Commander 4WD V8-4.7L VIN N (2006)
Low Fuel Lamp/Indicator: Description and Operation
LOW FUEL INDICATOR
DESCRIPTION
A low fuel indicator is standard equipment on all instrument clusters. This indicator is located above the fuel gauge and to the left on the cluster overlay.
The low fuel indicator consists of a stencil-like cutout of the International Control and Display Symbol icon for Fuel in the opaque layer of the
instrument cluster overlay. The dark outer layer of the overlay prevents the indicator from being clearly visible when it is not illuminated. An amber
Light Emitting Diode (LED) behind the cutout in the opaque layer of the overlay causes the indicator to appear in amber through the translucent outer
layer of the overlay when it is illuminated from behind by the LED, which is soldered onto the instrument cluster electronic circuit board.
The low fuel indicator is serviced as a unit with the instrument cluster.
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 hard wired 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, and that logic will only allow this
indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the LED
will always be 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 will turn on the low fuel indicator for the following reasons:
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Bulb Test - Each time the ignition switch is turned to the ON position the low fuel indicator is illuminated for about three seconds as a bulb test.
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Less Than 11 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 11 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 the appropriate position 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 indicates that the fuel tank is greater than about 14 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 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.
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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 for five consecutive seconds, 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.
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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 for five consecutive seconds, 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.
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. The cluster will store a Diagnostic
Trouble Code (DTC) for any fault detected in the fuel level sense circuit. 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.
