I have a 1992 jeep Cherokee and the fuel gauge is stuck on full. it worked perfectly the day before. what do I need to repair to fix it

 

You have a short somewhere in the gauge system.

Turn ignition on, then disconnect the sender at the fuel pump. Gauge should go to Empty. If it does, the sender is suspect.

You can measure the resistance of the sending unit.
Application Ohms
Empty ................................................ 105
1/2 Full .............................................. 33
Full .................................................. . 5

If it's still pegged, check for any visible signs of shorts along the wiring. You may have to pull the gauge cluster and then check the resistance above at the gauge. If the resistance checks out there, it's the gauge. If it's still 5 ohms or lower, you have a short in the wiring somewhere.