Engine Coolant Temperature (ECT) Sensor OPERATION The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance low), the ECM monitors a low signal voltage which it interprets as warm engine.
TESTING
Fig. 1: Submerge the end of the temperature sensor in cold or hot water and check the resistance 
Fig. 2: Temperature sensor resistance values 
Remove the ECT sensor from the vehicle.
Immerse the tip of the sensor in container of water.
Connect a digital ohmmeter to the two terminals of the sensor.
Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance illustration.
Repeat the test at two other temperature points, heating or cooling the water as necessary.
If the sensor does not meet specification, it must be replaced.
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