Malfunction Indicator Lamp - Transfer Case: 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 is located below the minor gauge set on the left side of the cluster.

The service 4WD indicator consists of a stencil-like cutout of the text "SVC 4WD" 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 "SVC 4WD" text 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 service 4WD indicator is serviced as a unit with the instrument cluster.

The service 4WD indicator gives an indication to the vehicle operator when the Front Control Module (FCM) (also known as the Integrated Power Module/IPM) 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 FCM over the Controller Area Network (CAN) data bus.

The service 4WD 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 service 4WD indicator for the following reasons:
- Bulb Test - Each time the ignition switch is turned to the On position the service 4WD indicator is illuminated for about two seconds as a bulb test.
- Service 4WD Lamp-On Message - Each time the cluster receives a service 4WD lamp-on message from the FCM, the service 4WD indicator will be illuminated. The indicator remains illuminated until the cluster receives a service 4WD lamp-off message from the FCM, or until the ignition switch is turned to the Off position, whichever occurs first.
- Communication Error - If the cluster receives no messages from the FCM for ten seconds, the service 4WD indicator is illuminated by the instrument cluster to indicate a loss of FCM communication. The indicator remains controlled and illuminated by the cluster until a valid message is received from the FCM.
- 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 bulb check portion of the test to confirm the functionality of the LED and the cluster control circuitry.

The FCM continually monitors the electronic transfer case control circuits and transfer case shift motor/mode sensor assembly to determine the condition of the system. The FCM then sends the proper lamp-on or lamp-off messages to the instrument cluster. For further diagnosis of the service 4WD indicator or the instrument cluster circuitry that controls the LED, .

For proper diagnosis of the FCM, the CAN data bus, or the electronic message inputs to the instrument cluster that control the service 4WD indicator, a diagnostic scan tool is required.