Automotive temperature sensors are used to measure temperature at many places in an automobile, and usually consist of either a thermistor (thermally sensitive resistor), thermocouple, resistance temperature detector (RTD), or infrared device. Thermocouples are usually used for higher temperature measurements (such as exhaust gas temperatures), and infrared sensors are used where direct contact with the object being sensed is undesirable.
Thermocouples operate on the principle that a voltage that changes with temperature can be produced by a junction of two dissimilar metals joined together. Thermocouples can be used when very fast response times are needed.
RTDs rely on certain metals' (usually platinum, nickel, or copper) thermo-resistive behavior to detect changes in temperature. RTDs respond to temperature changes very linearly and can achieve good accuracy.
Thermistors are made from a semiconductor material whose resistance varies with temperature. Thermistors have two variations: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). These types refer to the direction of resistance change with respect to an increase in temperature. For instance, a PTC type's resistance will increase with a rise in temperature. An NTC type will behave in the opposite manner. Thermistors generally have lower temperature limits than a thermocouple or an RTD, but they are less prone to signal interference.
Engine Temperature Sensor
This sensor is installed in the coolant circuit. The engine control module uses its signal to determine the temperature of the engine. The measuring range of this type of sensor is usually from -40 degrees to +130 degrees Celsius.
Air Temperature Sensor
The air temperature sensor is installed in the intake manifold and is used to measure the temperature of the engine intake air. The engine control module uses the air temperature and pressure to calculate the intake air mass. The measuring range of this type of sensor is usually from -40 degrees Celsius to +120 degrees Celsius.
Engine Oil Temperature Sensor
This sensor is used to calculate the temperature of the engine oil. The measuring range of engine oil temperature sensor is generally from -40 degrees to +170 degrees Celsius.
Exhaust Gas Temperature Sensor
The exhaust gas temperature sensor usually employs a platinum measuring resistor. Information from this sensor is used to regulate the exhaust gas recirculation system in an engine. This sensor is mounted in the exhaust system of the engine. The measuring range is usually from -40 degrees to +1000 degrees Celsius.
Fuel Temperature Sensor
Fuel temperature sensors are located along the fuel delivery system, and provide feedback to the PCM as to the state of the fuel system. Temperature ranges of these sensors is between -40 and 160 degrees Celsius.
Transmission Fluid Temperature Sensor
This sensor is located on the valve body of automatic transmissions and is used to measure the temperature of the transmission fluid. The measuring range of this sensor is approximately -40 degrees to 210 degrees.
Tire and Brake Disc Temperature Sensor
These sensors are usually located in an area around the tire envelope, where the sensor can "view" the radiation emitted by either the tire or the brake disc. Since direct contact with these elements is not desirable, infrared temperature sensors are usually employed (see McLaren Electronic Systems link in more information below).
Auxiliary Heater Temperature Sensor
Some auxiliary systems (e.g. seat heaters) that may be installed on certain vehicles will also have an associated temperature sensor to accompany them. These sensors provide feedback necessary to control these systems.
