Found this also on the American auto wire site:
"General Motors Fuel gauge troubleshooting"
"Fuel gauge troubleshooting begins with isolating the problem either to the gauge, sending unit, fuse or wiring. The most common gauge problems are either a bad ground at the tank or a sender problem. As with any electrical troubleshooting it is best to check the fuse and all wiring connections are clean, tight and free of corrosion.
The following process must be preformed with the ignition key turned to the “on” position. The first place to start is at the fuel tank sending unit. Ensure the fuel tank sender ground has a good connection by removing the wire from the frame to clean the surface of the terminal and the frame. Next, remove the wire from the sending unit that is connected to the fuel gauge (typically a tan wire) connect this wire to a good ground. Check the gauge, if the needle moves toward full, replace the sending unit. If the needle does not move, remove the same wire from back of the gauge and connect a jumper from the terminal on the back of the gauge to a good ground. Check the gauge if the needle moves toward full, the wire from the gauge to the sending unit has an open or bad connection. If it does not move, connect a test light by grounding one end to a good known ground and the other end to the positive side of the gauge. If it does not illuminate, connect a jumper wire between the ignition switch and the positive side of the gauge. If the gauge now works, replace the defective wiring between the gauge and the ignition switch. If the test light illuminates there is 12 volts present then replace the fuel gauge"
