ABS Warning Lights

[zth2000]

I have a 1995 Chevy Tahoe 4x4 with an EBC310 and am having some ABS issues. The sensors, fuses, and motor all check good. The symptoms are that with the engine running, when the brakes are applied first the ABS then The Brake light illuminates. If you hold the pedal down the lights go off. As soon as you move the pedal at all the lights come back on. There are no error codes and the system acknowledges the brake pedal is pushed, i.e., code 12 without the brake code 13 with the brake. I was suspecting the proportional valve was the culprit and decided to disconnect it's switch. With the connector disconnected the symptoms were exactly as they were when it was connected. I tried shorting the terminals in the harness which resulted in a steady Brake light, and no ABS. This would indicate that the switch on the proportioning valve is working correctly.

 

I can envision a scenario where the pintle is stuck or the switch defective so that it was in a continuously open mode. The controller may not show this as a fault until it receives a signal from the brake pedal switch. The problem with this thinking is that the lights go out with the brake depressed if there is no pedal movement! This would imply that the brake switch is either detecting motion or absolute position rather than a limit condition, or there is a pressure sensor somewhere in the system (which appears not to be the case). Neither of these possibilities feels right.

 

Any help that can be given would be appreciated.

[GM-Tech]

That's a strange one, particularly the part about hold the brake pedal. I can tell you that the switch in the combination/proportioning valve is simply a ground for the red brake light on the dash. It's not any sort of pressure sensing input to the ABS.

 

If I had to guess, I'd say your problem is in the BPMV (brake pressure modulator valve). LOTS of problems with those older ABS systems in that area. I'm surprised there's no DTCs though. I'm also not aware of any code 13 in the ABS system. Where/how did you get those codes? I really don't recall if it gives a 12 if there are no codes like the old OBD 1 used to do.

[zth2000]

That's a strange one, particularly the part about hold the brake pedal.  I can tell you that the switch in the combination/proportioning valve is simply a ground for the red brake light on the dash.  It's not any sort of pressure sensing input to the ABS.

 

If I had to guess, I'd say your problem is in the BPMV (brake pressure modulator valve).  LOTS of problems with those older ABS systems in that area.  I'm surprised there's no DTCs though.  I'm also not aware of any code 13 in the ABS system.  Where/how did you get those codes?  I really don't recall if it gives a 12 if there are no codes like the old OBD 1 used to do.

 

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Thanks for the response. The codes are less common but are as follows:

12 = No Problems

13 = No Problems - Brake pedal engaged

14 = No Problems - ABS engaged

 

I am finding that the documentation is poor for the ABS systems. I am trying to locate a wiring diagram for the system or at least pin connections for the 10 pin connector. As for the BPMV - you have me on that one. What is it and where is it? Is it all hydraulic or does it have a sensor on it? I feel the problem is pressure related but can’t figure out where the ABS system gets pressure information.

[zth2000]

After finishing my reply I looked up the BPMV and realized it is what I have always called the ABS valve body. I am reluctant to change this given the painful bleeding procedure. I also don’t understand how an issue with the hydraulics is sensed by the control module thereby turning on the lights. The only point of feedback I can see is the proportional valve brake light switch. Are there any other pressure switches? I have inserted a new switch into the end of the connector therefore eliminating any pressure effects to that circuit and still get the same results.