The electronic control unit needs to know the temperature of the outside air, the cabin air temperature, the temperature of the air-conditioning evaporator, the temperature of the coolant circulating through the heater core and if the vehicle is in bright sunlight.
In most instances the temperature sensors used in the climate control air-conditioning system are thermistors. Although they may vary in appearance, their general operating characteristics are the same.
These are solid state electronic devices that change their resistance in response to temperature. As the temperature of the sensor increases the resistance decreases. They are known as NTC resistors, as they have a negative temperature coefficient.
When connected into a circuit the sensor changes the voltage at the electronic control unit. The electronic control unit reads the sensor voltage as the temperature and compares this reading with information stored in its memory.
The ambient air temperature sensor measures the temperature of the outside air. It is usually located in front of the air-conditioning condenser where the forward motion of the vehicle and the action of the condenser fans forces air over the sensor.
If the outside air is cold the resistance of the ambient air temperature sensor will be high and the voltage at the electronic control unit will also be high.
If the outside air temperature sensor is hot the ambient air temperature sensor resistance will be low and the voltage at the electronic control unit will be low.
The cabin air temperature sensor is mounted in a tube type device called an aspirator. One end of the tube is open to cabin air and the other is connected to the interior fan. Operation of the interior fan creates a flow of air through the aspirator. The temperature of the cabin air flowing through the aspirator controls the signal sent to the electronic control unit.
