I have a 93 Dodge Ram diesel, When I bought it the speedometer didn't work and hence no overdrive. It also needed a new PCM. I threw one in from a gas job and the speedometer worked but was erratic. This verified that the wiring was all good before I purchased a new rebuilt PCM from Chrysler. I replaced the PCM and still no speedometer or overdrive. Thinking the PCM was defective I had the dealer get me another one. That is when I noticed that on a cold start if the check engine light came the speedometer worked and I had overdrive. As soon as the check engine light went out the speedometer stopped working. I took it to the dealer and 6 hours later the mechanic figured it was the PCM. He called Chrysler and asked them to bench test a PCM. They refused saying we needed to change the crankshaft speed sensor. $200.00 later and still no speedometer. On the test drive the mechanic put the tranny in neutral, shut the truck off and then turned the key back on. immediately the speedometer works. Does anyone have any ideas? I noticed a thread in here where some are installing separate switches to make the overdrive work, will this work without the speedometer working? PLEASE HELP! Thanks

 

Did anyone use scanner to see if road speed sensor was sending signal to PCM? Was PCM flashed to correct part number for your application? Were any codes present in PCM after road test? All fuses and relays check good, wiring to sensor and trans good no broken wires or corrosion in connectors?

 

Does your truck have ABS? IIRC, the speed sensor goes to the ABS module first, then to the PCM from there.

 

Have you replaced the VSS? Is the VSS tight? Is the drive on the VSS rounded off?

Are we talking 2wd or 4x4?

On a 1st gen the ABS has its own speed sensor, don't get sidetracked with the ABS.

 

This is a two wheel drive unit. Speed sensor was changed initially by tranny shop. PCM on a 93 does not need flashing. Ironically Speedometer and overdrive work when check engine light is on or when coasting with truck shut off and key on but engine not running.
Thanks

 

Check the harness down at the VSS. With the key on (engine running or not), the black/light blue wire should be a good ground and the tan/yellow wire should have 8 volts. Also check the crank sensor harness for 8 volt supply.

 

Checked the voltage at the connctor and my meter is saying 4.8 volts.