Evaporator Temperature Sensor / Switch: Description and Operation

If the evaporator temperature is allowed to cool below 1 °C, condensation can freeze on the evaporator. The ice then insulates it from the air passing over it, and it works much less efficiently. A temperature sensor is used to protect the evaporator from freezing, by signaling the appropriate control module to turn the compressor off, so that condensation cannot freeze on the evaporator The compressor is typically disengaged at 34 °F (1 °C). when the compressor is turned off, refrigerant flow is reduced and the evaporator temperature rises.

The temperature sensor is a Negative Temperature Coefficient (NTC) thermistor whose resistance varies according to the temperature of the evaporator core. Resistance is higher at lower temperatures, and decreases as the temperature rises.