Service 4WD - Still engages?

[Deviation]

I had the Service 4WD light happen earlier in the year. I've replaced the TCCM with one from a 2003 Envoy and that fixed it for a while (maybe 6 months). Now it's back. The switch lights up on start as I would expect, but all lights go out. I've done the "switch exercise" and I've tried a working switch as well. Last night I pulled the #8 fuse and also tried pulling the negative. Didn't magically fix it.

The odd thing is that I can get it to engage another mode; just not while the car is running. When the car is off, if I move the switch from 2HI to 4HI it will engage 4HI when the car is started. I really only use those two modes (A4WD is useless and I have no need for 4LO where I am). I don't recall being able to do this before the TCCM replacement.

Here's the work I've done lately (in case it's relevant) that I can recall:

• New Front Wheel Hubs
• New Struts
• New Tie Rods
• New Upper & Lower Bushings
• Replaced TCCM

Can a place like Advance Auto/AutoZone read the code for the 4WD? If so, I'll wander that direction and see what it's actually telling me. I thought I read that it was something a dealer had to read.

 

[The_Roadie]

It takes a GM Tech II tool or a high end scan tool to talk to the TCCM. But it's not a dealer-only tool. Many independent shops may have invested in such a tool. Call around.

But in my experience, you just had another TCCM go bad. The Service 4WD light is illuminated by the PCM, if it loses communication with the TCCM. Might be a flaky TCCM connector or a wiring harness issue, but the TCCM is in a protected location, and its connections usually don't go flaky. Could be a bad position feedback sensor from the front axle actuator of the encoder motor, but those are relatively rare failure modes as well. You could replug both of those connectors down under the vehicle to see if they've gone flaky.

The "change to a new mode upon start-up" behavior that lets you burp it into 4HI is one sign of TCCM flakiness that I've read about here and on Trailvoy a few times over the years. Risk is that sometime it may leave you locked into an undesirable mode, like 4HI on dry pavement.

Best troubleshooting is to locate a local owner to swap TCCMs with. Or buy one on speculation, and sell it if it isn't the cure. You can probably buy another used one for what an hours worth of professional diagnostic time would cost.

 

[Deviation]

In the back of my mind, I was thinking that may be the issue. Guess it's time to search around again. They weren't too pricey. Do you happen to have the new(er) TCCM part number by chance? And yeah I'm definitely worried about getting stuck in 4HI.

Out of curiosity, do you know what makes it more complicated to read the codes like this? I've built a few OBD tools and wrote the software in the past. Might be a fun project.

 

[tbyoda]

Rock auto.com is your friend. this is for 05 gm #12590220, you are going to need a used one.


https://www.google.com/search?q=12590220 - MODULE&newwindow=1

 

[Deviation]

Rock auto.com is your friend. this is for 05 gm #12590220, you are going to need a used one.


https://www.google.com/search?q=12590220 - MODULE&newwindow=1

Thanks!

 

[Deviation]

Has anyone used the Dorman TCCM replacement? Buy Dorman - Renew Remanufactured Transfer Case Control Module 599-113 at Advance Auto Parts

I'm tempted to just go the eBay route again, but with promo code TRT30 it's only $126 (after core).

 

[Wooluf1952]

IIRC, a new TCCM will also cost to have the dealer program it.

 

[Deviation]

IIRC, a new TCCM will also cost to have the dealer program it.

This one is remanufactured. Allegedly it doesn't need programmed.

"Plug and play for easy, economical installation—no programming required"

 

[Deviation]

Just wanted to report back that, once again, the roadie is right. Swapped the TCCM with an '04 TB version and all is good. Thanks again.

 

[linneje]

Just wanted to report back that, once again, the roadie is right.

Do not stoke the fire ....

 

[The_Roadie]

Do not stoke the fire ....