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Helping a buddy with his '96 Dump Truck. I think it is an F650 but there a no markings. It is small as dump trucks go, but has air brakes and an automatic transmission.
The speedometer goes back and forth full scale as if a speedometer cable were bad. Problem with that is, my buddy says it has an electronic speedometer. About the only thing I can think of that would cause that problem is the Vehicle Speed Sensor.
The truck has a Cummins and Allison. I wasooking around under it yesterday but did not have on work clothes so that i could really get under it. I did not find a sensor on the rear axle. It must be on the transmission. I googled around and couldn't cind much about a VSS on an Allison.