Low Fuel Indicator - Operation

OPERATION

The low fuel indicator gives an indication to the vehicle operator when the level of fuel in the fuel tank becomes low. This indicator is controlled by a transistor on the instrument cluster circuit board based upon cluster programming and a hardwired input received by the cluster from the fuel level sending unit on the fuel pump module in the fuel tank.

The low fuel indicator Light Emitting Diode (LED) is completely controlled by the instrument cluster logic circuit. This logic only allows the indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the LED is always off when the ignition switch is in any position except ON or START. The LED only illuminates when it is provided a path to ground by the instrument cluster transistor. The instrument cluster turns on the low fuel indicator for the following reasons:

- Bulb Test - Each time the ignition switch turns to the ON position the low fuel indicator is illuminated for about two seconds as a bulb test.
- Less Than 15 Percent Tank Full 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. 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 fuel gauge needle is moved to about the one-sixteenth graduation on the gauge scale, the low fuel indicator illuminates, and a single chime tone is sounded. The low fuel indicator remains illuminated until the fuel level sense input indicates that the fuel tank is greater than about 15 percent full 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 turns to the OFF position, whichever occurs first. The chime tone feature only repeats during the same ignition cycle if the low fuel indicator is cycled off 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 E (or Empty) gauge needle stop position, the gauge needle moves to the low end of the gauge scale and the low fuel indicator illuminates 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 F (or Full) gauge needle stop position, the gauge needle moves to the low end of the gauge scale and the low fuel indicator illuminates immediately. This input would indicate that the fuel level sense input to the cluster is an open circuit.
- Self Test - Each time the cluster is put through the self test, the low fuel indicator turns on, then off again during the bulb check portion of the test to confirm the functionality of the LED and the cluster control circuitry.

The instrument cluster, also known as the Cab Compartment Node, (CCN) continually monitors the fuel tank sending unit to determine the level of fuel in the fuel tank. The cluster then sends the proper electronic fuel level messages to other electronic modules in the vehicle over the Controller Area Network (CAN) data bus. For further diagnosis of the low fuel indicator or the instrument cluster circuitry that controls the LED, Testing and Inspection.

Use a diagnostic scan tool for proper diagnosis of the fuel tank sending unit, the CCN, the CAN data bus or the electronic communication related to low fuel indicator operation or fuel level data processing. Refer to the appropriate diagnostic information.