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  1. #1
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    4WD not working? Step inside.

    There seems to be quite a lot of mystery to newer folks when their 4x4 functions stop working. Since crappy weather season is upon us a lot of people are trying their 4WD functions for the first time this year... and discovering all isn't as it should be.


    Here's my attempt at a troubleshooting checklist/flowchart. Hopefully this helps folks get a handle on what's wrong with their setup and how to fix it.


    Information in this thread applies to GMT360/370 trucks with selectable 4WD functions (TrailBlazers, Envoys, Ascenders?). Trucks with non-selectable AWD (Rainiers, 9-7s, Bravadas) may find some of the information here useful. However AWD trucks lack some of the parts discussed here.


    On to the show.




    Gah! My 4WD doesn't work! What do I do?




    Is the Service 4WD light on?



    Yes: Go to Section 1.


    No: Go to Section 2.






    Section 1: My Service 4WD light is on.


    Are there any lights lit on the 4x4 selector switch?


    Yes: Go to Section 2.


    No: Try these steps.
    With the ignition off, rotate the 4x4 selector switch from left (2HI) to right (4LO) and back again about 20-30 times. With lack of use, the contacts inside the 4x4 selector switch can get a light amount of oxidation on them. Repeatedly rotating the switch can sometimes clear this problem up. If this doesn't help, read step 2. If step 2 doesn't work for you, replace your 4x4 selector switch. If replacing the 4x4 selector switch doesn't work for you, try to find a local member with a verified working TCCM to swap in. If the problem goes away, return your friend's TCCM and buy a replacement for yourself. Search these forums for replacement instructions.


    On some 2002 model year trucks, the Transfer Case Control Module (TCCM) can have trouble waking up when the engine computer tries to talk to it. Fuse #8 in the engine bay fuse block provides power to the TCCM. Pulling this fuse and replacing it will usually wake the TCCM up and allow the use of 4WD for a time. Fixing this requires either a dealer software update (can be spendy, doesn't always work) or replacing the TCCM with a unit from a 2003 or newer model with selectable 4WD.




    Section 2: Checking the motors.


    With the ignition on, switch the 4x4 selector switch from 2HI to 4HI. You should hear two distinct mechanical whirring sounds. One from the middle of the truck (under you) and one from the front of the truck.


    I hear both whirring sounds: Go to Section 3.


    I don't hear the sound from under me: There is an issue with the Transfer Case Encoder Motor. Check the connector going to the motor(cylinder looking thing) on the transfer case for damage/corrosion. The motor may require replacement.


    I don't hear the sound from the front: There is an issue with the Front Axle Disconnect Actuator. It is plugged into the Front Axle Disconnect (AKA Splined Disconnect) located under the truck on the right side of the oil pan. Check the connector attached to it for damage/corrosion. If the connector is OK, replace the actuator.




    Section 3: Identifying mechanical failures.


    Turn the ignition on but don't start the truck. Place the 4x4 selector switch in 4HI. Using a jack/jackstand raise the LEFT FRONT wheel off the ground. Try to turn that wheel by hand.


    The wheel does not turn freely: That's the expected behavior. The wheel will be prevented from turning by the passenger side tire being on the ground, and the transfer case connecting front to back and the rear tires on the ground.

    The wheel turns freely: Crawl under the truck so that you can see the front driveshaft. Have an assistant turn the left front wheel by hand. Can you see the driveshaft turn? If so, you have an internal transfer case failure. Most likely due to worn out clutches. If not, read on.


    It is likely your Front Axle Disconnect is not engaging properly. This can be caused by worn bearings or a broken shift fork. You can check for worn bearings by grabbing the inner tripot joint(closest to the Front Axle Disconnect) of the right side axle and trying to wiggle it. Movement indicates worn bearings. Even if your bearings aren't overly worn, there is a possibility of a broken shift fork, spring, or other components.



    There is a great guide on the teardown and rebuilding of the Front Axle Disconnect here: http://www.offroadtb.com/articles/te...4wd-disconnect



    There is an alternate way of fixing the shift fork issue here: OffRoad TB - View topic - Front axle disconnect fix... with a twist.



    If you rebuild/replace your Front Axle Disconnect or are 100% sure it is functioning properly and still are at this point in the troubleshooting guide, you may have an internal front differential failure (likely) or broken intermediate shaft (extremely rare, couldn't find an instance of this by searching). Now is the time to make a post detailing all the steps you followed in this guide as well as any other observations made.




    There you have it. I hope this guide points you in the right direction.


    Additions/corrections welcome!
    Last edited by The_Roadie; 02-11-2014 at 08:27 AM. Reason: Fixed a section that had the wrong flow
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    Reserved for additional resources.
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    wow... very indepth. I really hope i dont ever have to use this thread though.

