Teknikmobil.com – The ECT sensor or engine coolant temperature sensor is a temperature variable resistor used in car cooling systems . These sensors usually have a negative temperature coefficient. The sensor is a two-wire thermistor submerged in the radiator coolant water which is capable of measuring its temperature. The ECU will use the signal from this sensor as the main correction factor when calculating advance and injection duration.
How the ECT Sensor Works
(Credit: Rick’s Free Auto Repair Advice )
The working process is to convert ECT resistance variations into voltage variations for further processing by the ECU. The ECT sensor is connected in a circuit that has a reference voltage of +5 volts. When the engine is cold and at a temperature of around 20 degrees Celsius, the sensor resistance is between 2000 ohms and 3000 ohms. After the engine is running, the radiator water temperature will increase. The ECT will also gradually heat up and resistance decreases proportionally. When the temperature reaches 90 degrees Celsius, the ECT resistance will be in the range of 200-300 ohms.
Therefore, the variable voltage signal depends on the radiator water temperature to be sent to the car’s ECU.
Types of ECT Sensors
- With a negative temperature coefficient. This is the most popular sensor used in cars. The resistance of the sensor decreases with increasing temperature.
- With positive temperature coefficient. Used in some older systems, such as Renix. Here the voltage and resistance increase with increasing temperature.
Testing using AVO-Meter
The following are the steps for testing the ECT sensor using the AVO-Meter .
- Unscrew the protective rubber cap on the sensor connector.
- Connect the negative cable of the voltmeter to ground or chassis.
- Determine which is the signal terminal and the negative terminal of the sensor.
- Connect the voltmeter’s positive lead to the ECT signal terminal.
- Start with the engine cold.
- Depending on the temperature, the voltage reading should be in the 2V to 3V range. The relationship between voltage and temperature is shown in bold below.
- Check whether the ECT voltage signal corresponds to the temperature. You will need a thermometer for this purpose.
- Start the engine and leave it until the engine operating temperature is reached. During engine warm-up, the voltage should decrease according to the values given in Table.
- A common problem is that the output resistance (and voltage) changes erroneously outside its normal range. The normal value of the ECT sensor voltage is 2V on a cold engine and 0.5V on a ‘warm’ engine. A defective or damaged sensor can indicate a voltage of 1.5V when the engine is cold and 1.25V when the engine is warm, causing difficulty starting a cold engine and a rich fuel mixture when the engine is warm. This will not generate any error codes (if the onboard controller is not programmed to detect voltage changes) because the sensor continues to operate within its design parameters. If such conditions are found, the ECT sensor must be replaced.
- If the ECT voltage signal is equal to 0V (lack of power supply or there is a short circuit to ground) or if it is 5.0V – this indicates the presence of a short circuit.
Table 1.
The test above is a test for the negative temperature coefficient type ECT sensor which is most widely used in cars.