RAM 1500 Truck 4WD V8-5.9L VIN Z (2002)
Four Wheel Drive Indicator Lamp: Description and Operation
SERVICE 4WD INDICATOR
A service 4WD indicator is standard equipment on all instrument clusters However, on vehicles not equipped with the optional four-wheel drive
system and electronically shifted transfer case, this indicator is electronically disabled. The service 4WD indicator consists of the words
"SERVICE 4WD", which appears in the lower portion of the electronic gear selector indicator Vacuum Fluorescent Display (VFD) unit. The VFD
is part of the cluster electronic circuit board and is visible through a window with a smoked clear lens located on the lower edge of the
speedometer gauge dial face of the cluster overlay. The dark lens over the VFD prevents the indicator from being clearly visible when it is not
illuminated. The words "SERVICE 4WD" appear in an amber color and at the same lighting level as the gear selector indicator information when
they are illuminated by the instrument cluster electronic circuit board. The service 4WD indicator is serviced as a unit with the VFD in the
instrument cluster.
The service 4WD indicator gives an indication to the vehicle operator when the Transfer Case Control Module (TCCM) has recorded a
Diagnostic Trouble Code (DTC) for an electronic transfer case circuit or component malfunction. 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 TCCM over the
Programmable Communications Interface (PCI) data bus. The service 4WD indicator 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
indicator only illuminates when it is switched to ground by the instrument cluster circuitry. The instrument cluster will turn ON the service 4WD
indicator for the following reasons:
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Service 4WD Lamp-ON Message - Each time the cluster receives a lamp-on message from the TCCM, the indicator will be illuminated. The
indicator remains illuminated until the cluster receives a service 4WD lamp-OFF message from the TCCM, or until the ignition switch is
turned to the Off position, whichever occurs first.
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Communication Error - If the cluster receives no messages from the TCCM for five seconds, the service 4WD indicator is illuminated by the
instrument cluster to indicate a loss of TCCM communication. The indicator remains controlled and illuminated by the cluster until a valid
message is received from the TCCM.
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Actuator Test - Each time the cluster is put through the actuator test, the service 4WD indicator will be turned ON, then OFF again during the
VFD portion of the test to confirm the functionality of the VFD and the cluster control circuitry
The TCCM continually monitors the electronic transfer case switch and circuits to determine the condition of the system, then sends the proper
messages to the instrument cluster. For further diagnosis of the service 4WD indicator or the instrument cluster circuitry that controls the indicator,
(Refer to INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING). For proper diagnosis of the TCCM, the PCI data bus, or the electronic
message inputs to the instrument cluster that control the service 4WD indicator, a DRB III scan tool is required. Refer to the appropriate
diagnostic information.
