WAIT-TO-START INDICATOR
DESCRIPTION
A wait-to-start indicator is only found in the instrument
clusters of vehicles equipped with an optional diesel
engine. This indicator is located above the fuel gauge
and to the left of the speedometer in the cluster overlay.
The wait-to-start indicator consists of a stencil-like cutout of the International Control and Display Symbol icon for
“Diesel Preheat” 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 icon 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 wait-to-start indicator is serviced as a unit with the instrument cluster.
OPERATION
The wait-to-start indicator gives an indication to the vehicle operator when the diesel engine glow plugs are ener-
gized in their pre-heat 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 Con-
trol Module (PCM) over the Programmable Communications Interface (PCI) data bus.
The wait-to-start 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 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 heating and are too cool 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 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, then off again during the bulb check portion of the test to confirm the functionality of the LED and the
cluster control circuitry.
The PCM continually monitors the ambient air temperature and the glow plug pre-heater circuits to determine how
long the glow plugs should be energized in their pre-heat operating mode. The PCM then sends the proper wait-
to-start lamp-on and lamp-off messages to the instrument cluster. For further diagnosis of the wait-to-start indicator
or the instrument cluster circuitry that controls the indicator, (Refer to 8 - ELECTRICAL/INSTRUMENT CLUSTER -
DIAGNOSIS AND TESTING).
For proper diagnosis of the glow plug pre-heater control circuits, the PCM, the PCI data bus, or the electronic mes-
sage inputs to the instrument cluster that control the wait-to-start indicator, a diagnostic scan tool is required. Refer
to the appropriate diagnostic information.
8J - 146
INSTRUMENT CLUSTER
KJ