1. Identify the Problem:
* Intermittent or Inaccurate Reading: This is the most common issue. The gauge might fluctuate wildly, read low even when the tank is full, or be completely inaccurate. This often points to a problem with the fuel level sender unit, wiring, or the gauge itself.
* Gauge Always Reads Empty: This usually indicates a problem with the fuel level sender unit, wiring, or the gauge itself. A blown fuse is also a possibility.
* Gauge Always Reads Full: This suggests a problem with the fuel level sender unit (often a shorted float) or the wiring.
2. Troubleshooting Steps:
* Check the Fuse: Locate the fuel gauge fuse in your fuse box (consult your owner's manual). Replace it if it's blown.
* Inspect the Wiring: Carefully examine the wiring harness running from the fuel tank to the instrument cluster. Look for any broken, corroded, or loose wires. Pay close attention to the connections at both ends.
* Test the Fuel Level Sender Unit: This is the most likely culprit. It's located in the fuel tank and sends a signal to the gauge based on the fuel level.
* Access the Sender: This usually requires dropping the fuel tank (a significant undertaking), though some Jeeps might offer access from the bed of the truck or through a service panel. Consult a repair manual specific to your Jeep model.
* Check for Corrosion/Damage: Once accessed, inspect the sender unit for any obvious physical damage or corrosion.
* Test the Sender Resistance: You'll need a multimeter. The sender unit functions as a variable resistor. As the fuel level changes, its resistance changes. Consult a repair manual or online resources for your specific Jeep model to find the correct resistance values at different fuel levels. A faulty sender will show incorrect resistance readings. If the resistance is always low or high, the sender is likely bad.
* Test the Sender's Float: If possible, visually check the float within the sender. Make sure it moves freely and isn't stuck or damaged.
* Check the Fuel Gauge Itself: If the sender unit and wiring check out, the problem might lie within the gauge itself. Testing the gauge requires more specialized equipment and knowledge, and it's often easier and more cost-effective to replace the entire gauge cluster.
3. Repair/Replacement:
* Repair Wiring: Repair or replace any damaged wiring. Use automotive-grade wire and connectors. Solder connections for a more reliable repair.
* Replace the Fuel Level Sender Unit: This is the most common fix. Purchase a replacement sender unit specifically designed for your 1990 Jeep model. Installation typically involves removing the fuel tank (a potentially challenging task).
* Replace the Fuel Gauge: This is the least likely solution unless all other components have been thoroughly tested. Replacing the entire gauge cluster is a more involved job.
Important Safety Precautions:
* Disconnect the battery negative terminal before working on any electrical components.
* Work in a well-ventilated area when working with fuel.
* Handle fuel carefully to avoid spills and fires.
* Consult a repair manual specific to your 1990 Jeep model year and trim for detailed instructions and diagrams.
When to Seek Professional Help:
If you're uncomfortable working on your fuel system yourself, or if you've completed the troubleshooting steps without success, it's best to take your Jeep to a qualified mechanic. Incorrect fuel system repairs can be dangerous.
