Evaporator Temperature Sensor
The evaporator temperature sensor is a temperature sensing element located at the coldest point on the face of the evaporator. The evaporator temperature sensor prevents condensate water on the evaporator coil from freezing which can block airflow. The probe is a thermistor inside a metal tube which is wedged between the evaporator fins. The metal tube is tightly held by the evaporator fins so that the thermistor in the tube will detect the temperature of the evaporator. The thermistor will change resistance as the temperature changes.
The Body Control Module (BCM) will send a Programmable Communications Interface (PCI) bus message to the Powertrain Control Module (PCM), which will check engine, coolant temperature, and refrigerant pressure temperature before turning on the A/C compressor clutch. Turning on the A/C compressor clutch will allow the system to cool the evaporator. The BCM will send a message to the PCM when the evaporator temperature becomes too cold. The PCM will then turn off the A/C Compressor Clutch, before evaporator freeze up occurs. The DRBIII(R) scan tool can be used to monitor this operation. The temperature set point at which the clutch is turned off varies with the outside ambient temperature. The temperature at which the clutch is turned on is preset to 2°F above the OFF set point mentioned above. See Fig 1 for the correct set point.