Evaporator Temperature Sensor / Switch: Description and Operation

NOTE: The following applies to RS models only. RG models use an expansion valve mounted evaporator temperature sensor for both manual and automatic temperature control systems due to A/C heater control calibration requirements.

Evaporator Temperature Sensor - MTC System:

The evaporator temperature sensor used for the manual temperature control (MTC) system is installed on the top of the HVAC housing behind the instrument panel and measures the air temperature downstream of the evaporator. The sensor is an electrical thermistor in a plastic housing that is inserted into the HVAC housing. Two terminals within the 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.

Evaporator Temperature Sensor - ATC System:

The evaporator temperature sensor used for the automatic temperature control (ATC) system is installed on the top of the expansion valve in the right rear corner of the engine compartment and measures the temperature of the evaporator coils. 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 si I i cone-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 A/C
evaporator. The sensor will change its internal resistance in response to the temperatures it monitors. The A/C-heater control 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 control decreases. The control uses this monitored voltage reading to an indication of the evaporator temperature. The A/C-heater control 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 A/C compressor clutch as necessary to optimize A/C system performance and to protect the system from evaporator freezing. The external location of the sensor allows the sensor to be removed or installed without disturbing the refrigerant in the system. The evaporator temperature sensor is diagnosed using a DRBIII(R) scan tool. Refer to Body Diagnostic Procedures.