Four Wheel Drive Indicator Lamp: Description and Operation

SHIFT INDICATOR (TRANSFER CASE)
A part time indicator is standard equipment on all instrument clusters. However, on vehicles not equipped with the standard equipment Selec-Trac four-wheel drive system, this indicator is electronically disabled. The part time indicator is located near the right edge of the instrument cluster, to the right of the speedometer. The part time indicator consists of the words "PART TIME 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 indicator from being clearly visible when it is not illuminated. The words "PART TIME" appear 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. The part time indicator lens is serviced as a unit with the instrument cluster lens, hood and mask unit.

The part time indicator gives an indication to the vehicle operator whenever a part time operating mode of the four-wheel drive transfer case is selected. On vehicles with the Selec-Trac four-wheel drive system, the part time indicator illuminates when the transfer case is engaged in the "4 X 4 Part Time" or "4 Lo" positions. 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 part time 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 bulb will always be off when the ignition switch is in any position except On or Start. The bulb only illuminates when it is provided a path to ground by the instrument cluster transistor. The instrument cluster will turn on the part time indicator for the following reasons:

- Part Time Lamp-On Message - Each time the cluster receives a part time lamp-on message from the PCM indicating that a part time position of the four-wheel drive transfer case has been selected, the part time indicator will be illuminated. The indicator remains illuminated until the cluster receives a part time lamp-off message from the PCM or until the ignition switch is turned to the Off position, whichever occurs first.

- Actuator Test - Each time the cluster is put through the actuator test, the part time 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 transfer case switch to determine the driveline operating mode. The PCM then sends the proper part time lamp-on and lamp-off messages to the instrument cluster. If the part time indicator fails to light during the actuator test, replace the bulb with a known good unit. For further diagnosis of the part time indicator or the instrument cluster circuitry that controls the indicator, refer to Instrument Cluster Diagnosis and Testing. For proper diagnosis of the transfer case switch, the PCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the part time indicator, a DRBIII scan tool is required. Refer to the appropriate diagnostic information.