Kyle’s reply;

    Could be a TCCM problem, encoder motor problem or a transfer case problem — all of which has been covered within this thread.

    Kyle

  79. Kyle,
    I am greatly indebted to you. My new wife’s 2000 Blazer lost her 4wd a year ago. SHe found your web site about the TCCM module corrosion, printed off the information and her mechanic fixed it. He was also impressed.
    but… this year her dash light 4wd started blinking, (oh Nooo $$$$), and since we are recently married, (happily I add), I took on the task with the help of your web page.
    I cleaned the TCCM contacts, then was able to hear it click, but alas, the 4 HI and 4 LO lights flashed when I attempted to engage them. Rats! Step 2, checked all the fuses.. All OK, then found a broken actuator vacume tube broken near the actator itself. Fixed that.. Feeling good with myself now…but no 4wd. AHHH. Step 3…pulled the battery, the acuator was working. Now I’m thinking transfer case may be bad… mega dinaro, (Oh Nooo again)but in the back of my mind I remembered your 30 minute battery disconnect and flip the key 3 times. Voila!!!
    We now have 4WD. My wife is impressed with me and my hat is off to you sir. You saved us big time. Will gladly fund your retirement account for what you have saved us. Let me know how.
    Bill and very relieved wife Karen.

    Kyle’s reply;

    Hi Bill (and Karen)!

    That’s a great story! Thanks for sharing that with me… which is all I need! I’m not asking for anything, but hearing stuff like this makes me feel good. Good that I was able to help someone else out… and save you some money in the process.

    Hopefully, it also gives others the encouragement / confidence to tackle jobs like this on their own!

    Regards;
    Kyle

  80. I have a 1992 k1500 blazer (looks like a tahoe) and my 4×4 worked fine until yesterday.Ive read most of your posts. do any of your fixes apply to the 1992 blazer? To engage the 4×4 there is a floor shifter and lights that light up after it warms up. Yesterday the light never came on and no 4 wheel drive. great blog, thanks.

    Kyle’s reply;

    Hi Rich;

    It doesn’t sound like it’s the same type of system, so I’m not really sure what to recommend to you. My system is a purely electronic one (where you push a button, which activates a motor to change modes for you)… yours sounds like a mechanical system where you engage the 4WD manually (floor shifter) and the light comes on simply as an indicator. However, if that was the case, you’d think it would go into 4WD because you’re manually putting it there with the floor shifter.

    Sorry I can’t be of more help.

    Regards;
    Kyle

  81. Hey, I came across your website while trying to get more information about my 4wd that is not working. My “service 4wd” light is on, but it just came on about 1 week ago. Now I have had this used truck for 3 months now and it has never engaged in 4wd. Now that snow is coming I need to get it fixed NOW!! The indicator lights come on when I start my car but none of them stay illuminated. I had my tccm checked and was told it is good. Any info on what to do next??

    Kyle’s reply;

    As far as I know, if the “Service 4WD” light comes on, there is a problem in the transfer case, or the sensor attached to the transfer case. This can also happen if the sensor gets wet (ie. driving through a deep puddle). Quite a few people report this problem as clearing itself if the vehicle sits for a while (like over night). This is a problem on Silverados, Suburban, Tahoes and Avalanches. (“Control Unit” replacement is reported to solve the problem… I can only assume that “Control Unit” is referring to the TCCM.)

    This could or could not be related to your problem, but one thing I can tell you is to get it looked at ASAP. Do a Google search for this problem and you will see that people report that once the sensor produces an error, if it’s not looked at, it starts taking out other components of the 4WD system.

    Regards;
    Kyle

  82. well i brought my truck in and told them they could replace the encoder motor, not sure what was exactly wrong with it, they said it was enguaging only half way then kicking back out. well not sure what could cause this, i asked them to inspect all wires to the tccm and see if the wiring could be the problem, and they said that when they fixed my tccm last year they should have also replaced the encoder motor. They did say they replaced the tccm wiring harness also, so they probaably just ripped me off. Not even caring what was wrong with the actual tccm last year i blew up at the guy and got my encoder motor replaced FREE of charge. guess i should have found out what was wrong with my tccm last year before i got mad? Oh well everything is all good now. But thanks for the advice, next winter when it goes out for the 3rd year in a row, ill at least know where to start looking, haha. Thanks again!!

    Kyle’s reply;

    Free is good! Glad everything is working for you.

    I don’t know where you are, but around here (Toronto, Ontario, Canada), we’re getting dumped on starting tomorrow. 15-25cms (6-10 inches) on Friday and another 10-20cms (4-8 inches) on Sunday. Glad the 4×4 is working! (Actually, I drive a 4WD Nitro now, but my dad still drives the Blazer.)

