Negative Temperature Coefficient Sensors
Negative Temperature Coefficient (NTC) sensors resist the flow of voltage when they’re cold and allow it to flow more freely when hot. The Engine Coolant Temperature (ECT) sensor in the illustration is an example of an NTC sensor. It’s a thermistor, a type of variable resister 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 ground.
As resistance changes, the PCM uses this signal that correlates voltage to temperature. Based on this input it makes output decisions based on engine temperature. If the engine is too hot, the PCM will disengage the compressor.
Automotive air conditioning systems also 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 (ATC) monitors the air temperature outside of the vehicle. The EATC compares these values and makes output decisions based on the difference.