Evaporator Temperature Sensor / Switch: Description and Operation

The evaporator temperature sensor is a thermistor which is a device that translates temperature into an electrical resistance. Therefore, thermistors are connected into electrical circuits to determine temperatures in critical areas. Thermistors are used in engine control systems such as Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors. They are also widely used in air conditioning systems to determine temperatures at the evaporator core. If evaporator core temperatures drop to about 0°C (32°F), frost and ice will start to form on the evaporator fins. Ice formation on the evaporator fins seriously impedes the airflow through the evaporator and reduces the ability of the system to cool the vehicle's interior.

In order to prevent ice formation on the evaporator fins, this system incorporates a thermistor which is electrically connected to the A/C amplifier. When evaporator temperatures drop to 0°C (32°F), the A/C amplifier reads the corresponding thermistor resistance as a system cutout signal. When this' signal is received, the A/C amplifier shuts down the compressor until evaporator core temperatures increase to 3°C (37°F).