I have a 1997 Ram 2500, 5 spd 4:10 gears. I had to swap the rear axle, and I went from a dana 60 to a dana 70. When swapping the axle the m/c went empty, but for only the rear. I now have the ABS, and brake light on. It goes out when the brake is pushed down. I have plenty of pedal, not spongy, with good feel, no air in the line. I already swapped the speed sensor. I disconnected the battery, to erase the codes, that didn't work.
I read that there is a pressure switch, in the system, that when the rear lines went empty the switch quits working. To get it to operate properly, you have someone press on the brakes, while you open the bleeder screw on the front. This then lets the pressure equalize, and everything is good. Has anyone ever heard of this?

 

I have heard of that. It's called pressure differential switch.

 

When I unplug the pressure differential switch at the m/c the lights go out.

 

Sounds like it's working as intended then. The reason the ABS light is on is because the module is detecting a fault in the base brake system.

The switch looks for pressure differences between the two branches of your braking system. When it detects an issue, it activates. It's actually a very simple valve that sits between the two branches. The pressure difference pushes the valve one way or another, and if the difference is great enough, the valve makes contact and completes the circuit. It's a pressure switch basically.





Bleeding the brakes will help to re-center the switch.

 

Usually when people get there truck fixed we never hear what it was that fixed it, well here is mine. Thanks issakar, for the picture, it explained everything to me. I pushed the pressure relief at the end of the pressure differential switch, moved the plunger back and everything works good.


Thanks

 

Good deal man, glad you got it fixed up