    Just a quick note… if your system only engages half way and kicks out, it can also be caused by “plastic” bushings in the transfer case that wear out. Hopefully, yours is working as it should, and we can write off your problem to the encoder motor. If it happens again (fingers crossed it doesn’t), have your transfer case checked.

    Regards;
    Kyle

  83. The subject field says it all… I put the truck in 4WD and now it won’t change back to 2WD….

    Suggestions? It’s been one thing after another now for 5 years….Im so close to returning it to chevy…. right thru the front window

    Kyle’s reply;

    Yep, you either have a bad TCCM, bad encoder motor, broken vacuum lines, bad actuator or a problem with your transfer case — all of which has been covered within this thread.

    Good luck.

    Regards;
    Kyle

  84. I am working on a 1999 jimmy .I have read your page but have not solved my problem yet so I need your help .The lights on the buttons do not light up at all to show 2hi and nothing happens when you push any of the buttons . When you first turn the ignition on all the lights will light up together for a second then they go out and are NEVER seen again . I pull the TCCM but it and the plug and prongs look new .I then checked with a test light and found power at the button panel and the TCCM . Any help would be great !THANKS

    Kyle reply;

    Well, I guess it’s time to close down the comments again… people just don’t get it…

  85. I want to thank you for all of the information in this BLOG. My 4hi would not work although my 4lo would work. I tried the easy approach first, which was to unhook my battery and let the blazer sit for 30 minutes. When I hooked the battery back up everything worked fine. There’s no telling how much money you saved me by having this BLOG. I’m so glad I found it.

    Kyle’s reply;

    Glad the information here helped you — that’s why it’s here.

    Kyle

  86. my issue is I can not dis-engage 4×4 in my 95 four door Blazer.
    It is stuck in low.
    I tried running in reverse ( worked for my friends 95 Blazer)
    disconnected battery ( figured it might be electrical)
    I need to know is this module you describe in the floor?
    Or below the glovebox?

    the reverse ‘seemed’ to work but the engine revs up to 2000 rpm and does not go very quickly. When I do get to 25 mph it drops into 4×4 AGAIN!
    Argggg.. Could it be the cold? It has been below freezing for two weeks now and to compound the issue I have no heat either in the vehicle.
    Any help appreciated as I need vehicle to work.

    Kyle’s reply: Hi Spencer;

    I still don’t have pics yet, I’ve just been busy with a ton of stuff right now — but I can go over a few things for you to check/do.

    I’m not 100% on the 1995 model year, whether the controls are in dash or what (I know they are on 98-05). My in dash control has 2, 4HI then 4LO, each with their own light to indicate which mode is engaged. When you engage 4 LO, if you are not in neutral (or reverse, but under 5MPH), the 4HI and 4LO lights should flash and then 4HI will kick in instead of 4LO — this is what I think may be happening to you and why 4 wheel drive is kicking in at 25MPH (4HI will kick in at much higher speeds than 4LO). This may indicate a module wiring problem, it’s hard to really say, but a good place to look.

    OK, the module location — I can’t really describe it better than I did for the last comment poster, but in case you missed that part… sit in the passenger seat — swing your right foot back and forth while keeping your heel on the floor — you’ll hit your left foot one way, and the plastic panel where the module is located the other way. Some older pickup trucks had a “vent” pull handle located there, if that helps!

    As for the cold causing the problem — could be. That’s the problem — there are so many things it could be.

    There is, what is called an “encoder module”, on the side of the transfer case that controls which mode you are in (2, 4HI, 4LO) — if water got in there and froze, it could be preventing the encoder from changing modes. The encoder motor could also be bad. Anyway, if worse comes to worst, you can unscrew the encoder module and manually shift the transfer case into any mode you wish. Unfortunately, I can’t tell you which way is which — you’ll have to figure that out with trial and error! Don’t worry, you can’t hurt anything — you’ll either be in 2 wheel drive, or one of the 4 wheel ranges. If it’s not 2 wheel, try again until it is! (There will be metal “tab”, for lack of better words to describe it, sticking out of the transfer case — put a wrench on it and it should move with minimal effort. NEVER FORCE ANYTHING! Take it for a drive and see what you’re in. Hopefully, you’ll get it on the first try!

    Without knowing what is going on with your controls, I don’t know what more I can offer. If your controls work, and you here the Transfer Case Control Module (TCCM – the module under the passengers kick panel!) “click-click”, then it’s working — have someone do that while you listen under the vehicle for the encoder motor. If you hear that, then you have issues further along the system, such as the servo or actuator (vacuum operated, located under the battery).

