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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.