Over time, the contacts can wear, leading to an open circuit. When the vehicle is in motion, the sending unit is in constant motion, constantly rubbing the variable resistor. Sending Unit Failure is the most-common cause of a gas gauge not working.Here are four ways that the gas gauge can fail. The gas gauge is a relatively simple circuit, but its simplicity means each component is essential to its function.

The fuel pump module includes the fuel sending unit, used by the gas gauge to detect and display fuel level. Some gas gauges are directly controlled by voltage feedback from the sending unit, while others are controlled by the instrument cluster, which itself gets voltage information from the sending unit. The gas gauge in the instrument cluster is the visual indication of activity in the fuel tank and sending unit. Most modern sending units are grounded to the electrical system, but some older cars were grounded to the body or frame. The gas gauge circuit connects the battery, sending unit, gas gauge, and ground. Other systems have opposite wiring (high fuel level corresponds to high resistance and low voltage) but nonetheless go through the same process. The system thus outputs higher voltage at high fuel level, gradually dropping voltage as fuel level drops. Some systems are wired so that high fuel level contacts the low-resistance section, gradually increasing resistance as fuel level drops. The fuel float contacts run on a potentiometer, or variable resistor, whose resistance changes depending on float level, effecting a change in output voltage. Input voltage, typically from the ignition circuit, is modified by the fuel sender. The sending unit is usually part of the Fuel Pump Module: a combination of parts that includes the fuel pump, fuel strainer, fuel filter, and fuel float. A fault in one or more of these parts will lead to gas gauge failure. The gas gauge system can be broken down into three basic parts: gauge, sender, and circuit. This cutaway fuel tank shows where the fuel sending unit is located.

2005 expedition fuel pump install