Various combinations of:

C0310 Motor A/B Open
C0323 Lock Low
C0324 Lock High
C0387 Unable to shift
C0327 Encoder malfunction

I fixed most of that by cleaning up dirty connections, but the 324 persisted. Chasing that one led me to believe the TCCM really was toast. ( that and sometimes it wouldn't talk to me at all until I reset it )

Back to my original beef about lack of good info - the two most ambiguous statements in the GM troubleshooting process ( for the DTC C0387 Unable to shift diagnosis ) are :

Step 2:
1-Connect a Scan Tool and select the transfer case device control.
2-Turn the ignition to RUN.
3-Use the scan tool to command through all the shift modes.
4-Monitor the states of the Encoder Circuit P, A, B, and C.
Do all the Encoder Circuits change between ON and OFF?

(This is where the state table would be handy, and think about it - if you had intermittent encoder channels, they will still change - they'd just be wrong - how would you know? )

and

Steps 3 & 8:
1-Turn the ignition OFF.
2-Remove the encoder motor. Refer to Motor/Encoder Replacement .
3-Place a wrench on the end of the transfer case sector shaft and inspect for binding.
Does the transfer case sector shaft bind?

( a description of what to expect, number of detents, how hard to "force" it, etc )

Hey, you know, it just occurred to my how we might be able to help this problem for the future - if someone had a GOOD truck with NP8/NV236/Auto4WD that they could hook up to a real Tech 2 ( mine is not, I can only read the data, not what it really means ) - shift the tcase into each of the modes, 2HI, 4HI, 4AUTO, N, 4LO and read the encoder states (A, B, C, and P) for each of these modes. That way we could document this once and for all to share. It might save somebody a lot of money to have good info ( almost too late for me, though ).

 

Everything is working fine now. Got everything back together and took it out in the back lot on the wet grass to test it ( lawn job!!! )

Found a neat way to test auto 4wd. I was on a slightly downhill slope, nose down, wet grass. In Auto, I put it in reverse and had my head out the window to watch. Slight gas and the rear would slip for just a fraction of a second then the fronts would get more torque and no more slip. This was distinctly different than in regular 4HI as it never slipped at all. Knowing the theory and seeing it happen are pretty cool.

All modes work, no error codes.

It was the TCCM, for sure. I got a hold of another encoder/motor and mapped out the encoder on that too. The P channel only has the three segments that correspond to the three physical detents. That is different than the NV233 documents from RS Gear, so don't go by them for this unit. I had my doubts that my original encoder was bad after discussing it here, but it is not.

The new TCCM part number was 12577412 which is an updated version of my old unit which was 15154323. It also behaves differently on power up, it doesn't click the relays as much as the old one did.

I'm on to another problem ( leaky oil cooler line ), but when I get some more time I'll either check the encoder channels against all shift modes and publish the results for those who are interested. Unless someone beats me to it. It's still a mystery to me what the exact differences are in the positioning of the motor/encoder between 2HI/4HI/4Auto ( of course the auto4wd torque management will move it slightly, but generally it would seem to be in the same region ).

thanks all.

pete