ENGINE COOLANT TEMPERATURE SENSOR (ECT)
General description
The engine coolant temperature sensor is temperature-variable resistor, which usually has a negative temperature coefficient. It is a two-wire thermistor immersed in coolant and measures its temperature. The onboard computer uses the signal of ECT as the main correction factor when calculating the ignition advance and the injection duration.
Appearance
The ECT sensor is shown on fig. 1.
Fig. 1
Working principle of the ECT sensor
In order to convert the ECT resistance variation to voltage variation, which is further processed by the ECU, the ECT sensor is connected in a circuit typically supplied with a reference voltage of +5V. In cold engine and an ambient temperature of 20 ºC the sensor resistance is between 2000Ω and 3000Ω. After the engine start, coolant temperature begins to rise. ECT gradually heats and its resistance reduces proportionately. At 90 ºC its resistance is in the range of 200Ω to 300Ω.
Thereby, a coolant temperature dependent variable voltage signal is send to the onboard computer.
Types of ECT sensors
Procedure for verification the functionality of the ECT sensor
— Sensor with negative temperature coefficient —
Test with voltmeter
Temperature, ºС |
Resistance, Ω |
Voltage, V |
0 |
4800 - 6600 |
4.00 – 4.50 |
10 |
4000 |
3.75 – 4.00 |
20 |
2200 - 2800 |
3.00 – 3.50 |
30 |
1300 |
3.25 |
40 |
1000 – 1200 |
2.50 – 3.00 |
50 |
1000 |
2.50 |
60 |
800 |
2.00 – 2.50 |
80 |
270 - 380 |
1.00 – 1.30 |
110 |
|
0.50 |
Broken circuit |
5.0 ± 0.1 |
|
short circuit to the ground |
0 |
Possible sensor failures:
-- The voltage of the ECT signal terminal is equal to 0V.
-- The voltage of ECT signal terminal is equal to 5.0V
The voltage has such a value when an open circuit is present and may be obtained in one of the following conditions:
-- The voltage signal or the reference voltage is equal to the car battery voltage.
Check short circuit in the wire, connected to the positive terminal of the car battery or the power supply wire.
— Sensor with negative temperature coefficient —
Check with ohmmeter with ECT sensor detached from the car
— Sensor with negative temperature coefficient —
Voltage measurement with oscilloscope
Fig. 2
—Sensor with a positive temperature coefficient —
ECT sensor with a positive temperature coefficient of resistance is a thermistor whose resistance increases with the rising of the temperature. It is used in a small number of systems (mainly in Renault cars).
The overall method of verification is similar to the method for checking a sensor with negative temperature coefficient, described above. Received measurement data can be compared with the data given in Table 2 showing the relationship between the resistance and the temperature of the sensor.
Temperature, ºС |
Resistance, Ω |
Voltage, V |
0 |
254 - 266 |
|
20 |
283 - 297 |
0.6 – 0.8 |
80 |
383 - 397 |
1.0 – 1.2 |
open circuit |
5.0 ± 0.1 |
|
short circuit to ground |
0 |