  5. #5
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    Nicely written sir.
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  6. #6
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    Great write up

  7. #7
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    my service 4wd came on after I put it back into 2wd during the snow we had in PA last month. Light was not on the next time the truck started up, I put the selector into 4wd just to test it and hear if it was engaging still(it was thankfully!)...put it back into 2wd and went on my way. I think it was just a warning letting me know i'm going to have issues this winter.
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  8. #8
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    Quote Originally Posted by tbuckalew14 View Post
    my service 4wd came on after I put it back into 2wd during the snow we had in PA last month. Light was not on the next time the truck started up, I put the selector into 4wd just to test it and hear if it was engaging still(it was thankfully!)...put it back into 2wd and went on my way. I think it was just a warning letting me know i'm going to have issues this winter.

    I discovered that my actuator is sometimes picky if it's cold and/or hasn't been used in awhile. A little exercise may help that problem. I've since removed most of the shift components from my splined disconnect and run a solid AWD sleeve, so I haven't had to fiddle with it since.
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  9. #9
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    Mine was a bit flaky awhile back but after several cycles I have not had any further issues
    Keep the earth below my feet -- For all my sweat, my blood runs weak
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  10. #10
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    I have had the same issue before with the random service 4 wheel drive light. I shut the truck off and it goes away. I wouldn't overly worry about it.
    Twenty years from now you will be more disappointed by the things that you didnít do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover. - Mark Twain

  11. #11
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    great write-up! wish i had this two months ago. i couldve stopped in section 1 with a simple switch replacement- was lucky it happened during the summer and not the winter.

  12. #12
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    I too have discovered the selector switch gets dirty. Once a month I run the switch through the settings and drive a bit in AWD & 4HI just to keep everything moving.
    -- Mark
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  13. #13
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    In the event of needing a front diff rebuild, is it possible to remove the front diff (with 4WD interconnect mechanism), front driveshaft (TC to front diff) and both front half shafts and have the vehicle run in 2WD mode..??

  14. #14
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    That plan will work just fine, as long as you haven't replaced the hub bearings with the new cheaper kind some places offer that REQUIRE the presence of a torqued down CV shaft and nut to keep them together. If you ever see a hub bearing offered that's listed for 4WD ONLY, that's the kind to avoid. I use the 2WD kind if I have a choice because they will also work with 4WD. Some 2WD SS vehicles came with little stub CV ends to accomplish that purpose, since Chevy figured SS drivers would push their vehicles harder.
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  15. #15
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    First of all thanks to everyone for all the info and this wonderful site. My wifes 02 TB has no 4wd and thanks to this site I believe that the TCCM is the culprit.( Thanks to the Roadie's post of schematics in another thread I plan to double check the switch, power, and grounds with a meter to confirm the other stuff is good.) At first I had nothing, no lights on the selector switch, no service 4wd light on the dash, and no sounds from under the TB when it was switched to 4wd, so I pulled the #8 fuse under the hood to reset the TCCM and now the switch lights come on when you first turn the ignition to accessory then go out and now the Service 4wd light is on. Even tho the 4wd selector switch gets regular use I tried rotating it several times but still no 4wd. Like most everyone else I plan to buy one from a junk yard but my question is will one from an 08 work in my 02 or do I need to get 1 from between 03 - 05? Also I know the TCCM is in the dash near the light switch but what is involved in pulling it out? Is it bolted down or just clipped in? Can I get it out thru an access door on the driver side of the dash or do I need to pull off some panels? Thanks for all the help.

  16. #16
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    Quote Originally Posted by Brian@work View Post
    ... now the switch lights come on when you first turn the ignition to accessory then go out and now the Service 4wd light is on. ...
    Pretty much that's the definitive test of a bad 2002 TCCM. If the TCCM lights up the switch lights during the lamp test, but the PCM can't communicate with it within a certain time, the PCM sets the Service 4WD light on the dash. Sometimes, this sort of failure can be cured by a dealer reflash, but they typically charge more than a junkyard TCCM. Greedy and short-sighted policy that is.

    The TCCM is clipped to the lower dash closeout panel, but you can also get to it by the little 5"x5" access hatch in the side of the dash that's hidden by the door when it's closed. I prefer to take off the lower dash panel. The '08 will work. All 2003+ TCCMs are the same except for programming, so make sure you get one from a true 4WD Trailblazer or Envoy and not one of the AWD models. The dealer would have you believe it needs reprogramming for the vehicle's gear ratio, but that's a fib also because all it does is adjust the engagement speeds for the A4WD function, which is a silly mode I recommend against ever using, and the 4LO interlock speed, which is best engaged at zero MPH anyway.

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    Awesome thanks very much for the help.

