I have a 1989 F-350 crew cab XLT Lariat, 460, 38,000mi. Yesterday after a short stop at the bank I started the truck and the gas gage pegged out (past normal max). Today it seems to move only up to 7/8 mark and stay. I have two tanks one is full and the other is approx 3/4 (40 miles on it). The gage does not change when I shift tanks. With the ignition off the needle moves back to about 3/4. I reviewed the archives and I know it's not the IVR because of my model year, but I am not convinced it's a sender due to when switching tanks it doesn't move or change. What I really need is a wiring diagram of the complete circuit (my chiltons doesn't show the dual tank set up) so I can troubleshoot the problem.
thanks,
Joe

 

Sounds like its grounding out. check to see if any insulation has rubbed of the wires going to the sender. Also unplug the wires off the sending unit then turn the ignition forward to the point of almost starting, if its not grounding out the gauge should read empty.