you need to run a ground wire from the body to the sending unit. There a place on the outside of the sending unit, that aspade wire conector goe's over. Any time that the fuel gauge stay's on full its a ground issue. Any time the gauge stays on empty, its a voltage problem. Good luck let me know, how it went. Good luck.

The gauge has a ignition wire (key hot) and the other goes to the gauge, sending unit should also be grounded. I'd pull sending unit, keep it grounded and run the float up and down. The needle should move accordingly. Corrosion is a pretty big culprit with the gas these days.