Terry1218,
Here are some ideas. Without ANY wires going back to the tank, the gauge will read EMPTY when the key is OFF, and will read FULL when the key is ON.
Using the wire from gauge to tank, at the tank, with key ON you should see this. As Baldy stated, ground the wire to a known good clean ground and the gauge will swing to EMPTY.
Same drill, but dangle the wire in space! Gauge should swing to FULL.
If this happens, wire and gauge are good, and your sender or ground integrity is bogus. Don't get confused trying to match the fill level in the tank with a corresponding gauge reading...just prove one of the components, which will put the blame on the other. I think this info is accurate for a negative ground Chevrolet...I don't know if a positive ground system would reverse any of the readings, but I don't think so.
Stuart
