[Barnsnake]

The gauge needs a ground and the sender needs a ground.

Verify that the "ground" wire wire on the sender is really grounded.
Verify that the gauge is grounded and has power when the key is on.
If yes, momentarily ground the sender connector on the gauge. That should "peg" the gauge at full. If it doesn't, the gauge is bad.
If the gauge checks out, it's either the sender or the wiring to the sender.