    You could also unplug the TCCM and tap on the encoder module to see if it may just be stuck (a warm garage wouldn’t hurt either!) — leave the TCCM unplugged for at least 2 minutes, then plug it back in. Then, turn the key on to off 5 times, then to on and try it.

    You may also look for a friend, or mechanic, that can read your DTC’s (Diagnostic Trouble Codes). They are extremely helpful!

    Hopefully, this will give you something to start with. Let me know how you make out, or if I can answer any other questions.

    Good luck!

    Regards;
    Kyle

  87. Kyle:

    I had the same problem on my 2000 sonoma. The dash lights on the 4WD switched did not light up.

    I punched in the part number on Google and found your site.

    You called it right – pins 1 and 6 wires corroded and broken. I couldnt get the terminals on time, so I cleaned out the connector, also cleaned the pins, stripped off some fresh wire. Then, I put the harness back together, leaving out 1 and 6. I then inserted the newly stripped wire from the back side of the connector, then packed in aluminum foil down into channel 1 and 6. Works great.

    Thanks for posting – repairs like this save lots of time and money – total time to repair about 1 hour. Research time – about three.

    Have agood one – Pablo.

    Kyle’s reply;

    Hi Pablo — I’m glad that I was able to help you solve your 4WD problem!

    An interesting repair — way to use your head!

    Regards;
    Kyle

  88. hello, i came across your web stite and was impressed. i was hoping u might be able to help me out with my problem. as you can guess my 4×4 does not work. the service 4×4 light is on.i can here a clicking in the tccm. and when i turn my ignitionkey on. my push buttons light for a quick second but then go out. before the 4×4 went out i did have a problem with engaging the system and disengaging the system when i wanted to use the 4×4. any suggestions

    Kyle’s reply;

    Hi Roy;

    The service 4×4 light usually indicates a problem within the transfer case. Problems engaging a disengaging would seem to confirm this. Unfortunately, that is beyond the scope of this blog (and probably the average home mechanic) — you need a professional — take it to a transmission shop.

    Regards;
    Kyle

  89. The encoder motors commonly fail on the 2002+ autrotrac (4 4wb buttons) blazers and s10 p/u’s. A printed circuit board contact “cooks out” inside the encoder motor confimation switches. Just so you know, the A,B, and C switches tell the TCCM what general shift postion they’re in. the P switch is a singal to the TCCM when to stop the motor (final park postion). I am pretty sure A is 2wd, B is 4wd hi, and C is 4wd low. If you’re a little clever w/ electronics, you can also fix the TCCM problems w/ new relays. I totally ditched all the “electronic control” for the unit. Since I never use 4wd hi, I just manually operate the electric motor all the way when switching modes. Just back the motor up a tick to prevent the excessive shift fork wear. The vac operated front diff used to be operated by powering it w/ a switch. I removed the collar setup on my ZR2 by putting in a old bravada passenger axle stub. This eliminates the sldiing collar all together and makes it a full time front axle. I offroad with my truck and only drive it to spots, so this may be rather different than most people here.

  90. I have a 2000 blazer, and had the same problem as you described. I took the kick panel off, tightened up the wires, and it worked!! Thanks so much for the insight and guide.

    Kyle’s reply: Hi Timothy!

    That’s awesome! I’m glad that my experience was able to help you — that’s why I post them! ;)

    If you noticed any signs of corrosion, I would strongly recommend unplugging the module and applying some “dielectric gel” to the plug and then reconnecting it. Every module that I have looked at shows signs of moisture/condensation — a definite weak point that some dielectric gel will take care of. You can get it at most auto stores — Canadian Tire, PartSource, etc in Canada — Pep Boys, AutoZone, etc. in the US.

    Regards;
    Kyle

  91. I have no vacuum at actuator under the battery. When I put car into 4 hi I can hear something center hood “running” for several seconds. Does this indicate the tccm is ok?, The vac hoses run under what would looks like a distributor cap. This seems to be where the running motor noise comes from, but I can,t reach under there far enough to check for breaks in the vac lines. Can I safely start dissembling the ignition system to get at lines?

    Since October is prime fishing time on NC Outer Banks and 4wd is a must, can I manually put vehicle in 4 hi by pulling the cable that goes into actuator? for a temporary fix. Back diff seems to lock but is hard to tell around here, too flat and too dry.

    Thanks, John

    Kyle’s reply;

    Hi John;

    My first question would be, what are the lights on the dash doing (the 4wd pushbuttons)? And second, what year of vehicle are we talking? (I’m also assuming we’re still talking about a Chevy Blazer.)

