Negative Temperature Coefficient Sensors
(NTC) Negative Temperature Coefficient sensors resist the flow of current/voltage when they're cold and allow it to flow more freely when heated. The Engine Coolant Temperature (ECT) sensor in the illustration is an example of an NTC sensor. It's a thermistor, a type of variable resistor that changes resistance with temperature. Notice it has two wires that lead to the PCM; one is its 5-volt reference signal, and the other is the ground.
The PCM uses this signal to operate actuators like the engine cooling fan based on engine temperature. If the engine is too hot, the PCM will disengage the air conditioner's compressor.
Automotive air conditioning systems also use NTC sensors to monitor in-car air temperature. The (ICTS) In Car Temperature sensor is an NTC that monitors the air temperature of the passenger compartment. The (ATC) ambient temperature sensor monitors the air temperature outside of the vehicle. The EATC compares these values and makes output decisions based on the difference.