Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. ECT sensor is located on the top left of the engine. See Fig 1 . The Powertrain Control Module (PCM) supplies 5 volts to the signal circuit and a ground for the ECT low reference circuit. When the engine coolant temperature is low, the sensor resistance is high. When the engine coolant temperature is high, the sensor resistance is low. The PCM uses this High Side Coolant Rationality test to determine if the ECT input is skewed high. The internal clock of the PCM will record the amount of time the ignition is OFF. At restart the PCM will compare the temperature difference between the ECT and the Intake Air Temperature (IAT). Before failing this test, the PCM will perform a calculation to determine the presence of an engine block heater.

Engine coolant temperature signal is used in the control of most systems the PCM controls (i.e., fuel delivery, ignition timing, idle speed, emission control devices). After a vehicle has been parked overnight, ECT and IAT sensor signals (resistance and temperature) should be close to same reading. An ECT sensor which is out of calibration will not set a Diagnostic Trouble Code (DTC) but will cause fuel delivery and driveability problems. Failure in ECT sensor circuit (open or short to ground) will cause monitored voltage to swing high or low and should set a related DTC.

Fig 1: Locating ECT Sensor

Courtesy of GENERAL MOTORS CORP.