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I have a 1995, F-250 (4X4) with a PSD and automatic transmission. On a recent trip my speedo started to act erratic; about a 10-15 mph fluctuation at all speeds over 10 mph. I first looked for a speedo cable, but none found in-spite of the analog style gauge. This vehicle does have a Banks transmission controller.
By the way, the vehicle stalled several times yesterday. I replaced the CPS (had an extra) and the vehicle started fine, but haven't gone for a run yet.
The speedometer gets its input from the VSS (Vehicle Speed Senor) which is located in the front top or the rear differential. First thing I would try is to pull it out and clean it up. Alot of times they will be covers with metal shavings and gunk which will not let it pick up a good siganl from the exciter ring. Let us know what you find.
Since you installed your spare CPS I would order another spare off of ebay to have in the truck.
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