GM Truck Club
THE PREMIER CHEVY TRUCK & SUV FORUM
Founded in 2004 ~ We're the #1 Chevy Truck & SUV Forum.
Silverado & Sierra | Tahoe & Yukon | Suburban & Yukon XL | SUV & CROSSOVER
Page 1 of 2 12 Last Post byLast Post by
Results 1 to 10 of 18
  1. #1

    Default 4 wheel drive not engaging - recent transfer case swap- now grinding in reverse

    OK, now that the first snow of the year has flown I realized that the 4WD is not working on my 1999 Suburban 1/2 ton (NP 246 transfer case with pushbutton auto 4WD). Here are the facts:
    Swapped out the transfer case this fall with a factory new rebuilt unit that I bought from someone on Craigslist. It looked like it had never been installed and the transfer case doesn't make any unusual noise.
    When I select Auto 4WD, 4 Hi, 4 Lo the indicator light shows that it is engaged and I can hear the shift motor operating. However the rear wheels will spin and the front's are not engaged. (i.e. I can drive around in 4WD, it just doesn't engage the front wheels)
    There is no funny noise coming from the transfer case.

    I haven't crawled under to look at anything, but I'm curious if anyone has any suggestions of what I should look for. Will there be codes I can have pulled from the auto parts store? The check engine light is not on.

    Any feedback would be helpful. Thanks!
    Last edited by Family Force 6; 12-28-2012 at 09:58 AM.
    Current Rides: |99 Chevy Suburban- the family truckster |93 Lincoln Town Car- 250K miles & still churning -now a fan of the Panther platform |95 BMW 325is coupe- Wheeee!
    Former GM cars: |78 Suburban 454 |71 Buick Electra 225 (deuce & a quarter) |75 Monte Carlo- 400SB- heaviest doors GM every made |70 Impala coupe- my 1st car

  2. #2

    Default

    Well, there are a number of things that could be causing the problem.
    The most likely, imo, is the front passenger side axle disconnect. It lets the right front wheel turn freely from the front diff when in 2WD. When any of the 4x4 selections are made, the actuator causes the inner and outer halves of the axle to connect so the diff can turn the right front wheel.
    The most likely is the transfer case control module, the push buttons talk to the TCCM, the front axle disconnect talks to the TCCM and the transfer case endcoder motor talks to the TCCM.
    Assuming there is nothing wrong with the new transfer case, there could be a wiring problem, or a compatibility problem.
    And, of course, there could be a problem with the new t-case, usually this would involve the encoder motor.
    Ray

    '09 Avalanche LTZ - Black
    '05 Envoy XL (sold)

  3. #3

    Default

    Thanks for the response Ray. Where is the TCCM located? Is it inside the TC? I assumed that the black cylinder-shaped plastic piece on the outside of the TC was the encoder motor. I guess I don't have a real good understanding of how the 4wd system works on these. How would I test the TCCM?

    New Info. Yesterday my wife was backing out of the driveway the transmission (or TC?) started grinding VERY LOUDLY. She put it back in drive and it stopped. She pulled it back in the garage. Later on I went out checked the transmission fluid level (OK, and fluid looked good as I drained and replaced a good portion of it with some AMS Oil synthetic tranny fluid, filter and gasket) and started it, put in reverse and no noise. Didn't drive it around as I'm a little freaked out to go anywhere with it after I heard that terrible noise. Could this be related to the transfer case? Again it was only in reverse and made the grinding noise even when it wasn't moving.
    Last edited by Family Force 6; 12-28-2012 at 09:59 AM.

  4. #4

    Default

    Quote Originally Posted by Family Force 6 View Post
    Where is the TCCM located?
    Not sure on your truck, on my '05 Envoy and on a '96 Blazer, it was on the passenger footwell area, behind the trim panel.

    Quote Originally Posted by Family Force 6 View Post
    I assumed that the black cylinder-shaped plastic piece on the outside of the TC was the encoder motor.
    Correct.

    Quote Originally Posted by Family Force 6 View Post
    How would I test the TCCM?
    Usually if the TCCM has a problem, the "service 4wd" light will be on. Testing usually requires a GM Tech II tester.

    The tranny will not effect the 4x4 operation.

    Check the operation of the disconnect actuator in the front passenger axle. With engine running, put the transfer case in 4x4 hi, turn off the engine. Jack up the front of the truck (don't forget to use jack stands) and try turning the front passenger wheel. The inner half of the axle shaft should turn with the outer half. If it does not turn, the actuator (or the parts operated by the actuator) is the problem area.

  5. #5

    Default

    Quote Originally Posted by RayVoy View Post

    The tranny will not effect the 4x4 operation.

    Check the operation of the disconnect actuator in the front passenger axle. With engine running, put the transfer case in 4x4 hi, turn off the engine. Jack up the front of the truck (don't forget to use jack stands) and try turning the front passenger wheel. The inner half of the axle shaft should turn with the outer half. If it does not turn, the actuator (or the parts operated by the actuator) is the problem area.
    I did engage the 4wd and jack the front right tire. I crawled under and turned the front drive axle from the transfer case. The front tire turned so it appears the front axle is engaging properly.

    Is there a simple way to test the encoder motor without removing it from the transfer case?

    Secondly, I'm guessing that the grinding noise I heard while in reverse is a bad sign that the transmission is close to failing.
    Yikes- 200K may be all I can stand to hold on to this bad boy. Too many repairs in the last year and with engine leaks developing recently it looks like it's time to find a new family hauler.

