Grand Cherokee 2WD V8-4.7L VIN J (2004)
Fog/Driving Lamp Indicator: Description and Operation
Front Fog Lamp Indicator
FRONT FOG LAMP INDICATOR
A front fog lamp indicator is standard equipment on all instrument clusters, but is only functional on vehicles equipped with the optional front fog
lamps. The front fog lamp indicator is located on the left edge of the instrument cluster, to the left of the tachometer. The front fog lamp indicator
consists of an International Control and Display Symbol icon for "Front Fog Light" imprinted on a green 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 indicator from being clearly visible when it is not
illuminated. The icon appears silhouetted against a green 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 front fog lamp indicator is dimmable, which is adjusted using the panel lamps dimmer control ring on the
control stalk of the left multi-function switch. The front fog lamp indicator lens is serviced as a unit with the instrument cluster lens, hood and mask
unit.
The front fog lamp indicator gives an indication to the vehicle operator whenever the front fog lamps are illuminated. This indicator is controlled by a
transistor on the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Body
Control Module (BCM) over the Programmable Communications Interface (PCI) data bus. The front fog lamp indicator bulb is completely controlled
by the instrument cluster logic circuit, and that logic will allow this indicator to operate whenever the instrument cluster receives a battery current
input on the fused B(+) circuit. Therefore, the indicator can be illuminated regardless of the ignition switch position. The bulb only illuminates when it
is provided a path to ground by the instrument cluster transistor. The instrument cluster will turn ON the front fog lamp indicator for the following
reasons:
-
Front Fog Lamp Indicator lamp-ON Message - Each time the cluster receives a front fog lamp indicator lamp-ON message from the BCM
indicating that the front fog lamps are turned ON, the front fog lamp indicator will be illuminated. The indicator remains illuminated until the
cluster receives a front fog lamp indicator lamp-OFF message from the BCM.
-
Actuator Test - Each time the cluster is put through the actuator test, the front fog lamp indicator will be turned ON for the duration of the test to
confirm the functionality of the bulb and the cluster control circuitry.
The BCM continually monitors the exterior lighting (left multi-function) switch to determine the proper outputs to the front fog lamp relay. The BCM
then sends the proper front fog lamp indicator lamp-ON and lamp-OFF messages to the instrument cluster. If the front fog lamp indicator fails to light
during the actuator test, replace the bulb with a known good unit. For further diagnosis of the front fog lamp indicator or the instrument cluster
circuitry that controls the indicator, refer to Instrument Cluster Diagnosis and Testing. For proper diagnosis of the front fog lamp system, the BCM,
the PCI data bus, or the electronic message inputs to the instrument cluster that control the front fog lamp indicator, a DRB III scan tool is required.
Refer to the appropriate diagnostic information.
