Hi, new member here. I've been messing around with diesels for many years now but so far have only dealt with small VWs and big Ford/Navistars. Needed a hauler so finally broke down and bought a 6.5L van since the GM motor's fuel economy is much better than the Ford's.

My '94 Vandura runs and works great except for one issue. Increasingly often, something goes wrong with the electrical system involved with reading rear wheel speeds, I suspect. This causes a whole bunch of nasty problems in a lot of different places: speedometer, braking, and transmission. When the problem happens, the ABS light comes on and I notice the speedometer is staying on 0 even when moving. At the same time the truck bangs into 1st gear and refuses to shift out of it (makes sense, it thinks the car is going 0 mph...!). Last of all, it really messes with the braking system. It becomes impossible to modulate, the pedal is like an on-off switch. If you press it, the rear wheels will suddenly lock up and skid.

Now, every once in a while, it will suddenly work fine: I'll have a working speedo, nice smooth brakes, and a properly shifting tranny. But then it will slip back into its bad mode again and I'll be stuck doing 20 mph everywhere I go and hoping I don't have to brake.

I am suspecting an electrical connection issue. I got under the van today to try to find where the speed sensor(s) might be, but couldn't track it/them down. Didn't seem to be anything on the rear axle or hubs. Is it on the trans tailshaft? Or am I looking in the wrong place entirely? I am guessing either a frayed/broken wire, a bad plug connection, or some kind of grounding issue or maybe even just a bad sensor.

Anyone have any experience with this kind of issue, or any guesses where that sensor might be?

Thanks!