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

The ECT sensor is located in a coolant passage in the side of the cylinder head near the upper radiator hose. The ECT sensor is a two wire thermistor type sensor that varies resistance according to changes in engine coolant temperature. The PCM supplies a five volt reference through a resistor to the sensor, which is connected to ground. When the sensor is cold it has high resistance (high signal voltage at PCM). As the sensor temperature increases, its resistance decreases (low signal voltage at PCM). The PCM uses the signal voltage to determine engine coolant temperature.

The ECT sensor is used by the PCM for fuel control (calculation of temperature of air/fuel mixture at the intake valve in conjunction with the IAT sensor temperature), Coolant Temp telltale illumination, coolant temperature gauge position and cooling fan relay operation. The PCM also uses the ECT sensor for ignition timing, EGR operation, EVAP purge solenoid control, idle speed control and oil life monitoring.

COOLING FAN MOTOR CONTROL - The PCM controls the cooling fan motor by grounding the cooling fan relay. When the relay coil circuit is grounded, the switch side of the relay closes allowing current to flow to the cooling fan motor.

The PCM operates the cooling fan relay based on the reading from the ECT sensor. Under normal operating conditions, the cooling fan operates whenever the ECT is greater than 105.5°C (226°F) or the A/C relay is commanded On below 113 km/h (70 mph). The cooling fan will turn Off when the temperature drops below 100°C (212°F). If the engine is turned Off and the ECT is above 107°C (225°F), the cooling fan can run up to 4 minutes or until the ECT drops below 107°C (225°F). If a low/high ECT circuit fault (DTC P0117 or P0118) is set, the PCM will command the cooling fan relay On to protect the engine and transaxle until the condition is corrected.




COOLANT TEMP GAUGE - The PCM controls the coolant temperature gauge by sending gauge position messages to the BCM over the Class II link based on ECT sensor readings. The BCM averages the temperature over time allowing the gauge to move slowly and remain in a relatively stable position as temperature varies. The BCM then sends the averaged information to the I/P cluster over the link to position the gauge.

COOLANT TEMP TELLTALE - The Coolant Temp telltale is used to alert the driver that the engine may be in an overheated condition. The PCM controls the Coolant Temp telltale by sending a message to the I/P cluster over the Class II link. The PCM will send the message when the ECT sensor reading is above 118°C (244°F). If this condition exists, the PCM will set DTC P0217. If a high temp/low voltage fault exists in the ECT sensor circuit, the signal is considered invalid and the Coolant Temp telltale will not be illuminated. The Malfunction Indicator Lamp (MIL) and cooling fan relay will be commanded On, however.