Engine Coolant Temperature Sensor: Operation

• The coolant temperature sensor functions to convert the temperature of the engine coolant to voltage, and to input that voltage (as signals) to the engine control unit. The engine control unit, based upon those signals, regulates the amount of fuel injection and the fast idling speed when the engine is cold.
• The 5-volt power supply within the engine control unit is supplied, by way of the resistance within the unit, to the coolant temperature sensor. It passes through the coolant temperature sensor, which is a type of resistor, and is grounded at the engine control unit. Note that the resistance of the coolant temperature sensor decreases when the temperature of the coolant increases. See Fig 1 .
• The coolant temperature sensor terminal voltage becomes higher when the resistance of the coolant temperature sensor increases, and becomes lower when the resistance decreases. Consequently, the intake air temperature sensor voltage varies in accordance with the temperature of the coolant, becoming lower when the temperature of the coolant increases.

Fig 1: Identifying Engine Coolant Temperature Sensor Circuit

Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.