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

Engine Coolant Temperature Sensor

Description
The engine coolant temperature is determined by the use of a temperature sensor, fitted in the cylinder head. The sensor is a temperature dependent resistor (thermistor), i.e. the resistance of the sensor varies with temperature. The thermistor is an Negative Temperature Co-efficient (NTC) type which means that the sensor resistance decreases as the sensor temperature increases.

The sensor forms part of a voltage divider chain with an additional (pull-up to 5 V) resistor in the ECM. The voltage from this network changes as the sensor resistance changes, and hence the voltage measured by the ECM is related to the coolant temperature.

The coolant sensor information is used to adapt the fuelling and timing level of the engine with respect to engine temperature. This is to maintain performance and emissions as the engine temperature varies and is also used to ensure a good quality of engine start.

The data is also used by the instrument pack for the control of the coolant temperature gauge. This data is transmitted via CAN.

A fault condition is recognised if the ECT exceeds a minimum or maximum threshold, or the closed loop enable temperature has not been achieved within a temperature dependant time threshold.