Coolant Temperature Sensor/Switch (For Computer): Description and Operation

Engine Coolant Temperature (ECT) Sensor:

The engine coolant temperature sensor is a thermistor (a resistor which changes value based on the temperature) mounted in the engine coolant passage. A low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F) while a high temperature causes a low resistance (70 ohms at 130°C/266°F).

The VCM supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the VCM and then measures the voltage. The voltage will be high when the engine is cold. The voltage will be low when the engine is hot. By measuring the voltage, the VCM calculates the engine coolant temperature. The engine coolant temperature affects most Systems the VCM controls.

The scan tool displays the engine coolant temperature in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F), then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and the intake air temperature displays should be close to each other. When the VCM detects a malfunction in the ECT sensor circuit, the following DTCs will set:
^ DTC P0117 circuit low
^ DTC P0118 circuit high
^ DTC P0125 excessive time to Closed Loop
^ DTC P1114 circuit intermittent low
^ DTC P1115 circuit intermittent high

The above DTCs contain a table in order to check for sensor resistance values that are relative to temperature.