Inside a Car – Coolant Temperature Sensors

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A coolant temperature sensor is confined to the engine of a vehicle and measures the temperature of the vehicle’s engine coolant. The sensor feeds this information back in the form of an electrical current to the engine control unit (ECU). The ECU then responds to a change in the temperature difference and readjusts the engine’s fuel injection.

How Does a Coolant Temperature Sensor Work?

A coolant sensor, or car temperature sensor, is similar to a thermistor; it is based on the working principle that a change in electrical resistance will be the direct product of a change in the temperature of a wire carrying current.

Figure 1 illustrates the main circuitry of the coolant temperature sensor. With a coolant temperature sensor, the full sensor is located in a coolant passage that sits before a thermostat and is connected to the engine control and monitoring unit.

Figure 1. Structural features to the temperature coolant sensor. Image Credit: Schwaller A.E. (2005). Total Automotive Technology. USA, New York: Thomas Delmar Learning.

The coolant temperature sensor is fastened to the intake manifold of an engine coolant and is often referred to as a negative temperature coefficient type of sensor. In terms of resistance capacity, this temperature gauge has 20,000 ohms at 248 degrees Fahrenheit. There is an inverse relationship between the electrical resistance and the engine coolant (i.e. when the engine coolant temperature increases, the electrical resistance decreases and this also causes a voltage drop in the sensor).

A basic functional principle of a coolant temperature sensor begins with the ignition switch. When the ignition switch is ‘on’, a voltage passes through the resistor and heats it up. The resistor is connected to an electrical grid that behaves like a data logger measuring the temperature difference of the resistor.

In the event of air traveling through a vehicle engine, which can come as a result of heavy acceleration, the resistor to the sensor starts to cool down and drops its resistance charge. To control and maintain a balance in the engine temperature, the coolant sensor detects the drop in electrical resistance and emits a voltage to the resistor, a process that is completed in the space of a millisecond.

It is important to monitor engine temperature as this will determine exactly how much fuel is to be injected into the engine and can help maintain good engine performance. The engine control and monitoring unit assist in controlling the temperature of the engine by supplying 5 volts to the circuitry connected to the coolant temperature sensor, which allows for a measurement of change in the voltage drop between the resistor and the coolant temperature sensor.

If you imagine the coolant temperature sensor to be cold (i.e. in the event of light or no acceleration to the vehicle’s engine), the resistance of this sensor will be high. As soon as the driver puts their foot down on the accelerator, the engine will begin to warm up and the resistance of the sensor will drop.

How to Test a Coolant Temperature Sensor

A standard test is performed to test whether the coolant temperature sensor is working accurately. In order for a diagnosis, the ignition is switched off and the coolant temperature sensor connector is disconnected. An ohmmeter (electrical instrument to measure electrical resistance) is attached to the sensor terminal.

The coolant sensor can also be completely removed from the engine and submerged along with a thermometer into a water-filled container. Upon heating the water in the container, the sensor will demonstrate a particular resistance to a temperature change. It is advised to replace the sensor if it does not demonstrate a specified resistance to varying temperatures.

Another approach to measuring the specificity of a coolant temperature sensor is to remove the radiator cap (part of the vehicle’s cooling system) and insert a thermometer into the radiator followed by starting the engine. By running the engine, the coolant begins to warm up and, as soon as the temperature hits 97°C, the fan should start running. If the fan still doesn’t power up, the sensor then requires a full check. In order to check the sensor:

1. The coolant is drained from the engine
2. The ignition coil is removed
3. The electrical connector is disconnected from the sensor 
4. The sensor is dipped along with a thermometer into a container (connected to an ohmmeter) to measure the electrical resistance of this sensor to varying temperature levels (as discussed above).

Measuring the change in resistance is one method of identifying the specificity of a coolant temperature sensor. It is also possible to measure the voltage drop across sensor terminals while running the engine. Table 1 summarizes the engine coolant temperature sensor voltage drop range.

Source: Knowles D. Erjavec J. (2005). TechOne: Basic Automotive Service and Maintenance. USA, New York: Thomas Delmar Learning.

References and Further Reading

• Grainger R. (2008). Mazda MX-5 Miata 1.8: Enthusiast Workshop Manual. UK, Dorset: Rod Grainger & Veloce Publishing.
• Schwaller A.E. (2005). Total Automotive Technology. USA, New York: Thomas Delmar Learning.
• Gilles T. (2012). Automotive Science: Inspection, Maintenance, Repair. USA, New York: Delmar, Cengage Learning.
• Knowles D. Erjavec J. (2005). TechOne: Basic Automotive Service and Maintenance. USA, New York: Thomas Delmar Learning.

This article was updated on 13^th February, 2020.

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