LOW FUEL INDICATOR
DESCRIPTION
A low fuel indicator is standard equipment on all
instrument clusters. This indicator is located near the
upper edge of the instrument cluster, between the
minor gauge set and the speedometer.
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 icon to appear in amber through the translucent outer layer of the overlay when the
indicator 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:
•
Bulb Test - Each time the ignition switch is turned to the On position the low fuel indicator is illuminated for
about two seconds as a bulb test.
•
Less Than 12.5 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 12.5
percent full or less for ten consecutive seconds and the vehicle speed is zero, or for sixty consecutive seconds
and the vehicle speed is greater than zero, the fuel gauge needle is moved to about the one-sixteenth grad-
uation 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 15.5 percent full for ten consecutive seconds and the vehicle speed is zero, or for sixty 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 low fuel indicator will be turned 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 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 low fuel indicator or the instrument cluster circuitry that
controls the LED, (Refer to 8 - ELECTRICAL/INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING).
HB
CLUSTER
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