1980 F-250 DUAL TANKS BUT ONLY 1 TANK IS HOOKED up rear tank gauge reads empty how can i tell if it is the gauge,wiring, or sending unit it has two wires going to the sending unit in 1 plug. i rember something about grounding out the wire at the sending unit but which one? also how would you tell if the switching switch by the heater control is working or not? thanks Dave

 

How I found out the switch was bad on my 1980 was by taking the switch and flipping it up and down very rapidly about 5 or 6 times. The guage started working. About 3 or 4 months later it did the same thing again, and messing with the switch made it work, but I didn't want to rely on it anymore. So I wired around it. I too have only the front tank now.

I don't have my diagrams with me, but isn't one of the wires going to the sending unit colored, and the other one black? If so, the colored one is the one to ground with the key on, and watch the guage. And then take it off ground and watch the guage. If the guage swings back and forth during this test, then the wiring, switch and guage are good, the sending unit is bad.

 

so if i unplug the sending unit the gauge should move?

 

If the sending unit is "open" or in other words has failed by the wiper being broke, then it may not move. But taking a short piece of wire and hooking one end to a clean place on the frame, and taking the other end and poking it up into the sending unit plug should make the guage move. Be patient. The guage is a little slow. If this doesn't work, then I would suspect your switch.