  18. #18
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    The 08 was parted out before I got to it but I found a TCCM out of an 05 and put it in, everything is working perfectly now. $50 and good as new, man what a savings. Thanks again for all the info and help.
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  19. #19
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    Is there a thread anywhere about replacing the actuator. I hear the high pitch sound so actuator is my next step.

  20. #20
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    Quote Originally Posted by andy702252 View Post
    Is there a thread anywhere about replacing the actuator. I hear the high pitch sound so actuator is my next step.
    Have you looked down there yet? It's two-bolt-easy.

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  21. #21
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    Umm yea so can I have the written location where this is at I see so time of boot covering.

  22. #22
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    Galesburg, MI
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    4WD not working? Step inside.

    Okay my four wheel drive in not working. I followed these steps and the ones on http://www.offroadtb.com/articles/te...my-4wd-system/ . I heard both actuator and end coater engage. Jacked up passenger side the wheel rotated & front axle did not spin. Jacked up whole front end spun passenger tire driver side did nothing vise versa. My wife has a trailblazer also I engaged 4hi and checked passenger front it was locked in place. So I had it in 4hi the other day and was going in the driveway at the in laws when I was going I heard a whir-ing noise but not working. So I'm not sure if it's the disconnect, transfer case or whatever else. Just bummed out its messed up.
    2005 Trailblazer EXT
    2005 Trailblazer LT

  23. #23
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    Quote Originally Posted by jballentine View Post
    So I'm not sure if it's the disconnect, ...
    I am. If your switch changes modes and the lights aren't in blinky-land, then the TCCM is satisfied with what the front axle actuator and transfer case encoder motor are doing. So you have a mechanical issue, and the splined disconnect is statistically more likely than an internally broken front diff.

    The definitive article:
    Offroadtb.com Front Axle 4WD Disconnect
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    4WD not working? Step inside.

    Thanks roadie, I'm gonna be ordering parts and rebuild it see if I can get my dad to help since he has a heated garage. Well I'll have time to get them since it goes in the body shop from the deer that jumped out in front of me 2 weeks ago.
    2005 Trailblazer EXT
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  25. #25
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    4WD not working? Step inside.

    So another question. When I put the lifting blocks in the front to level it would have made it wear faster out than normal? How long would it take to change the axle disconnect? I'm pretty mechanically inclined and like to do it myself. I'm at 135k I'm wondering if I should remove the blocks off them and go back to factory level while I'm taking it apart. Thanks
    2005 Trailblazer EXT
    2005 Trailblazer LT

  26. #26
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    2003 GMC Envoy
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    4wd problem (no 4wd-hi, but yes a4wd) - replaced transfer case encoder motor

    i could not get into 4wd high during our recent icy and snowy conditions. the dashboard selector switch would indicate 4wd-hi, but the front wheels would not engage (obvious when the rears broke traction and the fronts did not). strangely enough, my a4wd did indeed work, and that is how i got through those sloppy conditions (albeit with a bit of driveline shock every time the front wheels would engage 4wd mode).

    since my a4wd worked, i ruled out the splined disconnect as a problem. since the dash selector switch seemed to work ok (no blinking or getting the "red N" indicator) i ruled that out too. as roadie would say, i basically shotgunned it and tried replacing the encoder motor on the transfer case first.

    purchased a new dorman replacement unit for $138 w free shipping on amazon.
    just completed the install this evening (my fingers are still thawing out, temp dropped to 24 deg F after sunset).

    jacked up the driver side only, and put jack stands underneath. this enabled easier access to slide underneath the vehicle.

    as previous posters have mentioned, removal/replacement is not that hard. however, there are some tips i can offer to make it less stressful.

    the electrical connector is snapped together, so to remove it you will need a medium flat blade screwdriver to slip underneath the snap and pull the 2 halves apart. sounds easy right, well momma forgot to tell you that the plug resides on top of the transfer case, so you will be working blind with both arms wrapped around the tc. be patient and it will eventually free up after much fiddling with the screwdriver.

    there is also a plastic nipple-keeper (plug side of the encoder pigtail) that snaps into a metal tang on top of the tc, again you'll be working blind. the keeper positions the wires so they don't jiggle around as you drive. i struggled to get this keeper off and eventually snapped it off and broke it. upon closer examination the keeper simply slides off 2 notches on the outside of the plug housing (duh couldn't see that - working blind). so instead of manhandling the plug and trying to push the nipple out of the metal retainer, try to slide the plug off the slotted nipple-keeper instead (slide the plug away from the captured keeper - as they are not molded together). some people might have gotten lucky and just pulled the plug off the keeper, not realizing how they connect.

    note - my new encoder came with a new slotted nipple-keeper. i used diagonal cutters to snip off the old nipple and throw it away.

