1999 Dodge 2500 4x4 w/Cummins and auto transmission.While towing a trailer with a car on it through Needles, CA I had some trouble. Coasting in overdrive down a long hill truck dropped out of cruise-control, ABS and Brake light came on, and speedo pretty much stopped working. Speedo only works when you let off the gas. As soon as I step on it the speed drops to around zero. The truck drives fine otherwise, but cruise doesn't work and the brake lights are still on. Brake lights are off when I start truck but come on and stay on as soon as the truck starts to move. I replaced the speed sensor in the transmission and no change to symptoms. The tranny shop hooked up their code reader and "couldn't communicate with the transmission". Not sure if that was because there were no codes for it, if their code reader didn't work right, or if the tranny is supposed to just communicate anyway. I checked all fuses that were labeled as related to the tranny and found them good. I guess the place to really start is find the sensor that causes the speedo to work. Figured that was the speed sensor, but I was told there is some kind of sensor in the rear diff too. I don't want to just chase this problem replacing random parts. I hope this is a common problem that other people have had and can help me out here. Thanks guys!

 

I have seen something on this before. I think the problem was wiring from the trans. It wasn't on this site but I have seen it check your wiring from the trans. if i remember correctly the wires for the speedo was smashed and when you stepped on throttle or hit a bump the speedo started working but when you coasted if wouldn't work or something like that. Maybe its reversed yours is working when coasting. the sensor in the rear end is for the ABS. the VSP is in the transfercase that works your speedo. can you have the codes ran on the vehicle by another shop or auto parts store and see if the ABS sensors are ok? But that really shouldn't have any effect of the speedo. unless it could short out computer???? I don't think it should be able too

In that wire harness for the trans does the wires for the brake lights run in the same loom? I am not sure but that's where i would start looking. It don't cost anything to trace wires.

Also if you have a programmer on truck I know my MAC Mentor won't connect to the truck with my bully dog on it. it says error unable to connect as soon as i take the bully dog off it works fine
I think you have a wiring problem or a ABS sensor is crapped out on you causing things to go crazy.

 

on a 99 the pickup recieves speed signal from the rear abs sensor(diff housing mounted) and this could be why abs light on ohm sensor and make sure not open also check wiring from sensor to frame rail and make sure they not broken or beat up along left spring area and you will not be able to communicate with any trans controler it is only the jtec controller engine controler/trans controler try that

 

defenatily a rear abs sensor failure you are encountering nuttn trans

 

Traced the wires and found a spot where the wire loom has been rubbing the rear stabilizer bar. Sure enough, two wires rubbed raw and grounding out. Easy fix.
Thanks a lot for the advice guys!