Temperature Warning Lamp/Indicator: Description and Operation

An engine coolant temperature warning lamp is used only in the low-line instrument clusters. The engine coolant temperature warning lamp is located in the right lower quadrant of the instrument cluster overlay, below the oil pressure warning lamp. The engine coolant temperature warning lamp consists of a red lens behind a round cutout in the instrument cluster overlay An International Control and Display Symbol icon for "Engine Coolant Temperature" is imprinted in black near the center of the red lens. The icon is clearly visible on the face of the instrument cluster when the lamp is not illuminated, and is silhouetted in a bright red glow when the engine coolant temperature warning lamp is illuminated from behind by a replaceable incandescent bulb and bulb holder unit located on the instrument cluster electronic circuit board. The engine coolant temperature warning lamp is serviced as a unit with the instrument cluster.

The engine coolant temperature warning lamp gives an indication to the vehicle operator when the engine coolant temperature is high. This lamp is controlled by the instrument cluster circuit board based upon the cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Chrysler Collision Detection (CCD) data bus. The engine coolant temperature warning lamp bulb receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (st-run) circuit whenever the ignition switch is in the ON or Start positions; therefore, the lamp will always be OFF when the ignition switch is in any position except ON or Start. The lamp bulb only illuminates when it is switched to ground by the instrument cluster circuitry. The instrument cluster will turn ON the engine coolant temperature warning lamp for the following reasons:
- Bulb Test - Each time the ignition switch is turned to the ON position the lamp is illuminated for about three seconds.
- Engine Temperature High Message - Each time the cluster receives a message from the PCM indicating the engine coolant temperature is about 124 °C or higher (about 255 °F or higher), the lamp will be illuminated. The lamp remains illuminated until the cluster receives a message from the PCM indicating that the engine temperature is about 122 °C or lower (about 252 °F or lower), 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 lamp will be turned ON and OFF in a prescribed sequence to confirm the functionality of the lamp and the cluster 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. If the engine coolant temperature warning lamp fails to light during the bulb test, replace the bulb with a known good unit. For further diagnosis of the engine coolant temperature warning lamp or the instrument cluster circuitry that controls the lamp, (Refer to INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING). If the instrument cluster turns ON the lamp after the bulb test, it may indicate that the engine or the engine cooling system requires service. For proper diagnosis of the engine coolant temperature sensor, the CCD data bus, or the message inputs to the instrument cluster that control the engine coolant temperature warning lamp, a DRB III scan tool and the appropriate diagnostic information are required.