To check the fuel guage (from the Haynes FSJ manual):

Set the ignition switch to ON.

If the gauge pointer does not move from empty, check the fuse. If the fuse is OK, locate the sender unit. Connect the sending unit connector to ground. If the pointer goes to the full position replace the sending unit. If the pointer stays in same position, use a jumper wire to ground the terminal on the back of gauge. If the pointer moves, the problem lies in the wire between the gauge and the sending unit. If the pointer does not move with the back of the gauge grounded, check for voltage at the other terminal of the gauge. If voltage is present, replace the gauge.

Checking the sending unit.

1. Raise the vehicle and support it securely on jackstands
2. If the fuel gauge on the instrument panel is malfunctioning, check the operation of the fuel level sending unit. If the fuel level sending unit operates in accordance with the following test, the problem may exist in the gauge or its circuit.
3. Before performing any tests on the fuel level sending unit, make sure the sending unit has a good ground electrical connection.
4. Raise the vehicle and support it securely on jackstands. [what, again?!?]
5. Locate the fuel level sending unit electrical connector. On most models, the connector is located near the back of teh fuel tank near the fuel filler hose and the fuel vapor hose. However, some late models are equipped with an access hole under the rear seat or trunk area. Lift the rear seat, remove the retaining screws and remove the access cover plate.
6. Position the probes of an ohmmeter into the electrical connector and check the resistance. Use the 200-ohm or low scale on the ohmmeter.
7. With the fuel tank completely full, the resistance should be approximately 110 ohms. With the tank half full, the resistance should be approximately 55 ohms. With the fuel tank empty, the resistance sof the sending unit should be approximately 1 ohm.
8. Note: a more accurate check of the sending unit can be made by removing it (fuel level sending unit of assembly) from the fuel tank and checking its resistance while manually operating the float arm.
9. If the readings are incorrect, replace the sending unit.

   Bill Kelsey
   Aberdeen, SD
   '79 widetrack Cherokee "S"