    What you describe sounds like the ported vacuum switch (NOTE: this switch is only on 1999 and up vehicles). I’m not sure on the “hearing something running” — what does it sound like? The vacuum switch shouldn’t make much noise, not enough to be confused with something running. What I suspect is you are hearing the encoder motor on the transfer case trying to shift it into 4wd.

    The drivers side, front axle is always engaged with the front diff — the passengers side, front axle is controlled by the vacuum switch, actuator and cable. The vacuum switch is a common component to fail.

    There are two problems trying to get your vehicle into 4wd with a failed vacuum switch;

    1) you can pull on the cable to engage the passengers side hub — but it will disengage when you release it as vacuum normally holds the diaphram closed, which holds the cable, which keeps the hub engaged.

    2) the vacuum switch also communicates with the TCCM — no vacuum and your lights will probably flash continually while trying to engage 4wd, but won’t engage until all 4wd components, once again, communicate with each other.

    You can test the vacuum switch (of course, once you get to it) by removing the two hoses from the switch and connecting them together — if the cable moves to engage the passengers side hub, the vacuum switch is bad. If you really don’t want to replace the switch, you can get a female/female hose adapter and join those hoses together (front hub will ALWAYS lock after doing this). Unfortunately, getting to the vacuum switch can be a problem, as you have found. I don’t want to say “yeah, start ripping stuff apart” as I cannot really answer that question — only you will know if you have the capability to disassemble things and be able to put them back together. If you feel you can remove a distributor cap, etc. and put them back after, go for it! You’ll save yourself a ton of $$$ doing it yourself.

    Oh, as a start, the vacuum comes off the drivers side manifold and goes to a reservoir (also on the drivers side), then to the vacuum switch (center firewall), then to the actuator (under the battery).

    Well, I’m heading off to work, so I hope this helps — if I think of anything else, I will post it here. If you have further questions, feel free!

    Regards;
    Kyle

    PS – here’s what you’re looking for (the vacuum switch, located on the firewall)


    Click the image for a full size view

  92. Thanks for this blog! My problem was would shift into 2hi and 4hi no 4lo Cleaned the corroded pins works perfect! Thanks agian.

    Kyle’s reply;

    Hello Mr. Reidel;

    No problem at all… that was my sole intention… to help others with a similar problem (and save a ton of money too!).

    Have a good one!

    Kyle

  93. Thank you for your post on the 4 wheel drive relay connector. I didn’t find it until after I replaced the in dash switch, the corosion was so small you never would have seen it if you were not looking for it. So thanks again Bill Shoner Whitmore lake Michigan

    Kyle’s reply;

    Hi Bill;

    Glad that I was able to help!

    Regards;
    Kyle

  94. Great Blog. I have a 2004 Blazer which would not engauge the 4×4. From all the info posted I was able to gather that my problem was not an electrical one. It turned out to be the vacuum line. It is encased in a plastic wire gathering material from the firewall to the front battery. It runs along the engine and then branches off to under the battery. Right at about the front of the engine the casing and vacum line itself had melted in half. Must have worked its way too close to the exhaust manifod. I inserted a small steel tube to each half of the hose to connect the vacume line. This worked. I now have 4×4.

    Best Regards to all,

    John

    Kyle’s reply;

    Great news, John! Glad that I could help.

    Regards;
    Kyle

  95. On my 2003 Chev S-10 with 6cyl automatic, I had to have the 4WD control module replaced once because I couldn’t make it shift into 4WD. Now about a year and a half later I am having the same problem. I have located the unit in the passenger area next to the front door, and here are all of the numbers which are printed on the unit: 25784 11:03-03
    15741322
    TV8N0723339808 . My problem here is that I cannot locate a replacement module anywhere, not at autozone.com, partsamerica.com, napaautoparts.com or even acdelco.com . Nobody is listing a module relating to four wheel drive or transfer case. If anyone can help with a supplier of this part, please email me at WeinCellar1811@aol.com . Thankyou, Dale

    Kyle’s reply; Hi Dale;

    First, the module is actually called a “TCCM” (Transfer Case Control Module) — that should help searching for one.

    Keep your eyes on eBay — I just saw one go for $135 that a guy bought brand new 6 months ago and is scrapping the truck. You can find them at the Chevy dealer, but they are right around the $500 mark — ouch!

