Hello,
I have a 94 dodge ram with a cummins that I've spend the last few years rebuilding. I'm currently working a few bugs out but I have an issue with the speedometer. When I start to move the truck the speed quickly moves up faster than the speed I'm doing and then stops and stays at 60kph....Yup I'm in Canada. The speed stays at 60 no matter how fast I go then quickly drops back down when I stop. Everything seems to shift smoothly except I'm not sure if it's going into overdrive. I've tried replacing the ABS sensor on the differential and have bought a new tranny speed sensor. Before I go any further I was hoping someone had some ideas rather that continuing to replace parts that I don't need to. I've seen people with issues of no working speedo but not any with my issue. Oh yes, the ABS and brake warning lights come on also.
Thanks

 

Check the tone ring for damage....rear wheels

 

Checked the tone ring in the diff and it looks ok. I replaced the speed sensor and it still has the same issue. Does anyone know if the crank position sensor has anything to do with speedometer?
Thanks

 

The crank sensor is completely unrelated to the speedometer.

If I'm not mistaken, the speed reading is taken from a gear drive in the tail shaft of the transmission. The tone ring and magnetic pickup in the rear diff is for the Rear Wheel Anti Lock (RWAL) system.

 

As pointed out above, your 94 doesn't use the sensor on the differential for the speedometer. Your speedo has its own sensor located on the side of the transmission, or if it's a 4wd, the sensor is located on the back of the transfer case.

It would be helpful if you had a scan tool that you could plug in and monitor the data stream while you drive. You would be able to see if the speed indicated on the scan tool mimics the speedometer, or if it reads correctly.