1999 S-10 Blazer with NP8 Auto 4WD ( and Hi, I'm new to this forum, but not to auto mechanics )

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!

 

Well, I would like to post the diagnostic sheet for the code you are experiencing, but the forum problems are making that problematic.

Anyway, this problem is most often caused by a faulty position sensor in the encoder motor. The magnet in the sensor fails to fully activate the hall effect sensor for the appropriate shifter position as requested by the TCCM.

There should be four positions on the shift lever - 2HI, 4HI, N, 4LO.

Here are the diagnostic charts anyway, hopefully these go through:

 

Swartlkk,
I have the GM manuals and such on DVD, but thanks anyway. I appreciate the response.

I know I have some electrical issues, and I am tracing them out. I can operate the motor/encoder off the case, for the most part. I am in the middle of setting up the ability to monitor some of the feedback from the encoder (LEDS), thinking that maybe I have an intermittent connection ( or bad sensor as you suggest ). If not, I have to narrow it down between the TCCM and the motor/encoder. I am also making some extension cables for the motor/encoder so I don't have to crawl under the car to mess with it. You have to have the encoder connected to the TCCM to get the right bias for the hall effects to work.

I can monitor the states of the encoder as I switch through different positions, but even the GM docs I have don't say which encoder channel A, B, C or P are on or off and what modes they correspond to. I've noticed that some of the details are often lacking in these docs, even though they are the best reference I've found. It would be nice if there was a table that said A = 2HI or whatever.

On the mechanics of the shifting, I found three detents. I am referring to the shaft on the tcase that is exposed when you pull the motor/encoder off. One extreme is 4LO, the center is N, and the other side is HI. I can go farther, but there is not a detent, so I imagine this is how the case and controls modulate the 4 Auto mode. Also, I thought 2HI and 4HI are really just whether the front diff is locked or not. Doesn't this case have both front and rear shafts always locked and turning ( except when in neutral ), right?

Do you know if this encoder is available as a separate part? I know on some other actuators, you can replace the encoder and there is a GM number for it. It would certainly be cheaper than replacing the whole assy, the motor and gear head is fine, as is the motor brake solenoid.

Thanks again.

 

If this helps at all, when it comes to the transfer case, 4AUTO is the same location as 4HI. The AUTO comes in when the TCCM opens the vacuum solenoid on the firewall to engage the front axle. In 2HI, there will be no movement of the front driveshaft. In 4HI or 4AUTO, the front driveshaft will move corresponding to the gear the transmission is in.

 

OK, are you saying that in 2HI, that the front driveshaft will not spin with the rear shaft? I thought I read somewhere that the front and rear shafts are always driven, regardless of the mode, and only the diff lock is enabled or disabled between the modes. I realize the front axle is not driven in 2WD, per se.

I have the front shaft out ( to allow me to install/remove the tcase motor/encoder ) and the front yokeof the tcase is never free to spin in any mode ( except neutral ) telling me that the above is true.

I'll recheck my sources, to make sure. It's been a while since I was into it, I should get back to it soon.

Any thoughts on whether the encoder itself is available by itself? I don't think so, but it doesn't hurt to ask.

I noticed you're not too far away, Waterloo eh? I'm near Rochester.

Thanks.

 

In 2HI, the front shaft should remain stationary. I am not sure if that means that it should be free to rotate as I have never tried that... That could mean that there is a problem internal to the transfer case...

If your encoder motor is at fault, the whole unit must be replaced.

And yeah, you are only about an hour to an hour and a half (depending on which side of Rochester you are on) away.

 

On the encoder, yeah, that's what I figured. It's weird that on some models you can replace just the encoder. My motor & lock solenoid work fine. I'm just not 100% sure of the encoder, so i am trying to check it before pulling the trigger on a new one. That's also why I was looking for a list of which encoder channels belong to which mode, etc.

Funny thing is that my local GM yard gets more for a used one than the auto store for an aftermarket Dorman or whatever brand unit.

I'll get around to continuing this later this week, if nothing else pops up.

I'll post findings later.

Thanks again

 

Please include the information here that you stated in your PM to me so that everyone can benefit. I have always known that the NV236 transfer case had a clutch pack for transition to and from 4wd, but I have always heard conflicting information as to whether it was a preloaded clutch pack or not. Many others have stated that it is not a preloaded clutch pack, but the information in your PM states otherwise. My comment above about the front driveshaft being free to rotate in 2HI is based on a non-preloaded clutch pack.

 

This might be a long post, but you might find something useful in here.

I've been chasing a similar 4wd fault as others, but I may have dove a little deeper in some areas in an attempt to avoid just swapping expensive parts.

I also have not found solid answers to my questions, but I have posted in other threads to try to help and continue learning.

Regarding the front drive shaft, as I posted in this thread, the front drive shaft is not free to rotate with an auto tcase. If it is free, then you may have a problem.

My questions are (still) how many physical detents are there in a 236 case sector shaft, I can find 3 which correspond to 4lo, neutral, and 4hi. I can't find any written documentation on the 236 case which shows the exact encoder channels that are on and off and what mode they correspond to. The GM troubleshooting docs only say to verify that the channels change state when shifting. (Great)

swartlkk posted a reference to rockland standard gear's website which has a bunch of great info, but none on the 236 case. The closest match is the 233, but that is not an auto case. This link refers to a set of encoder states (see fig 2) that seems close, but there is clearly a difference between 2hi and 4hi. This does not match my observations of the encoder for the 236. I removed the encoder from mine and mapped it out in 2.5 degree increments ( by properly supplying voltage and biasing the hall effect encoder channels and reading the output voltages for each channel ). I have the detailed link to my table here, if you are really interested.

I had multiple codes on my TCCM, so trying to debug this was tricky. The manuals and documentation clearly say not to operate the motor/encoder unit out the vehicle, or on the bench. No one really ever said why and just warned you that you'd be buying a new one if you did. It turns out that the encoder can only rotate through about 120 degrees of rotation. Any more and it will break. Without the computer connected, if you apply power to release the locking solenoid, then apply power to the motor, you have no control over the travel and you can destroy a very expensive part. Mystery solved.

I discovered this by completely disassembling my motor encoder unit, since I decided to get another one anyway ($130 used).

While testing the encoder states, I discovered that my TCCM was never unlocking the solenoid that allows the motor to move. So, I disassembled the TCCM module and traced the lock solenoid circuit out. I thought it might be as simple as an output transistor or a relay being bad, but no, it was much deeper into the circuit than I felt like fixing. So, I just ordered a new module ( ouch ). It is being shipped to me right now as I write this. For the record, the motor is controlled by two relays and the lock solenoid is controlled by a transistor. All of these circuits are monitored and can detect shorts and opens. If the self tests fail, the TCCM will set codes that you can read if you have a good scanner. You can debug them if you can read these codes and follow the GM diagnostic info in their shop manuals.

And -no- it is not my switch assy.

So, I'd still love to find out what the encoder states are for any given shift mode of the 236 ( to check my work ), how many detents there are when shifting this case manually with a wrench ( to make sure the tcase really is ok ), and optionally - why is there no good documentation on these things? I can't believe people enjoy swapping expensive parts and guessing what's wrong.

Other than that, I'll let you know in a few days how this turns out. I should have my new parts in by then.

Cheers!

 

I just found more information for you:

As far as the individual channel information for the various modes, all I have been able to find in my references as well as online has been centering around the NV233/NV243 transfer case, neither of which have an Auto mode. You could probably tell by disassembling an encoder. If/when I get some more time, I will take a look at your spreadsheet.

What codes were present in your TCCM?