I have a 1956 Ford F-100 pickup. The fuel guage reads full at all times. If truck is shut off it goes to empty. Float and sending unit work fine. Ground from sending unit to cab fine. Do I need to ground the guage in the dashboard itself? This used to work fine then just quit. Have put 3 different sending units in and always the same. Any suggestions? Thank you for your help.

 

If you have a light in the gauge and it's lighting up then it's probably already grounded. Besides, the fact that the gauge moves to empty indicates the gauge itself is okay. My guess is the sending unit is not grounded properly or the float is hanging up. On my truck I ran a ground wire from under one of the mounting screws to ground. Also, make sure the wire going from the tank sender to the gauge is not grounding out on something. You might also check to be sure the float arm is adjusted properly and is not hanging up on a baffle or the tank wall inside the tank.

 

Your have an unwanted ground. As GNW mentioned it's probly the wire going from the gauge to the sender.

The sender provides grounding. The gauge works when the float in the sender provides variable resistance at different levels.

Without the resistance of the sender, the gauge reads full all the time.

If the problem persists, I'd take out the sender and hook the top to ground and physically check movement of the needle by moving the float arm. If the gauge reade full all the time you definately have a chaffed sender unit wire that is grounding out somewhere.