I’m having an intermittent issue with my 1997 F350 Powerstroke. For the past two years at random times my OD light with start flashing and the E40D will either shift late hard from 1-2 or slip out of overdrive. I’ve had it scanned a few times and always get the P0500 VSS and also have got a P0603 KAPWR ( not even sure what that second one is) I’ve replaced the speed sensor three times, I currently have a motorcraft one in as well as the harness. Replaced shift lever, third brake light and harness, fluid/filter change, calibrated speedometer, and have also had the PSOM rebuilt by module mechanics. I notice the harness on the passenger side of the trans looks a little rough, would this cause a bad signal from the vss? Those things are expensive. Where are some grounds located that I should check? I have heard that the multifunction switch can cause interference? Mine doesn’t function properly as the wipers only work at certain speeds. Weird thing is after every time I try something it’s good for a while making me think I fixed it but inevitably it always comes back and then once it acts up once it starts doing it more and more often. Any help is greatly appreciated, this has been the most frustrating issue I’ve ever dealt with in a vehicle.

 

Have you checked the rear end itself? Maybe something is amiss there with the tone ring causing wear on your vss.

 

Yeah forgot to mention the tone ring has been inspected and looks good

 

Have you electrically checked the wiring to the VSS? Looking at the wiring means nothing. It has to be checked electrically for open circuit, short to ground, and short to power.

 

A bad alternator can cause VSS signal to go squirrelly. This happened to me intermittently after I got my BTS transmission, and Brian suggested I change the alternator. It solved my problems.

 

No I haven’t. What’s the best way to do that?

 

Interesting. Was your alternator actually bad or having a different brand made the difference? I have recently tested it and it seems to be working properly

 

the alternator charged fine, put out 13.5-14 volts. There were just variations in the power coming off of it that interfered with the VSS signal. It may have been an issue with the voltage regulator, which is built into the unit. I swapped it under warranty at the parts store and my problem went away.

 

Disconnect the harness at both ends. Probe the BACK of the connectors, NEVER the end that makes the connection. Check both wires for continuity from end to end, and for continuity to ground and continuity to power.

An alternator can output the proper amperage and to most testers looks fine. The problem is that if a diode in the alternator is failing there will be an AC component on top of the DC voltage. The PCM sees this as pulses from the speed sensors and therefor reads all the speeds incorrectly. The way to find this is with an oscilloscope. You can see the AC voltage. Sometimes you can see it with a standard multimeter set on AC voltage. When set on AC the reading should be near 0 volts. More than that indicates a problem.

 

So I replaced the whole cluster with one from the wreckers and the truck is driving perfectly. If I put the old in with the rebuilt PSOM the issues come back. Is there something else in the cluster that could be causing this or did Module Mechanics just do bad job of rebuilding it?

 

The PSOM is the only part that can affect the shifting.

 

I’m actually having this exact same issue. I found that the abs control module behind the glovebox has a short in it. Im assuming this is causing an issue with the signal being sent to the PCM from the PSOM. Like you, I checked all wiring (pulled the entire dash out) using a power probe ect3000 as well as a multimeter and breakout box. I also replaced my PSOM as you did and the issue was still there. It was only after I unplugged the abs control module that the short went away. I hope this helps.

 

For a P0500 I usually find the wiring between the sensor on the axle and where it to up to the frame has a problem, like a broken wire.

 

So I replaced the PSOM with one from the wreckers and the vss issue has disappeared. Apparently even though it’s rare a rebuilt psom can still have those issues even if it tests fine. Unfortunately last night I lost reverse and it won’t shift out of first. When I put it in reverse the backup lights come on but when I step on the pedal nothing. Driving home I was in first gear the whole time. No OD light, seems to be mechanical. Tried this morning same deal. Any ideas? Thanks

 

No reverse has to be mechanical. There is nothing electrical that can prevent reverse gear from engaging.