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4 wheel drive not engaging - recent transfer case swap (NP246)

Discussion in 'GM Powertrain' started by Family Force 6, Dec 26, 2012.

  1. Family Force 6

    Family Force 6 Rockstar 100 Posts

    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: Dec 28, 2012
  2. RayVoy

    RayVoy Active Member 2 Years 1000 Posts

    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.
  3. Family Force 6

    Family Force 6 Rockstar 100 Posts

    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: Dec 28, 2012
  4. RayVoy

    RayVoy Active Member 2 Years 1000 Posts

    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.

    Correct.

    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. Family Force 6

    Family Force 6 Rockstar 100 Posts

    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. a.graham52

    a.graham52 Rockstar 100 Posts

    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.
  7. RayVoy

    RayVoy Active Member 2 Years 1000 Posts

    The front diff is not a locking diff, as such, one wheel will always turn freely.

    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.

    This is good advice, if the truck advaces slowly with a lot of engine rpm, it is in 4LOW.

    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.

    @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: Dec 30, 2012
  8. a.graham52

    a.graham52 Rockstar 100 Posts

    [MENTION=54249]RayVoy[/MENTION] 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. RayVoy

    RayVoy Active Member 2 Years 1000 Posts

    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. a.graham52

    a.graham52 Rockstar 100 Posts

    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.

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