Service 4WD, no power to wheels

[southofyukon]

Hi everyone,

I posted on another forum about this and they specifically recommended posting here and tagging @NickTransmissions .

I bought a 2001 Tahoe about a month ago. 5.3, 180k or so miles, 4x4, with autotrac, G80.

4x4 had been working great. It would cycle quickly, including in the auto mode. Transmission shifted great.

This may or may not be related, but a few weeks ago I was trying to shift from neutral into drive around 30-35mph. I accidentally put it into reverse, which immediatelly stalled the engine. I put it into neutral and coasted. Thankfully, it seemed to run and shift fine after, but I was certainly concerned.

We have a long driveway with a bit of an uphill slope to get out. We got quite a bit of snow, and my wife took the Tahoe to try and see me. She cleared it off and made sure should could go back and forth in the snow (I wasn't there to plow it.) The 4x4 lights were not working.

She easily made it up to almost the top of the driveway, close to the road. She said that it lost power. I think she assumed she was stuck and needed 4WD. She pressed all kinds of combinations to try to get it into 4x4, but wasn't able to move in forward or reverse. Service 4WD message came up.

Later that day I got home and found absolutely no power to the wheels, so far more serious than I thought. It grinds going into park (when the engine is running.) It rolls in park. I've tried to shift the transfer case into 2 high or neutral with the engine running and in neutral, but no dice.

I got it up on jack stands and found that the transfer case appears to be in 4x4. When I rotate one driveshaft, the other turns. This is very puzzling. I was expecting to find the transfer case in neutral, or a neutral-like state.

What do you guys make of it? I'm not really sure where to go from here. There's a couple of transmission shops over an hour away from me. I'm not sure if this is all because of me stupidly shifting into reverse, or what it's from.

Thank you!

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

I cannot personally assist you with your issue, but @NickTransmissions and other members of this Forum much more knowledgeable than me in this area will chime in.

 

[southofyukon]

Hi Joseph,

Thank you for the warm welcome!

Unfortunately, I am not well versed in Chevys. This is one of my first domestics, so I don't have a whole lot to offer. That said, I did reply to one thread on here on a topic I'm more familiar with.

Here's a few photos I took yesterday.

Had to pull the Tahoe into the shop with a come-along.

Here it is up on jack stands. I think I need to lift it a little higher.

Is this the motor that controls which gear the transfer case is in?

I believe this is the motor that connects/disconnects one of the front axles. Is that correct?

 

[NickTransmissions]

I believe this is the motor that connects/disconnects one of the front axles. Is that correct?

Yes - it's called the transfer case encoder motor assembly. Inside of the motor there is a sensor, which often goes bad. The T-case motor's job is to shift the T-case in and out of gear (2H, 4H, 4L). If it goes bad a chassis DTC will set / store and you will lose transfer case functionality (you've already seen the 'Service 4WD' message in the DIC).

You will need a scan tool that's capable of reading and clearing chassis codes - plug the scan tool in and read the code. Then post it here. There's only a few different codes that will set for the 4WD system and most will cover the motor and/or its sensor or the switch in the dash. Once you replace the sensor (or entire assembly), you will need to clear the code prior to attempting to use the 4WD system again.

You may also want to drain the transfer case fluid to see if there's any metal pouring out w/the fluid - if so, the Tcase itself will need overhaul.

 

[southofyukon]

Hi Nick,

Thank you for replying! I appreciate it!

I have a very basic OBD-II scanner somewhere. What kind of scanner would you recommend?

Unfortunately, saw your post after we tore into things and not before.

The good news is that the transmission is fine. It does appear to just be a encoder motor and/or transfer case control module failure. Now we did find when shifting the transfer case by hand that it would not always want to go into neutral. And sometimes in neutral, it would make a weird noise when the input shaft was turning. Sort of a grinding noise, but not quite.

The transfer case oil was old but no signs of catastrophic failure. Same with the ATF.

We ended up pulling and opening the transfer case, mostly because of the neutral anomaly. Didn't see anything obviously wrong with it.

My biggest concern is that it shifted itself into neutral. I'm not sure if that was done from the encoder motor or if the transfer case itself shifted into neutral under load. I'm sure you have a much better idea on that than I do.

If it failed stuck in gear, I wouldn't be so concerned, but this could've happened to my wife anywhere. Thankfully, it was just at the top of the driveway!

Are you familiar with any other cases of it failing going into neutral? Or do you have any other thoughts on it?

At this point, my plan is to replace the transfer case with a NP241C. It seems to be more reputable, widely available, and bypasses the suspect electronics. This is what I'm using for a guide, though I do wonder if someone has a more detailed writeup. Seems fairly straightforward either way. https://www.gmfullsize.com/posts/3226090/