Grand Cherokee 2WD Limited V8-4.7L (2002)
Low Fuel Lamp/Indicator: Description and Operation
LOW FUEL INDICATOR
A low fuel indicator is standard equipment on all instrument clusters. The low fuel indicator is located near the left edge of the instrument cluster, to
the left of the tachometer. The low fuel indicator consists of an International Control and Display Symbol icon for "Fuel" imprinted on an amber lens.
The lens is located behind a cutout in the opaque layer of the instrument cluster overlay The dark outer layer of the overlay prevents the icon from
being clearly visible when the indicator is not illuminated. The icon appears silhouetted against an amber field through the translucent outer layer of
the overlay when the indicator is illuminated from behind by a replaceable incandescent bulb and bulb holder unit located on the instrument cluster
electronic circuit board. When the exterior lighting is turned ON, the illumination intensity of the low fuel indicator is dimmable, which is adjusted
using the panel lamps dimmer control ring on the control stalk of the left multi-function switch. The low fuel indicator lens is serviced as a unit with
the instrument cluster lens, hood and mask unit.
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 electronic circuit board based upon cluster programming and electronic messages received by the cluster from the
Powertrain Control Module (PCM) over the Programmable Communications Interface (PCI) data bus. The low fuel indicator bulb 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 indicator will always be OFF when the ignition switch is in any
position except ON or Start. The bulb only illuminates when it is switched 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 12.5 Percent Tank Full Message - Each time the cluster receives a message from the PCM indicating that the percent tank full is less
than about 12.5 (one-eighth), the low fuel indicator is illuminated. The indicator remains illuminated until the cluster receives messages from the
PCM indicating that the percent tank full has increased to greater than about 12.5 (one-eighth). The PCM applies an algorithm to the input from
the fuel tank sender to dampen the illumination of the low fuel indicator against the negative effect that fuel sloshing within the fuel tank can have
on accurate inputs to the PCM.
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Less Than Empty Percent Tank Full Message - Each time the cluster receives a message from the PCM indicating the percent tank full is less
than empty, the low fuel indicator is illuminated immediately This message would indicate that the fuel tank sender input to the PCM is a short
circuit.
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More Than Full Percent Tank Full Message - Each time the cluster receives a message from the PCM indicating the percent tank full is more
than full, the low fuel indicator is illuminated immediately This message would indicate that the fuel tank sender input to the PCM is an open
circuit.
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Communication Error - If the cluster fails to receive a percent tank full message for more than about twelve seconds, the cluster control circuitry
will illuminate the low fuel indicator until a new percent tank full message is received.
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Actuator Test - Each time the cluster is put through the actuator test, the low fuel indicator will be turned ON for the duration of the test to
confirm the functionality of the bulb and the cluster control circuitry
The PCM continually monitors the fuel tank sender input to determine the fuel level. The PCM then applies an algorithm to the input and sends the
proper percent tank full messages to the instrument cluster. If the low fuel indicator fails to light during the bulb test, replace the bulb with a known
good unit. For further diagnosis of the low fuel indicator or the instrument cluster circuitry that controls the indicator, refer to Instrument Cluster
Diagnosis and Testing. For proper diagnosis of the fuel tank sender, the PCM, the PCI data bus, or the electronic message inputs to the instrument
cluster that control the low fuel indicator, a DRB III scan tool is required Refer to the appropriate diagnostic information.