    the old encoder motor came off straightforward (after disconnecting the plug), 4 13mm bolts and a ratchet and 6" extension. notice that the old motor has 2 split locator bushings - these need to be removed and transferred to the new motor (mine didn't come with any). 1 bushings pulled out easy with pliers, the other was more stubborn (corrosion) and required heat-torch, and a hammer-punch from the backside. after removal, i cleaned them up and installed into the new encoder motor housing.

    my tc was in 2wd-hi, and i did not have any problems aligning the new tc encoder motor splines (female)with the splined tc shaft (male). there is a slight amount of play in the tc shaft, perhaps a 1/8" rotation. i did not have to manually turn the tc shaft like other members have mentioned.

    note - there was lots of blue loctite on the original 4 bolt threads (steel threads into aluminum tc housing). i used a little fresh loctite upon reinstallation.

    after bolting on the new encoder motor, i routed the new wiring harness over the top of the tc to mate with the other connector. the plug halves snapped into place with a little fiddling.

    tip - slide the new nipple-keeper off the new plug. after you connect the new plug, you can then slide the keeper into the side of the new plug, and then push the nipple into the metal retainer hole (empty hole since the original was cutoff - or if you didn't break it like me, use the old keeper). that was easier for me at least.

    it's dry outside so i was not able to fully test out the 4wd-hi mode. however, driving around the block i could "feel" and hear the 4wd-hi mode engage - mechanical gear mesh noises coming from the front end during slow speed corners. noise disappeared upon returning to 2wd. so i think it is fixed, but will test out when the snow comes down again.

    to repeat, this procedure is technically not that difficult, however there are some little things which can annoy and slow you down a lot. took me 3 hours this first time. bet i could get it done in a hour the 2nd time.

    just saved more than $500 at the dealer, cha ching.

  27. #27
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    Livonia, MI
    2002 GMC Envoy
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    Replaced spline disonnect and actuator, still not working - no lights on switch/dash

    Hello,

    Recently was doing an oil change and found the oil filter covered in grease. After looking for the source of the grease, I found the passenger side CV axle was broken (boot ripped). Took the axle out and then was worried about the spline disconnect so I removed that as well. Looked like the bearings in the disconnect was trashed. So ended up putting a brand new disconnect in as well as actuator.

    I put everything back together and now have no lights on 4x4 selector switch nor no tell-tale light on dash. Before started work, there was no tell-tale on dash but the 4x4 selector switch WAS illuminating lights for 2x2. Probably last winter was last time I switched into 4x4 mode and it worked then as well.

    Now through troubleshooting, I installed the old actuator and no change. I did the fuse 8 pull with and without engine running and no change (without engine running was also key off). Also disconnected battery for about a minute to truly make sure no power and still no change. Even with fuse pull and engine running, I did not get a tell tale on the dash. Shouldn't it of lit if the TCCM was not communicating?

    I also replaced the alternator less than 1 week earlier, so the battery was disconnected for an extended time. Could this have caused the TCCM to flake out?

    Thoughts? I've tried searching but haven't seen anything like this. Other posts show when TCCM not working the light on the dash is lit. If switch not working, also light on dash. In this case, I have no lights what so ever - dash or selector switch.

    I don't want to keep throwing money at it, already in pretty good with the disconnect. What are thoughts on the switch? I also have read the 2002 TCCM's are flaky, I can see about trying to find one to "borrow" to see if this helps.

    Any thoughts or ideas are very much appreciated.

  28. #28
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    Extended battery disconnection will not cause TCCMs to lose their memory. A bad 2002 model will *usually* light up all the switch lamps for a second (the lamp test function) and then just leave the 2HI lamp lit. But it will fail to respond when the PCM wants to start communicating, and then the dash warning light will light up. Sometimes, the behavior of the switch lights is different, but in all cases, if the module is one of the bad ones, the dash light will come on.

    Try using the Owner's Manual instructions to go into NEUTRAL transfer case mode. Its red light is fed by a different fuse (rear fuse #48) and may tell us something.

    Unless you want to check out the voltages at the TCCM connector and dig a bit deeper, I recommend just getting a replacement 2003+ TCCM.

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  29. #29
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    New Brunswick NJ

    Help same problem

    I have a 2006 Envoy. When I change the dashboard selector switch from Auto to 4WD the 4 wd light blinks for about 45 seconds then stays on. There is no sound at all from in front or under car, and it is still in 2 wheel drive. Same thing happens if I try to go to 4 WD low.
    Any Suggestions?

  30. #30
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    Did you read and follow the troubleshooting flowchart in post #1? What were the results?

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    From my Golden Age of Offroading (2005-2009): Trail Report & Picture Thread

 

 
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