To test a gauge for an AD truck, take the wire that goes to the sending unit off the gauge and attach a clip lead or wire to that post. With the wire dangling in the breeze (open) the gauge should go to full. Then ground the wire and the gauge should go to empty. If this works, your issue is with the sender or the wire to it. If it doesn't you have a gauge problem. If you have an original 6 volt gauge operating on a 12 volt system you must have a special voltage reducer installed. Most vendors sell them. You must also have your gauge well grounded. You can test this by touching a wire from a known good ground to the chrome bezel. Come back if this doesn't help.
