Temperature Gauge: Description and Operation

An engine coolant temperature gauge is standard equipment on all instrument clusters. The engine coolant temperature gauge is located in the lower right quadrant of the instrument cluster, below the oil pressure gauge. The engine coolant temperature gauge consists of a movable gauge needle or pointer controlled by the instrument cluster circuitry and a fixed 90 degree scale on the cluster overlay that reads left-to-right from "C" (or Cold) to "H" (or Hot) for gasoline engines except SRT-10. On SRT-10 models, the scale reads from "60"° C to "120"° C in markets where a metric instrument cluster is specified, or from "140"° F to "260"° F in all other markets. On vehicles with a diesel engine, the scale reads from "60"° C to "120"° C in markets where a metric instrument cluster is specified, or from "140"° F to "245"° F in all other markets. An International Control and Display Symbol icon for "Engine Coolant Temperature" is located on the cluster overlay directly below the left end of the gauge scale.

On all models except SRT-10 the engine coolant temperature gauge graphics are black against a white field except for two red graduations at the high end of the gauge scale. On SRT-10 the gauge graphics are black against a silver field except for a single red zone at the high end of the gauge scale. In either case, the gauge graphics are clearly visible within the instrument cluster in daylight.

When illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting with the exterior lamps turned On, the black gauge graphics appear blue-green and the red graphics still appear red. The gauge needle has internal optical illumination. Gauge illumination is provided by replaceable incandescent bulb and holder units located on the instrument cluster electronic circuit board. The engine coolant temperature gauge is serviced as a unit with the instrument cluster.

The engine coolant temperature gauge gives an indication to the vehicle operator of the engine coolant temperature. This gauge is controlled by the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) on vehicles equipped with a gasoline engine, or from the Engine Control Module (ECM) on vehicles equipped with a diesel engine over the Programmable Communications Interface (PCI) data bus.

The engine coolant temperature gauge is an air core magnetic unit that receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (run-start) circuit whenever the ignition switch is in the On or Start positions. The cluster is programmed to move the gauge needle back to the low end of the scale after the ignition switch is turned to the Off position. The instrument cluster circuitry controls the gauge needle position and provides the following features:
- Engine Temperature Message - Each time the cluster receives a message from the PCM or ECM indicating the engine coolant temperature is between the low end of normal [about 54° C (130° F) for gasoline engines except SRT-10, or about 60° C (140° F) for SRT-10 and diesel engines] and the high end of normal [about 122° C (252° F) for gasoline engines, about or about 116° C (240° F) for diesel engines], the gauge needle is moved to the actual relative temperature position on the gauge scale.
- Engine Temperature Low Message - Each time the cluster receives a message from the PCM or ECM indicating the engine coolant temperature is below the low end of normal [about 54° C (130° F) for gasoline engines except SRT-10, or about 60° C (140° F) for SRT-10 and diesel engines], the gauge needle is held at the graduation on the far left end of the gauge scale. The gauge needle remains at the left end of the gauge scale until the cluster receives a message from the PCM or ECM indicating that the engine temperature is above about 54° C (130° F) for gasoline engines except SRT-10, or about 60° C (140° F) for SRT-10 and diesel engines, or until the ignition switch is turned to the Off position, whichever occurs first.
- Engine Temperature High Message - Each time the cluster receives three consecutive bus messages from the PCM or ECM indicating the engine coolant temperature is above about 122° C (252° F) for gasoline engines, or about 116° C (240° F) for diesel engines, the gauge needle is moved into the red zone at the far right end of gauge scale, the check gauges indicator is illuminated, and a single chime tone is sounded. The gauge needle remains in the red zone and the check gauges indicator remains illuminated until the cluster receives a message from the PCM or ECM indicating that the engine temperature is below about 122° C (252° F) for gasoline engines, or about 116° C (240° F) for diesel engines, 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 check gauges indicator is cycled off and then on again by the appropriate engine temperature messages from the PCM or ECM.
- Communication Error - If the cluster fails to receive an engine temperature message, it will hold the gauge needle at the last indication for about ten seconds, or until the ignition switch is turned to the Off position, whichever occurs first. After ten seconds, the cluster will move the gauge needle to the low end of the gauge scale.
- Actuator Test - Each time the cluster is put through the actuator test, the engine coolant temperature gauge needle will be swept to several calibration points on the gauge scale in a prescribed sequence in order to confirm the functionality of the gauge and the cluster control circuitry.

On vehicles with a gasoline engine, the PCM continually monitors the engine coolant temperature sensor to determine the engine operating temperature. On vehicles with a diesel engine, the ECM continually monitors the engine coolant temperature sensor to determine the engine operating temperature. The PCM or ECM then sends the proper engine coolant temperature messages to the instrument cluster. If the instrument cluster turns on the check gauges 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 ECM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the engine coolant temperature gauge, a diagnostic scan tool is required.