Hi their I have a 1990 dodge w250 3/4 ton signal cab diesel. The anti lock light and parking brake light are on. I been working on it, on and off for 2 years and have read every post about brakes. I am ready to give up and take it in to a shop but was hoping someone could help me . The truck needs an out of Province inspection so I need the anti lock brakes to work. Here is what I have replaced.

Master cylinder

Front brake pads has newer rotors

New Drums, 3" Shoes, 1 Ton wheel cylinders, hardware, Bearings, seals and parking brake cables

New RWAL Hydraulic valve and RWAL sensor in rear diff



The Combination valve is stuck and the rear brakes never worked. I couldn't get it to re-center. I replaced it with a universal adjustable propositioning valve because I couldn't get a new combination valve and couldn't find a good used one that worked. So now I can lock the rear brakes up but the Anti-lock and parking brake light are still on. I have bleed the brakes 3 or 4 times with no air bubbles and the brake petal feels good.

On top of the stock combination valve there is a wire they call pressure differential switch. I read the shop manual and it states that the switch is triggered by movement of the switch valve. It states that a loss of fluid pressure in either hydraulic circuit will cause the switch valve to shuttle forward or rearward in response to a pressure differential.
Movement of the switch valve will push the switch plunger upward. This closes the switch internal contacts completing the electrical circuit to the warning light.

So my question is with the wire unplug it doesn't complete the circuit and it shouldn't turn the anti-lock light on. But with it unpluged I grounded the connecter behind the glovebox and get a code 8 which is "Dump output missing or valve wiring shorted to ground. What does this mean?

I than grounded the wire to the frame and I no longer get any codes but the Anti-lock and parking brake light is on. I am confused as I thought that it should do the opposite?

I also have the parking park cable switch unplugged and the low vacuum switch unplugged. Sorry for the long post and I hope someone can help me. Thank you in advance!!

 

Have you checked the vacuum at the booster? That can turn on the lights. I had the same situation as you, you probably read some of my threads and the last thing I did was put a new Bendix MC in (nla) and that is what finally turned the lights off. I hated these brakes until I did a disk conversion, but that still didn't turn off the lights, the mc did that...Mark

 

You have a double fault.

1 is that the combination valve is off center, indicating an imbalance front to back. As Mark said, it could be almost anything, including the MC.

2. When that fault is cleared (in this case by simply unplugging it) the RWAL attempts to work, and the second fault occurs (code 8). It may be related to the same hydraulic imbalance, or it could be something else (like a shorted wire).

 

Bleed the valve on the frame rail near the rear tire

That's what solved it for me.

I also replaced the the controller behind the glove box, it was $85 IIRC.

On my 1990 if I unplugged the controller the lights would stay off.

 

I'll try bleeding again. What I don't understand is with that switch unplug it can't detected if there is a leak in the system, or if you have a bad mc and other things so it must be the rwal valve or wiring?

 

That is very expensive logic. I replaced my master cylinder 5 times before I finally got a good one. I would condemn the mc before I condemn the rwal valve. I replaced mine and the combination valve, but I kept the old combination valve because I was sure that it was good, I had to send in the rwal valve to be rebuilt. You said you replaced the mc, was it new or rebuilt? Was the rod adjusted right. I would start with the mc, my experienced opinion. I had the exact same issue as you, good luck...Mark

 

New master cylinder, I am going to be taking it in to town pretty soon.

 

Just ignore the ABS diagnostic codes. They mean nothing.