    I am going to assume that you have read my post above and have looked at the connector and wires…

    The only other thing that I can suggest is a “home repair”. Obviously, I do not know what kind of problem you are having, but let’s assume that you have dash lights and the light DOES go to the 4×4 high (or low) position — at this point, listen to see whether or not you hear a definitive “click-click” coming from the TCCM. If you DO NOT hear the “click-click”, you have a bad relay in the TCCM. The good news — you can get a replacement relay at your local electronics store and replace it in the TCCM. You will need a soldering iron and some basic electronics skills, but it is very easily achieved (a couple of points to unsolder and then resolder the new relay in place).

    Now, if you DO hear the “click-click”, then your TCCM is working and trying to shift into 4WD — the problem, IMO, would then be with the module that is actually on the transfer case (I forget the term for it right now) — it is a simple “motor” setup that simply moves the transfer case gears to the proper mode.

    Heck, if ya want to “hack” it together, you could always remove the module from the side of the transfer case, cut a hole through the floor and put an actual shifter through the floor and attach it to the pin on the transfer case (I wouldn’t recommend this on an ’03… maybe leave that trick for one that is falling apart and you don’t want to spend another dime on it!) — then you can shift into 4WD manually (AGAIN… NOT RECOMMENDED on a street vehicle!!).

    Hope I’ve helped! If there is anything else I can do, feel free to ask away!

    Regards;
    Kyle

  96. I really wish I found your blog before I replaced my tccm. I live in Canada and my 4wd stopped working. I have a 2003 blazer and where I live 4wd is an absolute must. I took it to a shop and they could not figure out the problem, so then I took it to the dreaded dealer. They gave me a quote of $1800.00 to replace my encoder motor and transfer case control module. I certainly could not afford this so I took the estimate and found the parts on ebay, then I took it to the first shop and paid them $200 to install everything. I picked up the truck and everything worked great. I wish I had thought to check the connectors as if that was the only problem, then I could of saved a ton of money and probably could have gone on a week vacation. Anyway gotta love eBay, I got the tccm for $300(dealer wanted $550)and the transfer case encoder for $190(dealer wanted $680 for this). Great blog and great work. Thanks for posting

    Kyle’s reply;

    Hi Sherry!

    Ouch! I feel your pain. Great story though… thanks for sharing!

    Aren’t eBayers great? :)

    Regards;
    Kyle

  97. Okay so I got the kick panel off.. and yup, being i sunk half my truck lol, it is indeed corroded. I got the box off the frame but now i cant seem to get the main plug off the box its self. the Orange plug wont Unplug. and all those little wires they are supposed to connected right?.. i actually see one wire completely out of the main plug. cant see where it belongs tho because i cant get the plug off. Idk any help would be great.. and those pictures you said you were going to take would be awesome.. Thanks.. Jay

  98. I found on my 1999 GMC Jimmy the best way to check to see if the actuator under the battery was working is to simply pop the headlight assembly out. All you need to get it out is to pull up on the two strips locking it in place. There’s a hole behind the assembly that gives you a birds eye view of the actuator.

    Kyle’s reply;

    Hey Randy!

    AWESOME TIP! That’ll save people some time. Thanks!

    Kyle

  99. Ok So scratch that last comment. Now im Stuck in a different spot. got the module off and all is fine. my problem here now lies in the plug. there are 2 wires disconnected. And i cant figure out what spot they unplugged from.

    Kyle’s reply; Hi Jay;

    I will go out and take pics before I leave for work this morning or I will forget again. I will try to get a good shot of the connector to help you locate where the wires go. No being smart here, but you *should* be able to see where they go… because there will be a hole where the wire should go. The only problem you will have is because you have two wires disconnected, you will need to know which one goes to which hole (terminal).

    If you tell me the colors, I can try and locate them for you (be specific with wire colors — ie. white with light green stripe, pink with dark blue stripe, etc.).

    Regards;
    Kyle

    UPDATE – images have been added to the main article

  100. I have a 4WD 2000 BLazer that won’t disengage from 4HI. When I press 2HI I can hear the TCCM clicking and the lights flash on 2HI but goes back into 4HI. I also tried 4LO and that does the same thing. However, it does go into Auto 4WD with no problem. I’ve had the switch on the dash replaced (the letters on the old one wouldn’t light up). The vacuum switch on the firewall in fine and so is the vacuum diaphram under the battery. I checked the TCCM the other day and found at least two pins with the infamous corrosion and also corrosion on the female side. I cleaned them off as best as I could. Then I found your blog and read that some wires/connectors could be bad. I will check that tomorrow. Anything else I should consider taking a look at?

