I just purchased a 1995 F-250 with a Powerstroke diesel. The fuel selector switch seems to actually switch between tanks to supply the engine, but the gauge does not read for the front tank. It does indicate for the aft tank. I am looking for the wiring diagram for indicating system to see if I can establish what the problem is BEFORE I have to drop the tank to change the sending unit. I really hate 'shotgunning' a problem by buying parts until I finally land on the one that needed to be changed. Anyone have any advice?