Evaporator Temperature Sensor / Switch: Description and Operation

EVAPORATOR TEMPERATURE SENSOR - MANUAL A/C
The evaporator temperature sensor is a thermistor, which is a device that translates temperature into an electrical resistance. Thermistors are connected into electrical circuits in order to determine temperature in critical areas. A thermistor is used in engine control systems, such as Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors. A thermistor is used in the A/C system in order to determine temperatures at the evaporator core. If the evaporator core temperatures drop to approximately 0°C (32°F), frost and ice will start to form on the evaporator fins. Ice formation on the evaporator fins will impede airflow through the evaporator and reduce the system's cooling ability. In order to prevent ice formation on the evaporator fins, the A/C system incorporates a thermistor, that is electrically connected to the A/C compressor control module. When the evaporator temperature drops to 0°C (32°F), the A/C compressor control module reads the corresponding thermistor resistance as a system cutout signal. When the signal is received, the A/C compressor control module shuts down the compressor until the evaporator core temperatures increase to 3°C (37°F).