'07 GMC Sierra WT 4x4 (new body, not Classic) 4.8L - no comm with ABS module

Everything I've been seeing on the net is pointing me towards replacing the ABS module, but I want to be 100% sure before I do. I don't have access to SI, so I'm not sure if what I'm seeing here is normal or not - seems like it is NOT.

I've got power on both the 40A ABS-1 fuse, and the 25A ABS-2 fuse. Down at the module under the truck, I've got power on the red wire (ABS-2), but no power on the red/black (ABS-1) - according to my wiring diagram, I should have power here on BOTH wires. I hooked up a sealed beam headlight on high beam, and the red wire powered it bright, using each of the 2 heavy gauge ground wires in the plug. I went up and down the harness, and found nothing broken. In GM's infinite wisdom, they also run ANOTHER red/black wire through the SAME harness, all the way to the rear of the truck for trailer brake power. Whole time I was peeking through sections of the harness, I wasn't sure if I was ever on the correct wire. So, I was thinking, if I just supply the missing 12v to red/black from another fused source, I'll be golden. Tried that with a jumper, and the ABS motor ran non-stop!

The harness for the speed sensor in the right front is frayed right through to bare wires - that is getting repaired today.

So, is this no-comm the result of having no power on the red/black wire? Or, do I have 2 problems - a bad module, AND a break in the harness on red/black? Also .. am I correct in assuming that if I replace said module, it will need J-box reprogramming?

The loss of power on the RED/BLK (PIN 1) is one problem. There should be power there, all the time. If the fuse is good, but no power at the module, you have an open in the wire.

The pump running non-stop is another problem.



If you replace the Module you will not need a J2534. You WILL need an Upper-end scanner, though. I believe, the module will need to be INITIALIZED. A good bi-directional scan tool should be able to do this. You might even need a fairly large space/parking lot. I replaced the module on a Cobalt and had to do a driving maneuver to complete the process.

THANK YOU for confirming my suspicions! I really appreciate that.

I have an Autel 906TS - it's not the J-box compatible one. I'll have to poke around in the special functions, and see if I see anything.

The customer just spent a pile of money on a entire front end overhaul, plus U-joints, exhaust work, and a wheel stud on the right rear, plus my diag time on the ABS unit. Thing is, he needs this light off to pass inspection. I'll post back if/when I get the job.

Thanks again!!

Just an update - this one is fixed. The new module also fixed a no-comm code for the FSCM, as an added bonus. Apparently the comm lines run through the ABS module on their way to the FSCM. The module was all green inside, with a few capacitors looking like they were ready to pop. Those metal coils inside were all stuck on the plungers, too, when I removed it - they all broke free of the module. In hindsight I should've sold the entire ABS unit, as I spend WAY too much time extracting 2 busted off, rusted bolts.

New module did need to be programmed, but there wasn't a single shop in my area (aside from the dealer) that could do this! Guess I'd better invest in a J-box - I'll have the market cornered. I had to ship it with the light on for "configuration error".

Fine system GM has designed for milking every cent from their customers ...

Anyway, thank you again Pole71 for your help.