Engine Coolant Temperature (ECT) Sensor

On vehicles in all markets an ECT sensor is located in the water outlet on the cylinder head.

The ECT sensor provides the ECM and the Instrument Cluster (IC) with engine coolant temperature status.

The ECM uses the temperature information for the following functions:

• Fueling calculations
• Limit engine operation if engine coolant temperature becomes too high
• Cooling fan operation.

The IC uses the temperature information for temperature gauge operation. The engine coolant temperature signal is transmitted on the CAN bus by the ECM for use by the IC and other systems.

The ECT sensor circuit consists of a voltage divider which incorporates the NTC (negative temperature coefficient) thermistor within the ECT sensor. The input to the sensor is a 5V reference voltage supplied through a resistor within the ECM. The ground from the sensor is also connected to the ECM which measures the voltage across the sensor. As the coolant temperature rises, the resistance of the sensor decreases and vice versa. The output voltage from the sensor changes as the thermistor allows more current to pass to ground relative to the temperature of the coolant. The ECM calculates the coolant temperature from a map of sensor voltage against temperature.

The ECM adjusts fuel delivery for the measured coolant temperature to ensure optimum drivability at all times. The engine will require more fuel when it is cold to overcome fuel condensing on the cold metal surfaces inside the combustion chamber. To achieve a richer air/fuel ratio, the ECM extends the injector opening time. As the engine warms up the air/fuel ratio is leaned off.

If the ECT sensor fails, the following symptoms may be observed:

• Difficult cold starting
• Difficult hot starting
• Reduced engine performance
• Temperature gage inoperative or inaccurate reading.

In the event of ECT sensor signal failure, the ECM applies a default value of 80°Celsius (176°F) coolant temperature for fueling purposes.