Engine Coolant Temperature Sensors

The coolant temperature input to the engine control module is needed to provide fuel enrichment for cold cranking and during the warm-up phase. The sensor signal is also used as a base to provide the additional enrichment needed during cold cranking.

The coolant temperature sensor is an NTC resistor. It is located either in the coolant distribution housing near the thermostat or in the cylinder head. Base on the signal from the sensor the engine control module provides additional enrichment be extending the injector opening time.

Below a designated resistance threshold, normal operating temperature is assumed by the control module and enrichment is discontinued on M1.x systems.

The signal is also used for:
- ignition control
- idle speed control

On more recent systems (M5.x, MS 41.x) the sensor contains two NTC elements. One is the engine temperature input to the engine control module, the other sensor is used for the instrument cluster's temperature gauge.







If a malfunction occurs with the temperature input value, the control module will switch to a stored replacement value (FAILSAFE) for emergency running. The fault will be stored in memory and the Check Engine Lamp will illuminate when the OBD II criteria is satisfied