Temperature Warning Lamp/Indicator: Description and Operation

An engine temperature indicator is standard equipment only on all instrument clusters. This indicator is located near the upper edge of the instrument cluster, between the minor gauge set and the speedometer.

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 Controller Area Network (CAN) 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. The entire bulb test is a function of the PCM.
- Engine Temperature High Message - Each time the cluster receives a message from the PCM indicating the engine coolant temperature is above about 122° C (252° F), the engine temperature indicator will be illuminated and a single chime tone is sounded. 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. The chime tone feature will only repeat during the same ignition cycle if the engine temperature indicator is cycled off and then on again by the appropriate engine temperature messages from the PCM.
- 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 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 CAN data bus, or the electronic message inputs to the instrument cluster that control the engine temperature indicator, a diagnostic scan tool is required.