I've got a '00 Bravada with about 80,000 miles. Just noticed yesterday (when it got stuck in the wet grass on my front lawn) that the AWD is not engaging. I've done some poking around here and other places online and it seems that the culprit is likely a faulty encoder motor, but I did come across a few other suggestions as well. Can anyone give me some guidance on the best way to troubleshoot this problem so I can be sure to only fix what is actually broken? As I'm sure I dont need to tell anyone, times are tight and blindly tossing parts at it until it works isn't really an option.

Thank you in advance for any guidance you can offer.

Frank

 

The NV136 transfer case isn't so easy to diagnose. One thing that you could try that is fairly cheap is to change the transfer case fluid. If it is black/burnt, the clutch may not be operating properly. If changing the fluid doesn't help after driving it for awhile, then you may have to get it scanned to see if there are any stored codes in the TCCM. You will need to find someone with a full feature scan tool to be able to communicate with the TCCM. It is an serial bus communication device so generic scan tools cannot access it.

 

Update:

Starting with the least expensive fixes and working my way up, I flushed the transfer case with the correct fluid - no luck. I then replaced the encoder motor - still, no good. At this point, I took it to a local mech who ran a scan and found no codes. He put it up on the lift, and the damn AWD worked. Put it back on the ground, and nothing. The only thing I/we could come up with was worn clutches in the transfer case which were slipping when attempting to move the weight of vehicle?

Anyone have any other things to check out before I start shelling out $$$ for a transfer case replacement?

Thank you!

 

If you have access to a Tech2 or equivalent bi-directional scan tool, I think you can command the transfer case to lock up the front axle. If that still results in no real power being transferred to the front axle, the transfer case clutch is bad.

 

I just talked to the guy who scanned it, and I was wrong about there being no codes - he said it he came up with a code of C0308 - Motor A/B Circuit Low.

I read up on the code, and it seems to me that when the code is stored, the transfer case will no longer shift. However, he told me that when he put it up on the lift, the transfer case locked up the front axle on its own, he did not have to command it with the scanner.

 

The clutch inside the transfer case has ~100lb preload applied to it. So if all 4 wheels are off the ground, it will spin the front wheels regardless of the encoder motor position.

Here's the diagnostic sheet for the C0308 code:
C0308 - Motor A/B Circuit Low

 

I did not know that, but it sure explains a few things.

Thank you for the insight!

 

Verdict is in, supposedly. Mech said he had to put in a new (used) control module (I'm assuming TCCM, but not 100% sure - this info is from a voicemail he left for me). Would a bad TCCM account for the C0308 code?

 

It very well could cause a C0308 code to be set. Hopefully that works!