Temperature Switch (Warning Indicator): Description and Operation
ENGINE TEMPERATURE INDICATORAn engine temperature indicator is standard equipment only on vehicles equipped with a four-gauge instrument cluster. The engine temperature indicator is located in the Information Center area of the instrument cluster, to the right of center. The engine temperature indicator consists of a stencil-like cutout of the International Control and Display Symbol icon for "Engine Coolant Temperature" 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. A red Light Emitting Diode (LED) behind the cutout in the opaque layer of the overlay causes the icon to appear in red 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 engine temperature indicator is serviced as a unit with the instrument cluster.
The engine temperature indicator gives an indication to the vehicle operator when the engine temperature gauge reading reflects a condition requiring immediate attention. 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 engine temperature 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 engine temperature indicator for the following reasons:
- Bulb Test - Each time the ignition switch is turned to the On position the engine temperature indicator is illuminated for about two seconds as a bulb test.
- Engine Temperature High Message - Each time the cluster receives a message from the PCM indicating the engine coolant temperature is about 122 °C (252 °F) or higher, the engine temperature indicator will be illuminated. The indicator remains illuminated until the cluster receives a message from the PCM indicating that the engine coolant temperature is below about 119 °C (246 °F), 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 engine temperature 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 engine coolant temperature sensor to determine the engine operating temperature. The PCM then sends the proper engine coolant temperature messages to the instrument cluster. For further diagnosis of the engine temperature indicator or the instrument cluster circuitry that controls the LED, (Refer to ELECTRICAL/INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING). If the instrument cluster turns on the engine temperature indicator due to a high engine temperature gauge reading, it may indicate that the engine or the engine cooling system requires service. For proper diagnosis of the engine coolant temperature sensor, the PCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the engine temperature indicator, a DRBIII scan tool is required. Refer to the appropriate diagnostic information.