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

The coolant sensor is a thermistor (a resistor which varies resistance inversely with a change of temperature) mounted in the engine coolant stream. As coolant temperature increases, sensor resistance decreases. The temperature sensed is used by the ECM to determine when to go into closed loop mode. The information from this sensor is also used to change the open loop operation for fuel delivery (M/C solenoid control). The ECM provides a 5.0 volt source to the coolant sensor and measures the voltage in the sensor circuit. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the ECM knows the engine coolant temperature. Engine coolant temperature affects most systems the ECM controls.