A functioning sending unit will have a high resistance when the tank is full and a low resistance when the tank is empty. If the gauge reads higher than it should, make sure the wire attached to the electrical terminal on the tank sending unit is making a good ground connection. If necessary, clean the connection and reinstall the wire to the terminal, then check the gauge operation. If that didn't fix the problem, use a jumper wire to ground the electrical terminal on the tank sending unit to the tank unit housing (leave the wire to the gauge connected to the tank unit). Turn the ignition switch on. If the gauge reads empty or below empty, the sending unit is defective.
If the gauge still reads high, try grounding the tank unit electrical terminal to a clean portion of the frame. If this brings the gauge to empty or below, the tank unit is OK, but there is a bad ground between the gas tank and the body or chassis (or less likely, between the tank unit and the gas tank). Either way, find and correct the bad connection or merely run a permanent jumper wire from the frame to one of the screws holding the tank sending unit to the tank.
If the gauge still reads incorrectly, the wire from the tank unit to the gauge may be faulty. Check by using a jumper wire to ground the tank unit terminal on the gauge (the one not connected to the ignition). If the gauge now reads empty or below, the problem is in the wire leading to the tank (or its connection to the gauge). If grounding the tank terminal still doesn't cause the gauge reading to drop to empty or below, the gauge is faulty.
Sunday, July 20th, 2014 AT 7:49 PM