    Kyle’s reply;

    Hello confused — have a look at this post –> http://kylestubbins.com/node/11#comment-2666

    Regards;
    Kyle

  101. I have a 2001 S-10 with no 4wd. I can hear the transfer case engaging but nothing happens. checked the vacuum lines and there doesn’t seem to be any leaks. No problems with the actuator either. The actuator seems like it is pulling the cable connected to the differential. The cable connects to a “pin” on the differential. With cable disconnected from the “pin” on the diff. I can manually move it in and out less than 1/4″ in. This should pull out or push in closer to an inch or so right? Could this pin be flanged on the other end, not allowing it to pull out, or is there some other problem. Should I even open the diff. or just replace it.

    Kyle’s reply;

    Hi Daniel;

    Unfortunately, I have not encountered what you are describing and can only offer advise for things that I, personally, have done as I am not a mechanic. I do not know what this pin is, or how far it should move.

    Regards;
    Kyle

  102. Hi Kyle,

    Thanks for getting my account working. I still can’t figure out how to upload a picture. Anyway, great site, very informative. Like most here, I’m having a problem with my 4wd on my 2000 Blazer. I think it has something to do with the vaccum, but I’m not sure (been too cold out doors to really get into looking). Sure wish I could find a vacuum schematic. Anyway, on the drivers side, right at the firewall, I have a vacuum line loose. I took a picture, but like I said, I can’t figure out how to upload it. I read in one of the previous comments, that there is a canister on the drivers side? This line is coming off of a tee just in front of the driver going toward the drivers side fender. If someone can trace, and perhaps let me know where it goes, this may solve my 4wd problem.

    Best regards,

    Jim

    Kyle’s reply;

    Hi Jim;

    In the right-menu, click the link “create content” — from there you should have two choices; 1) create a forum topic, or 2) upload an image.

    If you choose to start a new forum topic, you can attach files to your post. If you choose to upload an image, it will create a new topic within my blog (which I will then move to be included within your original post).

    Or, as a last resort, please feel free to contact me via the contact form, leave your email address stating you’d like to send me some pics and I will respond with an address you can send them to.

    Regards;
    Kyle

    UPDATE – 03/07/08

    Hi Kyle,

    Thanks for the instructions on uploading pics. Can you please add this to my original post. I’d really like some help on this. We are suppose to have about 6″ of snow this weekend, and if this is my problem, I’ll have 4wd for it. The hose I’m referring to is the one sticking out over the fender.

    Best regards,

    Jim


    Click for a full size view

    Kyle’s reply;

    Hello again, Jim;

    I’ve got a 2003 Blazer, so I’ll pop the hood this morning and see if I can find the info you’re looking for.

    And I think I’ll go out now — we’re supposed to get 30 – 50 cm’s between now and Sunday (that’s 12 – 20 INCHES for you US guys!).

    Regards;
    Kyle

    UPDATE – 03/07/08

    I don’t have all the terminology, or know what all the parts are — but I can help you with the routing of the lines! The large black plastic thing you see IN the drivers side fender appears to be a vacuum canister — whether lines come off it INSIDE the fender, I do not know. It appears to only have one line going to it, as can be seen in the picture. The bottom hose runs to the middle of the firewall, presumably to the vacuum switch, which is what engages your front axle.

    Hopefully, this will help you figure it out!


    Click for a full size view

    Let me know how you make out.

    Regards;
    Kyle

  103. Thanks for the pic. Kyle.

    It’s 2:30 Saturday, 3/8, and we have 12 inches on the ground, and it’s suppose to snow for several more hours.

    Best regards,

    Jim

    Kyle’s reply;

    I hear you Jim — we got dumped on yesterday. We’ve had at least 12″ fall in the last 24 hours. Towns East of me we supposed to be hit harder — like up to 20″.

    Well, at least the sun is shinning this morning — what a beautiful site with the fresh fallen snow!

    Regards;
    Kyle

    UPDATE 03/16/08

    Hi Kyle,

    Thought I would give you an update on my 4wd situation. As I indicated in my original post, I had a vaccum line loose. I thought it went to a canister in the fender, what’s more, I thought it had something to do with my 4wd not working. Well, that was the case. I found the attachment for that line on the canister. I acquired a new vaccum line (the one I had was weathered), installed it and hooked it up to the canister. Then I went to a muddy area in a sub-division next to mine, drove into the mud in 2wd. I just sat the spinning the rear wheels. Then I pushed the 4HI button, the light flashed for a second, then came on solid. I pushed on the gas and drove around in the mud like it wasn’t muddy. MY 4WD WORKS! Thanks for your site, it’s great. I wouldn’t have ever thought that vaccum line would have kept me from having 4wd. By the way, my 4LO works as well. Of course it needs to be put into neutral before it will engage, per the owners manual. I’ve attached a diagram I received from the Chevrolet dealership parts department of some of the 4wd components on my 2000 Blazer. It did show me where the canister was and what to look for. Perhaps it will be helpful for someone else in the same situation.

