* Faulty Fuel Level Sending Unit: This is the most common cause. The unit itself might be malfunctioning, either due to age, corrosion, or a mechanical failure within the float mechanism. The float might be stuck, the potentiometer (variable resistor that sends the signal) could be broken or corroded, or there might be a wiring issue within the sending unit itself.
* Wiring Problems: A broken or corroded wire between the sending unit and the gauge cluster can also cause inaccurate readings. Check the wiring harness for any damage, especially where it enters and exits the fuel tank.
* Faulty Fuel Gauge Cluster: While less likely than a problem with the sending unit or wiring, the fuel gauge itself in the instrument cluster could be malfunctioning. This is harder to diagnose without specialized tools.
* Ground Issue: A poor ground connection somewhere in the circuit can disrupt the signal.
Troubleshooting Steps (in order of likelihood and ease):
1. Check the fuel gauge's float: This requires dropping the fuel tank (a major undertaking), but if you're comfortable with car repair and have the tools, this allows direct inspection of the float and potentiometer. Look for damage, corrosion, or anything that prevents the float from moving freely.
2. Check the wiring: Inspect the wiring harness leading from the fuel tank to the instrument cluster for any breaks, corrosion, or loose connections. Pay close attention to the connections at both ends.
3. Try a fuel gauge test (requires a multimeter): With the key on, you can test the voltage coming from the fuel sending unit using a multimeter. This will help determine if the unit itself is sending a faulty signal. You'll need to consult a wiring diagram for your specific Jeep Grand Cherokee to find the correct wire.
4. Replace the Fuel Level Sending Unit: If the wiring and gauge cluster check out okay, the sending unit is the most probable culprit. Replacement is usually the most cost-effective solution. This usually involves dropping the fuel tank, which is a labor-intensive job best done by a mechanic if you lack experience.
Important Safety Note: Working with fuel systems requires caution. Always disconnect the negative battery terminal before working on the fuel system. Gasoline is highly flammable, so take appropriate precautions. If you're not comfortable performing these repairs yourself, take your Jeep to a qualified mechanic.
