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Coolant Sensor basics | Coolant Temperature Sensor working

This page describes Coolant sensor basics and mentions Coolant sensor working operation. It is known as Coolant Temperature Sensor. The advantages and disadvantages of coolant sensors are also covered.

Definition :
It is one type of temperature sensor used to measure temperature of the engine coolant. Hence it is known as coolant sensor or engine coolant temperature (ECT) sensor. It is mounted into cylinder head of the engine. It is used to turn on cooling fan or activate emission controls.

The sensor consists of two wires which uses 5 Volt from PCM (Powertrain Control Module) and ground is connected back to PCM. The PCM is control unit used in vehicles. PCM consists of two modules viz. ECM (Engine Control Module) and TCU (Transmission Control Unit). The figure-1 depicts the structure of coolant sensor.


Coolant Sensor structure

Most of the coolant sensors are basically "thermistors" which change resistance as per change in the temperature of the engine coolant. Most of them are of NTC (Negative Temperature Coefficient) type.

Coolant Sensor Connection Diagram

The figure-2 depicts connection of coolant sensor with PCM. As shown sensor functions as variable resistance device similar to thermistor.

Coolant Temperature sensor working operation


Coolant Temperature Sensor Working curve

Let us understand working of Coolant Temperature sensor.
➤As mentioned it is NTC type of sensor. Hence resistance drops when the temperature increases and resistance increases when temperature decreases.
➤Due to above property of the coolant sensor, its voltage drops (due to decrease in internal resistance) as engine warms up or reaches to operating temperature. Moreover voltage increases when engine is cold. This is shown in the curve.
➤As mentioned 5V is connected from PCM to one pin of the sensor and return GND pin back to PCM. Due to change in resistance, voltage gets changed which returns to PCM. This returned voltage is used by PCM to calculate the coolant temperature.
➤The calcution is being applied as per application in the engine to switch ON fan or to activate other emission controls.
➤Examples of coolant temperature sensors with range:
GM coolant sensor: 10000 Ohms resistance at 32oF, 200 Ohms resistance at 200oF
Ford coolant sensor: 95000 Ohms resistance at 32oF, 2300 Ohms resistance at 200oF.

Advantages and disadvantages of Coolant sensor

Following are the advantages of Coolant Sensors:
• They just require two wire measurement of resistance in ohms.
• They provide fast response.
• They are very sensitive and provides large output change based on input temperature.
• They are more accurate compare to RTDs and thermocouples.
• They are inexpensive and available in small sizes.

Following are the disadvantages of Coolant Sensors:
• They have limited temperature range.
• They require current source for its operation.
• They are fragile.
• The output is non-linear function.
• They gets self heated. In order to limit the self heating error, care must be taken to limit the sensing current to lower value.

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