NP8 / NVG 236 Transfer Case Question - '99 Blazer

[doodad69]

1999 S-10 Blazer with NP8 Auto 4WD

I had a problem a few months ago where my service 4wd light came on after getting stuck in 4hi. I pulled the codes at the time and started the debug, but gave up and pulled the TCCM, the vacuum switch to disable the front dif lock, and the shift motor. I can't remember the exact code, but I think it was C0387 (Unable to perform shift) and maybe one other. Other distractions didn't allow me the time to follow up, but now I have it back in the garage and I have a few questions.

Everything electrically checks out, and the shift motor appears to function correctly. I think it is the transfer case shift selector shaft binding. I am aware of the Auto Trac fluid requirement for these units, and ultimately plan to switch to it (currently had red fluid) once I rule out junk transfer case.

My question is (finally) how many detents should there be when operating the sector shaft on the case manually (with a wrench)? I can find three easily, without forcing anything, with the center detent being neutral. I cannot find anywhere in the shop manual (GM) or any TSB that says anything other than "inspect sector shaft for binding".

I was able to unstick the shift sector shaft by loosening the shift detent plunger (after draining the fluid). I am planning on replacing that with P/N 12474072 referenced in bulletin 99-04-21-002 as being an updated part to reduce shift effort.

Basically, I was ruling out all the simple stuff prior to pulling the tcase. I would like some info from anyone who has been into these things at this level, or is otherwise experienced with these units. Without additional guidance, my goal is to replace the detent assy., refill the case with the proper fluid, reinstall the shift motor and try it out. Worst case I'm out the time and some expensive fluid.

It would make sense to me that if the mechanics of the unit were jammed, that I could throw a code like that. No fuses went, and nothing else appears to have been broken ( probably lucky ).

Additionally, if anyone has a table or list of the encoder channels that are on/off in each mode, that would be a bonus. Encoder has four channels: A, B, C, P which can encode many states. I am going to try to figure that out on the bench tomorrow, but someone could save me a bunch of time with that knowledge.

Thanks in advance!

[doodad69]

Whoops, my bad - I typed the wrong tcase model the first time I posted. I just corrected it to the correct 236 model, sorry for confusion.

I ordered the new poppet, it'll be here Thursday and I'll just try it out.

So, nobody has ever torn into one of these units for a rebuild, or even just out of curiousity?

If anyone is interested, I'll post my findings.