Bad TCCM or something else?

[Mike534x]

Hi guys. Having problems with our Envoy recently - again - (2002 SLT)

3 weeks ago we replaced the alternator (had it tested but there was no change in the amperage as engine rpm increased or when load was placed on it. Few days later it sounded like a baring went out and there was a strong smell of something burning) Prior to replacing it, the mode selector was flashing and then the lights went out completely. The best way for me to put it was that not all of the lights were flashing but the mode the nob was set at was. I always keep mine in 2WD and the amber light is usually solid for the most part. During the time it's flashing if you try to switch it into say A4WD or back to 2WD it'll flash and then the amber light will shut off completely and you won't seeing anything until you mess with the selector again. I know the 4WD/A4WD doesn't engage (I go by that noise from the front axle that usually lets me know its engaged ) while the light is out or when its flashing.

This is the second time I've noticed it do this. It usually happens after a cold start and it usually goes away after it sits for awhile. I drive the Envoy mostly to school or work, its belongs to my parents but since I'm the one driving it most of the time I've taken care of all its maintenance to help them out. We've been having issues since last year and my dad has reached the point on whether or not this thing is worth keeping and fixing.....or to just trade it in and be done with it.

If you guys are familiar with this problem or know of a fix, It would be great to hear it!

 

[Wooluf1952]

Not sure if it would help, but, have you cleaned the contacts in the switch by rotating the switch back and forth five or ten dozen times with the key in the OFF position?

Also, the 2002 is known to have problems with the original TCCM.

 

[The_Roadie]

Yep. If the "Service 4WD" light ever comes on in the dash, it's a bad design early 2002 TCCM. It could also be the switch, which can sometimes get dirty contacts from not being exercised. GM redesigned that around 2005, so newer ones are more reliable. But best of all is to listen for the TWO noises that happen underneath. The front axle actuator is a high pitched whine that happens for just over a second, and ONLY on 2WD->A4WD transitions and vice versa. The transfer case encoder motor is a lower pitched (larger motor) noise that happens for a shorter time during EVERY mode transition.

Could you try each of the transitions, and describe the motor noises and the mode switch light behavior very precisely? Thx.

 

[Mike534x]

Not sure if it would help, but, have you cleaned the contacts in the switch by rotating the switch back and forth five or ten dozen times with the key in the OFF position?

Also, the 2002 is known to have problems with the original TCCM.

We seldom use the mode selector unless we have a good amount of snow so maybe it could be dirty contacts? I just got done turning the selector between each mode with the truck off to see if it does it again over the next few days/weeks.

I just find it strange how this happens very randomly but then again the Envoy/Trailblazer seem to have a knack for that.

Yep. If the "Service 4WD" light ever comes on in the dash, it's a bad design early 2002 TCCM. It could also be the switch, which can sometimes get dirty contacts from not being exercised. GM redesigned that around 2005, so newer ones are more reliable. But best of all is to listen for the TWO noises that happen underneath. The front axle actuator is a high pitched whine that happens for just over a second, and ONLY on 2WD->A4WD transitions and vice versa. The transfer case encoder motor is a lower pitched (larger motor) noise that happens for a shorter time during EVERY mode transition.

Could you try each of the transitions, and describe the motor noises and the mode switch light behavior very precisely? Thx.

Sure. When switching from 2WD->A4WD I do hear the high pitched whine coming from the front axle actuator as well as the lower pitched noise when switching from 2WD->4Hi and I will hear it again vice versa when switching back in 2WD.

When switching the amber light seems to blink for around 4-5 seconds before turning solid after the high pitched whine. Whenever I do run into the issue like I previously stated the mode selector will flash as it normally does during each transition minus the high pitched whine or the lower pitched one when selecting 2WD->A4WD/4Hi and then after its finished flashing it the amber light will go out like you've shut the truck down or disconnected the selector. I have yet to have seen the Service 4WD light come on.

 

[Sparky]

FWIW, when my TCCM died, it just suddenly quit. It wasn't sporadic or intermittent. Turning the key off and on wouldn't even make the switch light up. My service 4WD light didn't come on either at first. I pulled the fuse and reinstalled it in an attempt to reset the TCCM, but all that did was make the switch light up at first ignition, then it went dark and the service 4WD light came on.

The fact that it shuts down entirely and not select the wrong mode leads me to think it is more likely a TCCM issue than a switch issue (usually a switch problem makes it select the wrong mode or jump around modes).

 

[bitz]

Hey Roadie... from the other site I went thru the TCCM stuff you posted. Today I replaced my 2002 TCCM with a 2004 unit. Didn't need to reprogram and all is well (kind of) I am having an issue mounting my unit back in. Do you have a pic of the view thru the side access that shows what it should look like installed in the bracket/body? Seems like the vehicle structure is blocking the installation and so I wonder if I have it backwards. It was hanging loose when I opened it, so not sure what it looked like then.

Also, I now have the 2002 unit and if you want it, (per an earlier post on another site) I will be happy to provide it to you. Shoot me an email to my gmail.




I just picked up a used TCCM from AAA auto in Rosemount, MN (I found it on Ebay from them). I paid the buy it now price of $55 and it was from a 2004. Plugged it in and works great.

Used Auto Parts - AAA Auto Salvage

above is the web site. I live about 10 miles from the place and stopped on the way buy.. super nice people and I thought the price very good.


Bitz