My ABS and Brake light comes on at 40 MPH and I cannot engage the cruise control on my 1999 RAM 1500 with about 94,000 mi on the odometer. I have replaced the differential sensor with a new one from Dodge, replaced gear lube in differential and checked that brake lights illuminate correctly. I have also stood on the brakes to check functionability. Aside from having the brakes replaced recently, no work has been done to this truck. The lights come on at 40 MPH after about 2 - 3 seconds. I'm almost at wits end with this problem. This problem is not intermittent or random, it happens the same way every time. Also the speedodmeter works normally with no indication of error.

 

I have the same issue. Mine Is completly Random. Seems to happen when the truck is warmed up though. Tonigh im going to check for any codes while im at school and change my tranny lol.

 

Dodgeman your problem is more than likely either the sensor on top of the differential , metal filings on it, or you have contaminated gear oil in the differential. Pull the sensor (one bolt with a floating nut then it pulls out but it is tight) check to see there is not black metal filings attached to it and try to look at gear oil for water contamination. Takes about 30 minutes to do.

 

80% its gonna be the rear speed sensor, now when that ABS light comes on, you dont have abs, i tested that out...

 

Yeah, 've been having this same problem, but mine happens at around 55-60, every time. I just replaced the sensor above the diff today, still came on. Replaced to diff fluid, still comes on. Rear wheels lock up if you stand on the brakes. I know I don't have rear wheel abs but that sensor is supposed to kill the rear brakes completely when the front abs kicks in, right? This has been happening since I bought the truck and I'm getting a liil tired of it!

 

Mine happened like that after i overloaded my 2wd. My dual exhaust crushed the first one, after i replace it no matter what I did the lights would come on at 25mph. It turns out i made where it goes in the dif sit diferent. An after I got moving the speed sensor dectected excessive wheel speed an triped the abs system. I had to have a special shim made for proper clearance.