Speedometer Issue...

[slimjim]

1995 F350. Before I state the issue, I want you to all know that I have used the search feature, and can't resolve the problem.

So, my speedometer starts jerking really bad when I get above 55 mph. Here is what I have done so far...

1. I have changed the VSS in the rear differential 3 times. Thought maybe I was just getting a bad sensor, but I think changing it three times eliminates that as a possibility.

2. I replaced the PSOM with one from a wrecking yard. It behaved exactly the same as my old speedometer, so I think the one from the wrecking yard must have been good since it behaved the same.

3. Last night I changed the differential fluid and checked the exciter ring. Everything looked fine, not metal shavings and no teeth missing from the ring.


So I'm thinking it must be electrical. I have visually inspected the wires, but don't see anything. Can anyone tell how to test the circuit, or where some areas I might check would be? Could the ABS module cause this problem? If so, how could I test it? Thanks for any input.

[slimjim]

Oh, forgot to mention, No ABS light or OD light flashing.

[kanuck15]

is it still shifting ok and whatnot?

[slimjim]

Yes, it shifts fine, Although I have always thought it shifted a little hard into overdrive. I thought that maybe if I could fix this it might smooth that out some too.

[kanuck15]

Thats weird, My truck does the same thing but I know for a fact its the VSS. I wonder if you have a bad speedo wire?

[slimjim]

yeah, it's just strange to me that it is so consistent. Doesn't jerk around at all until I reach higher speeds. I may just unplug all the harnesses and plug them back in and see what happens.

[kanuck15]

mine does the same thing, it start to go nuts at like 55 then anywhere over that is just it bouncing back and forth

[slimjim]

Well, let me know if a new VSS fixes the problem. If not, I have the psom I can send you if you want to try it.

[kanuck15]

whats a psom?

[slimjim]

It's the speedometer part of the gauges. Stores the mileage and has the speedometer on it. It's known to go bad and cause the speedometer to do weird stuff. When the VSS didn't fix the problem, I ordered it from a scrap yard since none of the ones around here had one. They are interchangeable between the f150, 250, and 350 of similar years.