Rebuilt the Transmission, now I'm in deep s***

rallysport096 · June 11, 2022

So back in March, a family friend and I rebuilt my 6L80 in my 2015 Sierra. I had taken it out, rebuilt it with him, put it back in, reprogrammed the TCM and such, truck runs great.

While I was without my truck, my Dad so graciously lent me his 2015 Sierra to borrow while I was working on mine. His had the same issue though (bad lockup in the tc/bad tc). Once mine was done, I decided to rebuild his. Disconnected the battery, took the trans out, overhauled it, put it back in the truck, reconnected the battery, went to reprogram the truck, and it would not show up on the reader (ACDelco Techlink with MDI). Would not populate the VIN. MDI would not read the truck.

Checked the program on my Sierra, the MDI and software are fine.

Back to my Dad's truck, as I turned the key on when I initially plugged the MDI in, I received the following messages in the dash:

- Action Required: Steering Assist is Reduced Drive with Care

- Fuel Level Low (gauge shows empty, but I know I had gas when I parked it)

- Service Stabilitrak

- Service Trailer Brake System

- Automatic Light Control On (This one popped up as I am writing this post... so new?)

In addition:

- The PRNDM don't have a red highlight (no shift indication)

- The shifter doesn't have the safety lock engaged (where you have to press the brake to shift)

- It won't start

My initial worry was, "Man, I f****d up the techm somehow." But, can the techm cause all of this?

The battery is a bit low (12.03V), but I've had it worse in my truck and nothing like this ever happened. Stabilitrak, for sure, but nothing else.

Finally thought to connect my VerusD10 scanner, after waiting the 10 mins to start WinXP, it got through to give me the U0100 and U0101 codes....

Any ideas? Thanks!

Edited June 11, 2022 by rallysport096

mjm-1957 · June 11, 2022

Make sure the connector at the transmission is fully seated and there are no bent or backed out terminals, the scanner should see the TCM at start up.If that checks out okay make sure the passthru in the case is fully seated and that the retainer is fulled locked.It should snap and click into place when fully seated.

rallysport096 · June 11, 2022

So that was something I checked, and I can see there's no pin for #13, but it's not bent nor does it look like it broke off. I can't tell if there was even a pin to begin with. Theres a wire in the pin 13 spot on the harness. Maybe it was a clean break?

mjm-1957 · June 11, 2022

Pin 13 is high speed communication circuit 2501. I cant figure out how to upload the schematic. If you have access to service information from GM the document ID number is 3627766. That should get you talking with the TCM again.

rallysport096 · June 12, 2022

I'll take a look at it. Thank you so much for your help!

Spurshot · June 12, 2022

I'm no techy with these devices but, I'd start with a fully charged battery. 12.03v sounds really low.

16LT4 · June 13, 2022

15 hours ago, Spurshot said:

I'm no techy with these devices but, I'd start with a fully charged battery. 12.03v sounds really low.

Throw in some poor grounds and a voltage drop, and that already low voltage gets even lower. On other brands, getting in to the DME with too low a voltage (and without a charger hooked up) is a good way to brick it, though I have ZERO experience with the computers on these trucks. Try again with a fully charged battery.

rallysport096 · June 16, 2022

So it was pin 14, not 13. Regardless, I replaced the techm and the truck talked to the computer. Reprogrammed, went through the shifting procedures, truck drives great.

So what does pin 14 do?

mjm-1957 · June 16, 2022

Pin 14 is circuit 2500 dark blue wire is high speed communication circuit, you were correct, pin 13 is blank on your model,sorry about that. If I could post the schematic you would have seen that. These old eyes aren't as sharp as they used to be. Glad ypu got it going.