* Bad Fuel Sender Unit: This is the most frequent culprit. The fuel sender unit is located inside the fuel tank and contains a float that rises and falls with the fuel level. It's a potentiometer that sends a resistance signal to the gauge. If the float is stuck, the arm is broken, the potentiometer is faulty, or there's corrosion, the gauge won't read accurately or at all.
* Wiring Problems: A break, short, or corrosion in the wiring between the fuel sender unit and the instrument cluster can prevent the signal from reaching the gauge. This could be anywhere along the wire's path.
* Faulty Fuel Gauge: The gauge itself could be malfunctioning. While less common than a bad sender, it's a possibility.
* Bad Instrument Cluster: Problems within the instrument cluster itself, where the gauge is located, could also prevent a reading.
* Grounding Issues: A poor ground connection in the fuel gauge circuit can disrupt the signal.
* Low Fuel: Sometimes, an empty tank can lead to erratic fuel gauge readings or a complete failure to display the level (the float might be resting at the bottom). Even if the tank isn't completely empty, extremely low fuel can cause problems.
Troubleshooting Steps:
1. Check the Fuel Level: The most obvious first step! Is the tank truly empty?
2. Visual Inspection: Carefully check the fuel gauge wiring for any visible damage, breaks, or corrosion.
3. Test the Fuel Sender Unit: This requires dropping the fuel tank (or accessing it through an access panel if your model has one). You'll need a multimeter to test the resistance of the sender at different fuel levels. There are many online resources showing how to do this specific test for a 1990 Mustang fuel sender. Be cautious working around fuel.
4. Check Ground Connections: Inspect the ground wires connected to both the fuel sender and the instrument cluster. Clean any corrosion you find.
5. Test the Gauge: If the sender unit checks out, and wiring looks good, you might test the gauge itself by using a jumper wire and applying a known resistance to simulate a fuel level (again, requires a multimeter). This helps determine if the gauge itself is faulty.
It's recommended to consult a repair manual specific to your 1990 Ford Mustang for detailed wiring diagrams and testing procedures. Safely working with fuel requires precautions. If you are not comfortable working on your car yourself, take it to a qualified mechanic for diagnosis and repair.
