Evaporator Temperature Sensor / Switch: Description and Operation

Fig.18 Evaporator Temperature Sensor:

The evaporator temperature sensor is a switch that is installed on the top of the expansion valve in the right rear corner of the engine compartment. The sensor has a small probe that is inserted in a small well in the body of the expansion valve that is filled with a special silicone-based thermal grease. A small molded plastic push-in retainer secures the sensor to a threaded hole in the top surface of the expansion valve. Two terminals within a molded plastic connector receptacle on the sensor connect it to the vehicle electrical system through a take out and connector of the HVAC wire harness.

The evaporator temperature sensor cannot be adjusted or repaired and, if faulty or damaged, it must be replaced.

The evaporator temperature sensor monitors the temperature of the evaporator through its connection to the top of the expansion valve. The sensor will change its internal resistance in response to the temperatures it monitors. The heater-A/C control module is connected to the sensor through a sensor ground circuit and a sensor signal circuit. As the evaporator temperature increases, the resistance of the sensor decreases and the voltage monitored by the module decreases. The module uses this monitored voltage reading to an indication of the evaporator temperature. The heater-A/C control module is programmed to respond to this input by sending electronic messages to the Powertrain Control Module (PCM) over the Programmable Communications Interface (PCI) data bus, and the PCM then cycles the air conditioning compressor clutch as necessary to optimize air conditioning system performance and to protect the system from evaporator freezing. The external location of the sensor and the use of a screw allows the sensor to be removed or installed from the expansion valve without disturbing the refrigerant in the system. The evaporator temperature sensor is diagnosed using a DRB III scan tool. Refer to the appropriate diagnostic information.