ABS Rear Circuit Open

[WICruiser-97]

I have a '97 C1500 2WD with the three channel ABS (single rear signal). The front ABS appears to be working as intended based on panic stop testing on a grass surface.

My scan tool indicates that the rear circuit is open but the signal for the rear is taken from the speed sensor in the tail section of the transmission and speedometer reads correctly. The wiring from the sensor (2 wires) appear intact and in good condition as far as I can tell before they disappear into the harness above the transmission.

Is there a connection from somewhere in the harness that leads to the ABS unit seperate from the speedometer signal?

I am trying to get the ABS system to bleed using the panic stop technique (I have a low but firm pedal) but I suspect tha tthe rear portion of the control is not actuating due to the open circuit fault.

 

[Schurkey]

I have a '97 C1500 2WD with the three channel ABS (single rear signal).

Is there a connection from somewhere in the harness that leads to the ABS unit seperate from the speedometer signal?

Download the service manual set for your vehicle. The vehicle wiring schematics will be in there...somewhere.

I dealt with front-wheel ABS issues (faulty LF speed sensor/frayed wiring) on my '97, but the rear worked fine--I didn't look at any of the rear wheel diagnosis.

Wild Guess: the vehicle speed sensor's signal goes to a DRAC module, which then processes/modifies and distributes the vehicle speed sensor signal to the ECM, speedo, odo, cruise, and ABS.

MAYBE there's a problem with the wiring between the DRAC and the ABS unit.

I am trying to get the ABS system to bleed using the panic stop technique (I have a low but firm pedal) but I suspect tha tthe rear portion of the control is not actuating due to the open circuit fault.

Among the downsides to that method is that after activating the ABS, you need to drive all the way back to your shop/garage to bleed the air that was in the ABS but is now "downstream" out of the brake tubes and rear wheel cylinders. Hopefully, the air doesn't go back into the ABS module on the drive home.

Scan tool auto-bleed is strongly recommended.

 

[WICruiser-97]

Thanks, I probably should download the service manual, just have not yet.

I am on the list for the community scan tool but my understanding is that it probalby will not work for bleeding the '97 ABS system.

 

[Schurkey]

It will not. You need a Snappy scan tool with newer software than was ever released for the MT2500, or a different brand of scan tool.

 

[WICruiser-97]

Planning to review the wiring from the brake control module (BCM) to the vehicle control module (? VCM). I believe the speed signal from the transmission goes to the VCM and from there to the speedometer, BCM, etc. so hoping that is the source of the open circuit.

If I can get the rear ABS to communicate I will try variosu bleeding methods again as a scan tool to do it per the book is not in my plans.