I have a 91 cew cab d350 with the cumins,my speedometer would not work either. i replaced the distance sensor on the trans, no change,checked continuity from sensor to the ecm and it was good for both wires, the same to pin 6 on the instrument cluster. I pulled the speedo and checked for power , i had 12 v between the center(B+) pin and the top(ground pin). The last step was to check the speedo over very carefuly and i found a discolored resistor and part of the printed circuit missing. I was able to find a speedo out of a 1990 and it looked to be ok, installed it and I now have a speedometer. You may have the same problem as these speedos are a pain.