RAM 1500 Truck 4WD V8-5.7L VIN D (2005)
Four Wheel Drive Indicator Lamp: Description and Operation
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 text "SERV 4WD", which
appears in the lower portion of the odometer/trip odometer Vacuum Fluorescent Display (VFD) unit.
The odometer/trip odometer VFD is soldered onto the cluster electronic circuit board and is visible through a window with a smoked clear lens located
on the lower edge of the tachometer 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 text "SERV 4WD" appears in an amber color and at the same lighting level as the odometer/trip odometer information
when it is 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 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:
-
Service 4WD Lamp-On Message - Each time the cluster receives a service 4WD 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.
-
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.
-
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. The TCCM then sends the
proper lamp-on or lamp-off messages to the instrument cluster.
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
diagnostic scan tool is required.
