1977 gmc fuel gauge

i have a 77 gmc sierra and the fuel gauge goes past full with the key on. what is causing this?

thanks

the fuel gauge works on resistance. You have a broken wire from the fuel tank to the gauge. Trace the wire and you'll find the break. Have you changed a starter or swapped the engine? The wire is in the harness with the starter wires at the bellhousing, and it's common to have it break there.