Ambient Air Temperature Sensor
The ambient (ATC) or outside air temperature sensor is an (NTC) negative temperature coefficient sensor that informs the semiautomatic/automatic temperature control system of outside air temperature. The NTC sensor's resistance decreases as the outside air temperature increases. The computer uses this input and other in-car temperature sensors to control temperature and blower speed. When this sensor has a problem, performance suffers, and the compressor's clutch may not engage.
Diagnose ambient air temperature sensor symptoms, like an inoperative clutch or input problems, with a scan tool and a multimeter. Factory and many generic scan tools can activate specific components; the compressor clutch is one of them. If the clutch engages with the scan tool, there is likely a problem with one of the inputs. Verify any suspicions with a visual inspection of the sensor and its connector. To test this sensor for resistance, use a multimeter and verify the manufacturer's specifications (~ 220-240 ohms at 70°-80° F).
Automotive air conditioning systems use NTC sensors to monitor air temperature. The In-Car Temperature Sensor (ICTS) is an NTC that monitors the air temperature of the passenger compartment.
The ambient temperature sensor (ATS) monitors the air temperature outside the vehicle. The EATC compares these values and makes output decisions based on the difference between these air temperatures.