4WD doesn't work

fechnerj

Original poster
Member
Apr 25, 2012
6
I have a 2003 GMC Envoy 4X4 that is having issues with the 4WD engaging (it does'nt). I have installed a new TCCM unit and a new transfer case encoder motor. I still do not have 4WD engagement. When I had it on jackstands (driver side both wheels off ground) changing out the the TC motor I noted that the rear wheel rotated in gear in 2WD and both front and rear wheel rotated in 4WD. But when all four wheels on ground the front axle is not getting drive power in 4WD. What I have not done and am thinking about is having the electronics flashed although the TCCM was a used unit. Also the transfer case lube is coming up on 50000 miles. Any thoughts?
 

mrrsm

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Oct 22, 2015
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If you have your own GM Tech2 or access to one... your suspicions are right on the money regarding the need to re-flash as per this link...when you see the script stop yellow line at the top just press "Stop It" and the rest of the page will completely open up... it is NOT necessary to interact with the on-line mechanic to obtain alll the information you require there:

https://www.justanswer.com/gm/5ys1b-transfer-case-my-04-will-flashed-reprogrammed.html
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Normally a used TCCM does not need to be reflashed. I seems to be working correctly and you're not getting the service 4x4 light (I assume). If your fronts aren't getting power but the drive shaft is spinning, I would be that either your disconnect is bad or the intermediate shaft stripped.
 

fechnerj

Original poster
Member
Apr 25, 2012
6
Normally a used TCCM does not need to be reflashed. I seems to be working correctly and you're not getting the service 4x4 light (I assume). If your fronts aren't getting power but the drive shaft is spinning, I would be that either your disconnect is bad or the intermediate shaft stripped.
I am not getting a 4X4 service light. Is there a clutch like device in the transfer case? I ask because as stated I got both front and rear wheel rotation with driver side on jack stands both wheels off ground. When wheels are down though no traction on front axle.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
So you get driver front wheel turning with just two wheels off the ground (which is dangerous if you have a G80 locker in the back) but with all four off the ground, fronts don't turn? With two off the ground, can you stop that wheel by hand? What about when you jack passenger sides? (do this only if you have a rear open diff).

I still think it's your disconnect or intermediate shaft.
 

fechnerj

Original poster
Member
Apr 25, 2012
6
So you get driver front wheel turning with just two wheels off the ground (which is dangerous if you have a G80 locker in the back) but with all four off the ground, fronts don't turn? With two off the ground, can you stop that wheel by hand? What about when you jack passenger sides? (do this only if you have a rear open diff).

I still think it's your disconnect or intermediate shaft.
To clarify I don't have locking rear diff. What specifically is disconnect and intermediate shaft and where are they located? With all four wheels on the ground I don't have front drive. With driver side two wheels off ground I have drive at both wheels. I did not try to stop the front wheel turning.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
The disconnect is on the passenger side of the oil pan and disconnects the right shaft. The intermediate shaft is what connect the disconnect from the diff to the disconnect.

This article on ORTB shows how to rebuild the disconnect but also shows where it is located.
Front Axle 4WD Disconnect Rebuild

This one shows transfer case operation:
Transfer Case Operation: NVG 226 (NP8)

And this one is especially good:
Diagnosing the 4WD System

We also have one but it's not as detailed:
4WD not working? Step inside.
 

fechnerj

Original poster
Member
Apr 25, 2012
6
The disconnect is on the passenger side of the oil pan and disconnects the right shaft. The intermediate shaft is what connect the disconnect from the diff to the disconnect.

This article on ORTB shows how to rebuild the disconnect but also shows where it is located.
Front Axle 4WD Disconnect Rebuild

This one shows transfer case operation:
Transfer Case Operation: NVG 226 (NP8)

And this one is especially good:
Diagnosing the 4WD System

We also have one but it's not as detailed:
4WD not working? Step inside.
Thanks for the great information. Will digest this and let you know what I find.
 

fechnerj

Original poster
Member
Apr 25, 2012
6
So I ran thru the diagnostic chart and it appears the TC is the problem. With pass front tire off ground I get tire rotation and prop shaft rotation. With both front off ground I get CW and CCW tire rotation and prop shaft rotation. With pass front tire off ground and driver front blocked from rotating I turned the prop shaft with a strap wrench. Wife could not stop pass front from rotating which tells me all is well from the output of the TC thru the front diff to both front tires. Looks like I'm shopping for a used/rebuilt TC but before I make that leap---- way back when I first started this I suspected the TCCM unit was bad so I replaced it with a NOS unit. After this all the lights on the switch responded correctly and the motors make the expected noises when engaging AWD 4Hi 4Lo. It is a bit more challenging to get into neutral but it does go just not as readily as it did with the original TCCM. Reading the operational info you got me thinking that there is a clutch involved with 4WD and maybe the new TCCM isn't doing the job of communication with the clutch pack. What do you think?
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
All the TCCM controls is the TC and disconnect. The internal clutches are engaged by the shaft rotated by the encoder motor. Have you had a look at the fluid yet? It is often forgotten, overlooked or neglected as it has a relatively short 50k mile change interval. It has been known to destroy TC's.

I would also put back the original TCCM.
 

fechnerj

Original poster
Member
Apr 25, 2012
6
Just to update the situation. I got a rebuilt TC and reinstalled the original TCCM. This resulted in only 2WD and 4Lo. Did not have any response from Auto 4Hi or Neutral.

Took the vehicle to GM service shop where they pulled several codes cleared the codes and reloaded the software. Vehicle now works in all 4X4 modes. The dealer didn't treat me too badly only charging $206 for a couple hours work which included the above and pulling the encoder motor to check the internal contacts. All ok there.

By the way pulling the original and reinstalling the rebuilt/used TC was a PIA. That job took me about 3 hrs to get the old one out and 12-16 hrs to get the replacement one installed. If you are going to do work that requires carefully positioning and lifting a TC or transmission invest in a transmission jack.

By the way when draining the oil from the original TC there were fine metal particles and one piece of metal about 1/2 inch long X 1/8 X 1/8 square in the drain pan. Definitely a problem going on there but I did not open it up because it was a return core.

Thanks for all the help and suggestions especially to Mooseman.
 
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