    Kyle’s reply;

    Awesome, Jim! I’m glad the posts here were able to help.

    Thanks for the vacuum diagram! That will be very helpful for others trying to diagnose vacuum problems.

    Regards;
    Kyle

  104. hi kyle, I own a 1995 chevy blazer. within the past week my four wheel drive would not engage, about two weeks ago I just replaced the distrubuitor rotor and cap, plugs and wires. at first I thought it would not engage because of the fuse but the fuse was good. I did a little research online and found out the most common problem – the vacume opperated acutator under the battery tray, so I replaced that and made sure the cable wasent binding or froze,everying worked ok but still no 4wd, so i did a vacume test of the system- 19 1/2 lbs per square inch which is perfect, then I did a vacume test on the hose that attaches to the accuator- 2wd = 0 lbs, 4 hi and lo = about 4 1/2 lbs of vacume. is that 4 1/2 lbs normal opperating range? i can hear the relay click when attemping to switch from 2 to 4 wheel, and i also hear the motor on the transfer case working, so my questin is, could it be the transfer case select switch be bad? or someting called the four wheel drive switch (HERE)? Please help me, i live in syracuse, where it snows ALOT!

    Kyle’s reply;

    Hello Ryan;

    Unfortunately, I don’t know the answers to most of your questions — I can guess, but that’s what it’s going to be, a guess.

    As for the 4.5 pounds of vacuum — it doesn’t sound right, simply based on the 19.5 pounds you get from the base system. I’d check your hoses for a leak.

    I am not familiar with the variances in model years — I own a 2003 blazer, so what may be applicable to my vehicle may not be to your ’95. Anyway, on the newer blazers, there is a vacuum switch, either located on the firewall (below the distributor), or on the top of the transfer case — this switch controls the front, passenger axle (via the actuator/diaphram that you have already replaced).

    As for the vacuum switch you linked to above — could very well be the problem. For $18.95, it’s cheap to find out! However, for pennies you can test it — NAPA (and probably others, like PepBoys, etc.) will have vacuum hose connectors — male to male connectors will allow you to stick 2 vacuum lines together. Remove the 2 lines from your switch and stick them together with a connector. This will allow vacuum to pass, regardless of the condition of the switch. Actually, seeing as you have a vacuum gauge — test the lines as you remove them from the switch (keep in mind, 1 should have no vacuum, which would be normal).

    Other than that, there are a couple of sensor switches — one to detect if the front axle is engaged for sure — which could also be a cause of failure (if the sensor says something is not right, it will kick it back out of 4WD and into 2Hi). Where they are located, I’m not 100% — especially on a ’95.

    Well, that’s about all I can think of right now — if I think of anything else, I’ll update this post.

    Good luck, and let me know what you find — in the hope that it will help someone else down the road.

    Regards;
    Kyle

  105. Your information has been very helpful, Thanks. My system is acting a little different than the postings I’ve read. My annunciator “Service Four Wheel Drive” comes on about two seconds after the ignition switch is turned on then goes out while starting the engine and comes back on as soon as the engine is running and the switch is released. All four lights on the 4wd selector come on when the ignition switch is turned on and then after a couple of seconds they dim. I have had problems with the four wheel drive in the past and discovered the corrosion at the connector on the tccm. Cleaning the terminals fixed it and another time I had flashing selector sightws and had to change the vacuum valve. The “Service” light Has come on befor but seemed to go out with the fixes. Any advice Kyle?

    Kyle’s reply;

    Hi Daniel;

    Unfortunately, I don’t have any real experience with the service 4WD light. As I said to my guest below (just the order I answered the posts), it usually indicates a problem within the transfer case — trouble codes should be set, so you may need to have it scanned by your mechanic. Other than that, I cannot really offer any further advise — all of my experiences with my Blazers 4WD system have been posted here.

    Regards;
    Kyle

  106. light comes on when it is put in 4×4 and comes off once you shut vehicle off for 5-10 seconds and you push 2 wheel drive button, any help would be appreciated

    Kyle’s reply;

    Generally, that is telling you that there is a problem in the transfer case — trouble codes should be set, so have your vehicle scanned by your mechanic. I have no experience with the 4WD service light, so other than that, I have no further information.

