Fuel Gauge: Description and Operation

A fuel gauge is standard equipment on all instrument clusters. This gauge is located in the lower left quadrant of the instrument cluster, below the voltage gauge. This gauge consists of a movable gauge needle or pointer controlled by the instrument cluster circuitry and a fixed 90 degree scale on the cluster overlay that reads left-to-right from E (or Empty) to F (or Full). On SRT10 vehicles, a 1/2 designation is also imprinted under the center graduation on the scale.

An International Control and Display Symbol icon for Fuel is located on the cluster overlay, directly below the right end of the gauge scale. An arrowhead pointed to the left side of the vehicle is imprinted on the cluster overlay next to the Fuel icon on the gauge to provide the driver with a reminder as to the location of the fuel filler access. On vehicles equipped with a diesel engine, text that specifies DIESEL ONLY is located across the fuel gauge below the gauge scale, but above the hub of the gauge needle. On SRT10 vehicles, text that specifies premium only is located across the fuel gauge below the gauge scale, but above the hub of the gauge needle.

On all vehicles except SRT10 the fuel gauge graphics are black against a white field except for a single red graduation at the low end of the gauge scale. On SRT10 the gauge graphics are black against a silver field except for a single red zone at the low end of the gauge scale. In either case, the gauge graphics are clearly visible within the instrument cluster in daylight.

When illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting with the exterior lamps turned ON, the black gauge graphics appear blue-green and the red graphics still appear red. The gauge needle has internal optical illumination. Gauge illumination is provided by replaceable incandescent bulb and holder units located on the instrument cluster electronic circuit board. The fuel gauge is serviced as a unit with the instrument cluster.

The fuel gauge gives an indication to the vehicle operator of the level of fuel in the fuel tank. This gauge is controlled by the instrument cluster circuit board based upon cluster programming and a hard wired input received by the cluster from the fuel level sending unit on the fuel pump module in the fuel tank.

The fuel gauge is an air core magnetic unit that receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (run-start) circuit whenever the ignition switch is in the ON or START positions. The cluster is programmed to move the gauge needle back to the low end of the scale after the ignition switch is turned to the OFF position. The instrument cluster circuitry controls the gauge needle position and provides the following features:
- Fuel Level Sending Unit Input - The cluster provides a constant current source to the fuel level sending unit and monitors a return input on a fuel level sense circuit. The resistance through the fuel level sending unit increases as the fuel level falls and decreases as the fuel level rises causing changes in the sense input voltage. The cluster programming applies an algorithm to calculate the proper fuel gauge needle position based upon the fuel level sense input, then moves the gauge needle to the proper relative position on the gauge scale. This algorithm is used to dampen gauge needle movement against the negative effect that fuel sloshing within the fuel tank can have on accurate inputs from the fuel tank sending unit to the cluster.
- Less Than 15 Percent Tank Full Input - Each time the fuel level sense input to the cluster indicates the fuel tank is about 15 percent full or less for 10 consecutive seconds and the vehicle speed is zero, or for 60 consecutive seconds and the vehicle speed is greater than zero, the gauge needle is moved to about the one sixteenth graduation on the gauge scale, the low fuel indicator is illuminated, and a single chime tone is sounded. The low fuel indicator remains illuminated until the fuel level sense input is greater than about 15 percent for 10 consecutive seconds and the vehicle speed is zero, or for 60 consecutive seconds and the vehicle speed is greater than zero, or until the ignition switch is turned to the OFF position, whichever occurs first. The chime tone feature will only repeat during the same ignition cycle if the low fuel indicator is cycled off and then on again by the appropriate inputs from the fuel level sending unit.
- Less Than Empty Stop Input - Each time the cluster receives a fuel level sense input that indicates the fuel level in the fuel tank is less than the Empty gauge needle stop position, the gauge needle is moved to the low end of the gauge scale and the low fuel indicator is illuminated immediately. This input would indicate that the fuel level sense input to the cluster is a short circuit.
- More Than Full Stop Input - Each time the cluster receives a fuel level sense input that indicates the fuel level in the fuel tank is more than the Full gauge needle stop position, the gauge needle is moved to the low end of the gauge scale and the low fuel indicator is illuminated immediately. This input would indicate that the fuel level sense input to the cluster is an open circuit.
- Actuator Test - Each time the cluster is put through the actuator test, the fuel gauge needle will be swept to several calibration points on the gauge scale in a prescribed sequence in order to confirm the functionality of the gauge and the cluster control circuitry.

The instrument cluster continually monitors the fuel tank sending unit to determine the level of fuel in the fuel tank. The cluster then sends the proper fuel level messages to other electronic modules in the vehicle over the Controller Area Network (CAN) data bus. For further diagnosis of the fuel gauge or the instrument cluster circuitry that controls the gauge, refer to testing.

For proper diagnosis of the fuel tank sending unit, the CAN data bus, or the electronic fuel level message outputs of the instrument cluster, a diagnostic scan tool is required.