Negative Temperature Coefficient Sensors
An (ECT) Engine Coolant Temperature sensor is a thermistor, a material that changes electrical resistance along with changes in temperature. It's an NTC or negative temperature coefficient sensor.
The ECT is located in the coolant stream, on or around the thermostat housing. Its job is to sense the temperature of the engine's coolant. When the engine has been sitting for a while, the coolant is cold. The temperature sensor senses this cold temperature and adds resistance to the circuit, creating a difference in voltage between the 5-volt reference and the ground circuit. The ECM makes actuator outputs according to this sensor's input.
As the engine reaches operating temperature, the sensor has less resistance, and there is less difference between the two circuits. The ECM recognizes this input and activates components like the engine's cooling fans to maintain proper operating temperature. The ECM uses different strategies programmed into its lookup tables for hot and cold operating conditions.
This signal will affect EGR flow, enrich fuel mixtures, and delay torque converter, or A/C compressor engagement. Excessive resistance in the connector or anywhere in this circuit alters the signal, increasing injector pulse width and advancing the engine's ignition timing.