I have no clue what's going on here?!?


unknowingly Knowing
May 31, 2020
British Columbia

2009 LTZ 2500 HD

I'm driving the truck, just in a parking lot to be safe. Watch the speedometer.

No one can gimme a straight answer! On what's going on here or where to start looking....

I have searched forums and YouTube at I have come by 2 instances where the description is the same, but solutions were different.

1) TCM needed to be replaced.....I did that and no change.
2) ELM 327 blutooth interface box was defective. <----This makes no sense to me I thought that was a reading tool not an actual part in my truck.
3) Just brought this to my mechanic and his best guess is....the Instrument Cluster needs to be replaced probably due to the heat that is generated, but he is not 100% sure and says there is no way for him to be sure.
4) Wires have been checked and none frayed or rubbing up on something.

So this only happens when it is hot out. Never happens in the winter.

Codes that come up:
U0101 - Lost Communication with TCM

On the dash:
Service Stabilitrak
Service Traction Control
Service Trailer Brake System
Park Assist off

If the radio is on it will get louder as the speedometer goes higher. Also two weeks ago it would not shift into gear until I actually cleared the codes.

Ryan Diesel Shop where I bought the TCM from suggested cleaning all the grounds. I just have to find them all. <---diagram anyone?

Any Ideas? I Live on Vancouver Island so there is rust in and about the bottom of the truck.

Thanks for taking the time to read this.

Ron Nielson

Oct 11, 2009
Berryton, KS
The ground most likely for your problem would be the EBCM ground which is on the vertical frame member, outside of the main frame rail under the driver's feet. Main battery grounds on side of the block, both sides, near the front. Look for where the battery grounding cable goes to the block. There are TONS of grounds. You really don't want to do all of them. TCM's do go out, but many are replaced because of faulty wiring between the transmission and the TCM. Wiring diagrams at the GMUPFITTER.COM website under Body Builder Manuals.

I would start by carefully removing the TCM connector, make sure all the pins, both sides of the connector, are even and not bent. Spray lightly with DEOXIT D% cleaner and re-attach the bale connector and make sure it's firmly and firmly closed. Wiggle the harness going to the TRANS and see if you can get the truck to act up. If so, the problem is somewhere in the harness. The problem may be hard to pin down exactly, but the good news is that it will cost little to nothing to repair.

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