    FF6

  6. #6
    Jr. Engineer
    Join Date
    Aug 2010
    Location
    eas' bum fuk, maine
    Posts
    207

    Default

    it sounds an awful lot like like the front diff is NOT locking. put the transmission on 4wL and see if it feels like low range. that will tell you if your transfercase is probably working.

    if you put it in 4wd and lift the right front wheel ONLY... you should NOT be able to turn the front wheel because it will be locked into the diff that will not turn (vehicle in park and the left front wheel cant turn either). the grinding noise COULD be the actuator on the front diff partially engaging. all it is is a gear that has a toothed collar the slides over it to engage the inner axle shaft to the diff and if its partially engaging it will grind (gears grind when they dont mesh).

    -------- Edit-------------

    after re reading what i first wrote and what you wrote last i would like to add this. put the transfer case in 4wL and drive it 10 or 20 ft so you can assume that under normal circumstances the transfercase should be locked in. i say this because if you did turn the front drive shaft with the front wheels off the ground then the transfer case is not engaging you may even want to try it with the engine running because it may or may not unlock the front diff when you shut it off.

    overall... if it goes into 4wd low then the transfer case is prolly ok.
    "i dont think you got the picture. i got a beautiful picture. this baby happens to have an extra turn of speed, which is the only thing i care about. you dont understnad what happens do you? they make cars. they make 'em exactly the same way. one or two of 'em turn out to be something special. nobody knows why. i know why. i may be kiddin' myself, but i think i can make somethin' out of that sad little bucket of bolts." ~ Tennessee Steinmetz

  7. #7

    Default

    Quote Originally Posted by a.graham52 View Post
    it sounds an awful lot like like the front diff is NOT locking.
    The front diff is not a locking diff, as such, one wheel will always turn freely.

    Quote Originally Posted by a.graham52 View Post
    if you put it in 4wd and lift the right front wheel ONLY... you should NOT be able to turn the front wheel because it will be locked into the diff that will not turn (vehicle in park and the left front wheel cant turn either).
    Under this senario, it does not matter which front wheel is lifted off the ground, it will turn freely. The diff does not lock and one wheel will always be able to turn freely.

    EDIT: I stand corrected, one wheel in the air while truck is in 4x4 will not turn.

    Quote Originally Posted by a.graham52 View Post
    put the transfer case in 4wL and drive it 10 or 20 ft
    overall... if it goes into 4wd low then the transfer case is prolly ok.
    This is good advice, if the truck advaces slowly with a lot of engine rpm, it is in 4LOW.

    Quote Originally Posted by Family Force 6 View Post
    I did engage the 4wd and jack the front right tire. I crawled under and turned the front drive axle from the transfer case. The front tire turned so it appears the front axle is engaging properly.
    Yep, I agree, the actuator appears to have connected the inner and outer axles.

    EDIT: while posting farther down in this thread, I realized that if you can turn the front driveshift (under the above conditions), the t-case is not in 4 wheel drive.

    Quote Originally Posted by Family Force 6 View Post
    Is there a simple way to test the encoder motor without removing it from the transfer case?
    @a.graham52 suggestion of using 4LOW is a test. The encoder motor should make some motor turning noise as the selector switch is moved to different positions (most noticeable between 2WD and 4HI).
    Last edited by RayVoy; 12-30-2012 at 04:16 PM.

  8. #8
    Jr. Engineer
    Join Date
    Aug 2010
    Location
    eas' bum fuk, maine
    Posts
    207

    Default

    @RayVoy i know what your saying about an open diff but in my experience iv worked with on vehicles with stuck ebrakes on one side, lifted the truck (with the trans in park) and one wheel locked solid (ebrake stuck) and you could not spin either wheel. now if the vehicle was in neutral i should be able to spin the one unstuck wheel and it willl in turn spin the drive shaft.

  9. #9

    Default

    Let's talk 2wd, rear drive, with an open diff. The make up of the open diff lets only one axle gear connect to the ring gear at any given time, however, the axle side can switch back and forth at ease. The one thing that doesn't change, is that the power always goes to the axle (wheel) with the least resistance. That is why, you can have one rear wheel on dry pavement and one one black ice and you are stuck, the wheel on ice will always spin, taking all of the engine power.

    If you jack up both rear wheel, with the tranny in PARK, one wheel will turn, the driveshaft will not, The other wheel will not turn, because it is connected to the driveshaft and the tranny will not let the driveshaft turn.

    Put tranny in N, and one wheel will turn the driveshaft.

    The front works the same.

  10. #10
    Jr. Engineer
    Join Date
    Aug 2010
    Location
    eas' bum fuk, maine
    Posts
    207

    Default

    The side with the least resistance (least traction) gets power from the driveshaft. So the side with the least resistance is the side you have jacked up and in theory will be the side spin when u put it in gear. So put it in park (lock the driveshaft) and go spin that jacked up wheel. Tell me how well that works.

Page 1 of 2 12 Last Post byLast Post by

Similar Threads

  1. NP246 246 Rear Transfer Case Half w/ Case Saver NEW!
    By parts bot in forum Chevy Parts eBay
    Replies: 0
    Last: 09-09-2012, 03:50 AM
  2. 4 Wheel Drive not engaging
    By 96Z71VORTEC in forum Chevy C/K Truck Forum
    Replies: 0
    Last: 06-05-2012, 12:25 AM
  3. np246 transfer case problem? Or something else
    By smcdo9nnell1 in forum GM Powertrain
    Replies: 2
    Last: 07-21-2011, 08:56 PM
  4. engaging itself into 4 wheel drive
    By toddk in forum GM Powertrain
    Replies: 2
    Last: 11-28-2010, 12:09 AM
  5. engaging 4 wheel drive
    By BugShooter in forum Chevy Suburban Forum (GMC Yukon XL)
    Replies: 2
    Last: 02-09-2010, 01:35 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •