ABS and BRAKE Lights on 2006 GMC Sierra

THH2079 · August 9, 2014

Ok I’m stumped, I have been chasing this problem now for three weeks and can’t seem to figure this out. Before I give up and take this to the mechanic I want to post this and see if anyone has any other suggestions.

I have a 2006 GMC Sierra SLE extended cab 4x4. I am a Ham Operator and approximately three weeks ago installed some additional radios in my truck.

Prior to installing the additional radios, I had one radio installed. Radio was wired to the battery by passing through the firewall at an already existing rubber grommet where the wiring harness passes through. Radio was grounded to the frame via the metal dash framework with no problems. In addition, radio had options for an ignition switch and was tied into the fuse for the Truck Body Controller IGN 0. No problems experienced.

I wanted to add additional radios and constructed a wooden box on my trans hump under the dash. I removed the original power wire from the battery and replaced it with 6 gauge pos and neg wires running from the battery to the box where they are bolted to a quarter inch stud one each for pos and neg. One additional stud supplies an ignition source for the internal switching in the radios.

The morning after I installed the new power cables I noticed that my ABS and Brake Lights had gone on while on the way to the Parts Store for supplies. I finished installing radios and switched the ground from the battery to the engine block, thinking I had made a mistake. The ground was connected to a different location on the engine block from the battery ground. The lights went out, drove truck without problems.

1 day later lights come back on. The next day I deduce that in mounting the chassis of one of the radios in the truck, I have inadvertently put the screws through the wooden mounting base I built, all the way into the frame creating an additional grounding location. When I corrected this, the lights went out.

The next morning is a very dewy and damp morning when I leave for work. No lights. Truck was driven approximately 1.5 miles to work without warning lights. 8 hours later I come out of work start up my truck and ABS and brake lights come on. I have an Innova Scanner that is capable of reading ABS codes. The scanner will connect with the computer but cannot get the keyword to read the codes and says that it does not support codes for this model. I stopped by my dealership where my buddy is the service manager, and they first put their simple scanner (similar to mine ) on the truck. Same problem, will connect but not read ABS codes. So they get out the big scanner which will also not read and we all scratched our heads. My buddy asks me if I want them to dig into this and I said no for now, knowing that I still have regular brakes.

I go home and do some internet research and come up with a service bulletin concerning grounds. Followed instructions and cleaned ground wires for ABS EBCM unit very carefully.

Next morning I leave for work at zero dark thirty and drive through town to work about three miles taking the long way for coffee. No lights. This time I brought my code scanner along and when I stopped for coffee hooked up the scanner and got the following codes.

C0040 1. Brake pedal switch “A” subfault. 2. Right front wheel speed sensor circuit malfunction GM only

C0035 1. Right Front wheel speed sensor supply subfault 2. Left front wheel speed sensor circuit malfunction GM only

C0201 DTC definition not available.’

Reset the trouble codes and drove from gas station to work. No lights. Read codes a second time and there were no codes stored.

At some point previous to this, when the lights were out, I had captured the same codes, but now I’ve been fighting with this for three weeks my memory is a little foggy. Bottom line, when the ABS and brake lights are on, the code reader won’t get anything.’

Since then I’ve done a ton of internet research across all kinds of forums and done the following tests.

1. Tested wiring harnesses for both front wheel sensors. All have continuity from connector to harness at EBCM unit

2. Tested AC voltage on both front wheel speed sensors as researched and instructed both appear to be ok. Have not yet attempted to clean them.

3. Tested power supply to EBCM unit. Shows 12 volts with ignition and engine off.

4. Tested EBCM ground wire for continuity, checks good, cleaned grounding point.

5. Tested for continuity from EBCM harness to OBD2 Scanner connection point. There is a purple, black with white stripe, black and orange wire at the scanner test port, the purple and black/white striped wires show continuity to the blue/blue stripe wire on the EBCM harness. Have not been able to locate schematics that tell me if this is correct or not but I am deducing that the EBCM unit is capable of communicating with something based on this.

6. Tried disconnecting ignition wire to radios and disconnecting radios completely from power.

7. Tested brake light switch at brake pedal for continuity with EBCM unit, appears to be working.

8. Replaced EBCM unit with exact model of same unit, I’m told by a mechanic buddy that this does not have to be reprogrammed?

9. Pulled off sensor wire from airflow sensor to trigger a check engine light. Connected scanner and got appropriate code.

10. All other electronic, and diagnostic stuff on the vehicle appears to be working fine, but scanner will not talk to ABS unit.

Currently I have replaced the EBCM unit, and radios are grounded to the body in the engine compartment. I’ve tested the ground and it appears to be good. Radios are powered directly from the battery at the top post, the rest of the vehicle is powered from the side post.

I’m stumped, so I’m putting this out to the car guys and the ham guys to see if anyone has had a similar issue. Or any wiring schematics for this vehicle so I can keep checking.

Thanks