1. Check the Fuel Sender Unit: This is the most common culprit. The sender unit is located in the fuel tank and sends a signal to the gauge. It's a relatively inexpensive part and a common cause of inaccurate readings.
* Symptoms of a bad sender unit: Inaccurate reading (always low, always high, erratic fluctuations), gauge pegged at empty even when the tank is full, or gauge not moving at all.
* Troubleshooting: You likely need to drop the fuel tank to access the sender unit. This is a messy and potentially dangerous job requiring care with fuel fumes and a fire extinguisher. If you're not comfortable working with fuel systems, take it to a mechanic. *Before* dropping the tank, disconnect the battery's negative terminal.
* Visual Inspection: Once the tank is dropped, inspect the sender unit for any obvious damage (corrosion, broken wires, etc.).
* Testing the Sender Unit: You can test the sender unit's resistance with a multimeter. The resistance should change as the float moves (indicating different fuel levels). You'll need a wiring diagram to identify the correct wires. If the resistance readings are incorrect, replace the sender unit.
* Replacing the Sender Unit: Carefully remove the old unit and install the new one, ensuring a proper seal to prevent leaks.
2. Check the Fuel Gauge Wiring: Broken, corroded, or loose wiring can disrupt the signal from the sender unit to the gauge.
* Symptoms of bad wiring: Intermittent readings, gauge fluctuating wildly, or no reading at all.
* Troubleshooting: Carefully inspect all wiring related to the fuel gauge, starting at the sender unit and following the wires to the gauge cluster. Look for broken wires, corrosion, loose connections, and chafing. Repair or replace any damaged wiring.
3. Check the Fuel Gauge Cluster: The gauge itself could be faulty. This is less common than a bad sender unit or wiring, but possible.
* Symptoms of a bad gauge: Always shows empty or full, regardless of fuel level, or is completely inoperative.
* Troubleshooting: This requires removing the instrument cluster and potentially testing the gauge itself with a multimeter or replacing the cluster entirely (expensive option). Often, this is the last thing checked since it is less likely to cause a problem and is expensive to repair.
4. Check the Fuel Pump Relay (Less Likely): A faulty fuel pump relay can sometimes affect the fuel gauge reading, though this is less common.
* Troubleshooting: Locate the fuel pump relay (check your car's manual), and try swapping it with another relay of the same amperage rating (for testing purposes). If the gauge works correctly, replace the fuel pump relay.
Important Considerations:
* Safety: Working with fuel systems is dangerous. Always disconnect the battery's negative terminal before working on the fuel tank or fuel lines. Work in a well-ventilated area and have a fire extinguisher nearby.
* Repair Manual: A repair manual specific to your 1991 Ford Aerostar will be extremely helpful, providing wiring diagrams and detailed procedures.
* Professional Help: If you are not comfortable working on your vehicle's fuel system, take it to a qualified mechanic. This job can be tricky and potentially hazardous if not done correctly.
By systematically checking these areas, you should be able to diagnose and fix the problem with your 1991 Ford Aerostar's gas gauge. Remember to prioritize safety throughout the process.
