The coolant temperature sensor is used to measure the temperature of the engine coolant of an internal combustion engine. The readings from this sensor are then fed back to the Engine control unit (ECU). This data from the sensor is then used to adjust the fuel injection and ignition timing. On some vehicles the sensor may be used to switch on the electronic cooling fan. The data may also be used to provide readings for a coolant temperature gauge on the dash. The coolant temperature sensor works using resistance. As temperature subjected to the sensor increases the internal resistance changes. Depending on the type of sensor the resistance will either increase or decrease.
There are two common types of coolant temperature sensors in use on automotive engines. Negative Temperature coefficient (NTC) and Positive temperature coefficient(PTC). The difference between the two is when the sensor is exposed to heat. In the case of Negative temperature coefficient sensor the internal Electrical resistance will decrease as it is exposed to more heat, whilst the opposite is true in a Positive temperature coefficient sensor. Most Automotive coolant temperature sensors are NTC sensors.
The ECU sends out a regulated reference voltage typically 9 volts to the Coolant Temperature Sensor, through the sensor where the voltage is decreased in relation to the internal resistance within the sensor which varies with temperature. This voltage is then returned to the ECU via the signal wire. The ECU is then able to calculate the temperature of the engine, and then with inputs from other engine sensors uses lookup tables to carry out adjustments to the engine actuators.