Evaporator Temperature Sensor / Switch: Description and Operation

A/C Refrigerant (Evaporator) Temperature Sensor

The A/C refrigerant temperature sensor is a thermistor that translates temperature into an electrical resistance. Thermistors connected to electrical circuits in order to determine temperatures in critical areas. Thermistors are used in engine controls systems such as Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors. Thermistors are used in A/C systems in order to determine temperatures at the evaporator core. If the evaporator core temperatures drop to about 0°C (32°F), frost ice will start to form on the evaporator fins. Ice on the evaporator fins will seriously impede airflow through the evaporator and will reduce the ability of the system to cool the vehicle interior.

In order to prevent ice from forming on the evaporator fins, the system incorporates a thermistor that is electrically connected to the A/C compressor control module. When the evaporator temperatures drop to 0°C (32°F), the A/C compressor control module reads the corresponding thermistor resistance as a signal in order to shut down the system. When the A/C compressor control module receives the signal, the module shuts down the compressor until the evaporator core temperatures increase to 3°C (37°F).