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

Engine Coolant Temperature (ECT) Sensor:

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

OPERATION
The VCM supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the VCM and 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. Engine coolant temperature affects most systems the VCM controls.

The scan tool displays engine coolant temperature in degrees. After engine start-up, the temperature should rise steadily to about 9O°C (194°F) then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other. When the VCM detects a malfunction in the Engine Coolant Temperature (ECT) sensor circuit, the following Diagnostic Trouble Code (DTC)s 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 to check for sensor resistance values relative to temperature. Refer to Specifications / Electrical / Temperature vs. Resistance Table.