Coolant Temperature Sensor/Switch (For Computer): Description and Operation

PURPOSE
The Engine coolant temperature sensor is a Thermistor type unit which sends information to the ECM on the temperature of the coolant. This signal is used to determine when the engine has reached operating temperature and affects when the engine mixture control function enters "closed loop."

OPERATION
The engine coolant temperature sensor functions to convert the engine coolant temperature to voltage, and to input that voltage (as signals) to the engine control module. The engine control module, based upon those signals, regulates the amount of fuel injection and the fast-idling speed when the engine is cold.
The 5 V power supply within the engine control module is supplied, by way of the resistance within the unit, to the engine coolant temperature sensor; it passes through the engine coolant temperature sensor, which is a type of resistor, and is grounded at the engine control module. Note that the resistance of the engine coolant temperature sensor decreases when the temperature of the engine coolant increases.
The engine coolant temperature sensor terminal voltage becomes higher when the resistance of the engine coolant temperature sensor increases, and becomes lower when the resistance decreases. Consequently, the engine coolant temperature sensor terminal voltage varies in accordance with the temperature of the engine coolant, becoming lower when the temperature of the engine coolant increases.