I own a 94 f250 4x4 with a 7.3 idi diesel with 158348 miles on it. First problem I am having is the tach, when the engine is idling it reads 0 but at highway speed {55} it shows about 1500rpm it seems to jump up at 1000 rpm?? Second question I have is with the trans when I let off the throttle at highway speed like when slowing for a speed zone the o/d off light flashes but when I pick up the throttle again it stops flashing and is not lit up?? any ideas.

Problem solved it was a bad rpm sensor