I just put a tank in my truck from an 89 bronco. When I turn the key on the fuel gauge reads past full while the fuel pumps run then the gauge reads correct. If I start the truck then the gauge goes past full and stayes there till I shut truck off and cycle key on then it does the same as above. Any ideas as to what is causing this. The wiring on the truck was a 4 wire connector for the fuel pump and the bronco tank had three wire connector. When I wired it the black wire and orange wire from truck side are hooked to black wire on tank. Could that be the problem?

 

The fuel guage reads resistance(from the sending unit in the tank). When you ground out the sending unit wire, it will read FULL on the guage. So, if you got the wiring hooked up incorrectly, it's apparently getting a ground signal with the key on. I'm not familiar with what the wiring diagram is, so can't help you with it's possible to make it work correctly.

 

I disagree.....an open circuit will make the fuel gauge read full/past full. A short in the fuel gauge circuit will make it read empty.

22.5 ohms (resistance of an empty tank) is closer to zero ohms, therefore a short will make the gauge read empty. An open circuit is above 145 ohms (resistance with a full tank), plus an open circuit will pick up induced voltage which in turn makes the gauge read full.

 

I agree with rla2005. Wirings either improper or theres a problem with the sending unit. Rust can do this in the tank, my old 72 galaxie read full all the time. I had to have the sending unit replaced, the gas tank removed, soaked in an acid bath I think, and sealed with epoxy. Then, it worked great.

 

Sorry, got my head on backwards. When I was typing that I was questioning it myself if I was wording it backwards. Apparently I was. Been quite a few years since I've had to work on a fuel guage and test it.

 

Been there a time or twelve myself!

I have had this issue in the recent past, my mind is still fresh on this subject.

 

Maybe I'm thinking of my older stuff. Seems my '64 Galaxie works that way. Seems I've grounded the wire to see if the gauge moves(to full). I'm going to have to go out this weekend and try it.