    Regards;
    Kyle

  107. Hi Kyle
    I cannot download any of the files/documents which you posted from Raymond Dunne (2008-01-03 19:30).
    Indication I get is that the files are no longer available.They may have been deleted by the uploader, or expired.
    Is it possible for you to relist them? From the titles these documents would appear to be very useful in diagnosing Blazer/Jimmy 4×4 problems.
    I thank you in advance.
    Les

    Kyle’s reply;

    Hi Les — I didn’t realize that Raymond had uploaded them to a file server — I thought they were part of a site that stored that information (ie. an auto info site), or I would have saved them on my server. My bad.

    However, I looked and do have an email for Raymond… I’ll send him a message and see if it’s possible for him to email the files to me.

    Regards;
    Kyle

    UPDATE — 03/20/08

    Hi Les — I’m going to have to say no as Raymond no longer has them — again, I apologize for not backing them up.

    Regards;
    Kyle

    UPDATE — 03/20/08

    Hold onto that thought for a few days — some software is on the way that will allow me to retrieve the files.

    Regards;
    Kyle

  108. I am having a problem with my 4WD not going into 4HI, I press the button and nothing happens. I put the truck in neutral and hit 4LO it goes in, then I press 2HI and it goes back. When I have it in neutral and press 4HI both 4HI and 4LO lights flash and then goes back to 2HI. I read that if you jumper pin 13 on the data link cable to ground, then turn key to the ON position the lights on the 4WD buttons flash a code, so I tried it and sure enough a code of 2 flashed meaning motor/encoder failure. I haven’t changed it yet so I don’t know if that is the problem. I am going to check the TCCM connections. Just to let you know there are only 4 codes. The first is 1 flash meaning RAM stand by power to the TCCM was lost. 2 flashes means motor/encoder failure. 3 flashes means motor circuit failure. 4 flashes means TCCM needs to be replaced. I hope this helps you in some way and if I am incorrect please let me know. Thanks Rudy

    Kyle’s reply;

    Hi Rudy;

    That’s some pretty good information to have.

    Please do let me know how you make out with the encoder motor replacement.

    Regards;
    Kyle

  109. ok no corrosion no wetness at all also when i ask it to shift i still here it click at the tccm. Also a new thing I forgot it has to be in nuetral to shift into or out of 4 low when its in nuetral i can also here the small electric motor on the transfer case my next thought is to remove the motor from the transfer case to see if I can shift it maunlly but that involves alot of dissmanlting there’s a lot of things in the way is this necessary. If it is I just might wait untill its warmer there has been very little snow in denver area this year.

    Kyle’s reply;

    Hi Scott;

    I’m not totally sure what causes the lights to simply blink and then not go into gear. The first time I put the Blazer in 4Hi this winter, it did just that…. I had to put it in neutral for it to go into 4Hi. Since then, it will shift on-the-fly. Why? I don’t know. There has to be sensors either in the TC or the encoder motor to let it know when it has reached it’s intended mode… I’d suspect it’s part of the encoder motor, but I’m just guessing.

    Your idea of removing the encoder motor to manually shift it is a good idea, one that I explored and was helpful to me. However, mine was easy… just a couple of bolts and off it came… nothing in the way (mines a little newer and doesn’t have the “auto” feature).

    I wish I could be of more help, but it sounds like you’re on the right track.

    Good luck!

    Regards;
    Kyle

  110. Hello I wanted to know if you can point me in the right direction. My 4×4 wont work I replaced the switch on the transfercase and the accuator under the battery but still nothing. I thought that I was losing vacuum, just because when I go up a hill my heat if selected to be on the floor vents will change to the top vents the back to the floor/or where ever I have selected them before they changed on there own. This will happen when the rpms go up to or pasted 2000. One other thing can you tell me were I can purchace the cable that goes to the front axle or at least the name. Thank you Edward

    Kyle’s reply;

    Sorry, I don’t know the answer to your questions.

    Regards;
    Kyle

  111. I’m in 4 wheel low, this was after a snowstorm , we never have used the 4 wd before.
    I have change the dash selector, checked out the tccm module it clicks but will not come out of lo 4 wheel drive, I disconnected the vaccume module in the center of the fire wall.

    Still it shows in “four wheel low” I feel my next move is removing the front drive shaft , and pulling the servo shift motor on the transmission , it looks as if the are around 9 wires going to that servo motor, I’m I on the right tranck ? or should I test other things? I have tried to reset the computer, also stood on one foot and jumped around three times ” that didn’t help either” !!!

    I would love your input if I may ask?

    Randy

    Kyle’s reply;

    Hello Randy — have a look at this post –> http://kylestubbins.com/node/11#comment-2666

    Regards;
    Kyle

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