1. The Sending Unit: This is the most frequent cause of fuel gauge inaccuracy or failure. Located in the fuel tank, it's a float mechanism that sends a resistance signal to the gauge based on the fuel level.
* Problem: The float could be stuck, corroded, or leaking. The float arm might be bent or disconnected. The resistor inside the sending unit could be faulty. Corrosion in the tank can affect the sending unit's connection.
* Diagnosis: This requires dropping the fuel tank (a significant undertaking). Once out, inspect the float, the arm's connection, and test the sending unit's resistance with an ohmmeter. Compare your readings to a chart specifying resistance at various fuel levels (often found in repair manuals). A visual inspection for corrosion is also crucial.
2. The Fuel Gauge Itself: The gauge in the instrument cluster can malfunction.
* Problem: Internal shorts, broken connections, or a faulty gauge movement are possibilities.
* Diagnosis: While the gauge is in the cluster, testing it directly is difficult without specialized equipment. You can try temporarily swapping it with another gauge (if available) to rule it out.
3. Wiring and Connections: Corrosion, loose connections, or broken wires anywhere in the circuit can interrupt the signal.
* Problem: Wires can be chafed, damaged, or corroded at the terminals on the sending unit, in the fuel tank, within the wiring harness, or at the gauge itself.
* Diagnosis: Visually inspect all wiring and connections from the sending unit to the gauge. Look for corrosion, breaks, or loose connections. Use a multimeter to check for continuity in the circuit.
4. Grounding Issues: A poor ground connection anywhere in the circuit can lead to erratic readings or no reading at all.
* Problem: Corrosion on grounding points, loose bolts, or broken ground wires.
* Diagnosis: Check all grounding points associated with the fuel system and the instrument cluster. Clean and tighten connections.
5. Instrument Cluster Issues: Problems within the instrument cluster itself, beyond the fuel gauge, can affect multiple gauges or other functions.
* Problem: Faulty circuit boards, bad solder joints.
* Diagnosis: This usually requires specialized knowledge and tools to properly troubleshoot.
Troubleshooting Steps (in order):
1. Visual Inspection: Carefully examine all wiring and connections associated with the fuel gauge system. Look for obvious damage, corrosion, or loose connections.
2. Check the Ground: Ensure good ground connections at the fuel tank and instrument cluster.
3. Ohmmeter Test (Sending Unit): This is the most critical test. Once the tank is dropped, test the sending unit's resistance at different fuel levels.
4. Check Wiring Continuity: Use a multimeter to test the continuity of the wiring between the sending unit and the gauge.
5. Gauge Replacement (Last Resort): If all else fails, a new fuel gauge might be needed. Finding an exact replacement for a 1966 Mustang can be challenging, depending on the trim level.
Important Considerations:
* Safety: Work with gasoline requires caution. Always disconnect the negative battery terminal before working on the fuel system. Adequate ventilation is crucial.
* Repair Manual: A 1966 Mustang repair manual is invaluable for wiring diagrams, specifications, and troubleshooting guidance.
* Professional Help: If you're not comfortable working on your car's fuel system, it's best to seek professional help from a mechanic experienced with classic Mustangs.
Remember to systematically check each component to isolate the problem. The sending unit is the most likely culprit, but don't rule out the others.
