Wait-To-Start Indicator: Operation

The wait-to-start indicator gives an indication to the vehicle operator when the diesel engine glow plugs are energized in their preheat operating mode. 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 Powertrain Control Module (PCM) over the Programmable Communications Interface (PCI) data bus. The wait-to-start 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 wait-to-start indicator for the following reasons:

• Wait-To-Start Lamp-On Message  - Each time the cluster receives a wait-to-start lamp-on message from the PCM indicating that the glow plugs are not properly heated for efficient and reliable engine starting, the wait-to-start indicator will be illuminated. The indicator remains illuminated until the cluster receives a wait-to-start lamp-off message, until the PCM detects the engine is running 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 wait-to-start 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 ambient temperature and the glow plug pre-heater circuits to determine how long the glow plugs must be heated in the pre-heat operating mode. The PCM then sends the proper wait-to-start lamp-on and lamp-off messages to the instrument cluster. If the wait-to-start indicator fails to light during the actuator test, replace the bulb with a known good unit. For further diagnosis of the wait-to-start indicator or the instrument cluster circuitry that controls the indicator, refer to DIAGNOSIS AND TESTING - INSTRUMENT CLUSTER  . For proper diagnosis of the glow plug pre-heater control circuits, the PCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the wait-to-start indicator, a DRBIII(R) scan tool is required. Refer to the appropriate diagnostic information.