Coolant Sensor Basics: Working, Advantages, and Disadvantages

This article explains the basics of coolant sensors, also known as coolant temperature sensors, and how they operate. We’ll also cover their advantages and disadvantages.

What is a Coolant Sensor?

Definition: A coolant sensor is a type of temperature sensor used to measure the temperature of the engine coolant. It’s often called an Engine Coolant Temperature (ECT) sensor.

The sensor is mounted into the cylinder head of the engine and is used to turn on the cooling fan or activate emission controls. The sensor typically has two wires, using a 5-volt signal from the PCM (Powertrain Control Module) with the ground connected back to the PCM. The PCM is the vehicle’s control unit, consisting of two modules: the ECM (Engine Control Module) and the TCU (Transmission Control Unit).

Figure 1: Coolant Sensor Structure

Most coolant sensors are thermistors, meaning their resistance changes with the temperature of the engine coolant. Most of them are NTC (Negative Temperature Coefficient) type.

Figure 2: Coolant Sensor Connection Diagram

The figure above depicts the connection of the coolant sensor with the PCM. As shown, the sensor functions as a variable resistance device, similar to a thermistor.

Coolant Temperature Sensor Working Operation

Figure 3: Coolant Temperature Sensor Working curve

Let’s understand how a coolant temperature sensor works:

• NTC Type: As mentioned, it’s an NTC type sensor. This means its resistance drops as the temperature increases, and resistance increases as the temperature decreases.
• Voltage Changes with Temperature: Due to the NTC property, the sensor’s voltage drops (due to a decrease in internal resistance) as the engine warms up to its operating temperature. Conversely, the voltage increases when the engine is cold. This is shown in the curve above.
• PCM Connection: A 5V signal from the PCM is connected to one pin of the sensor, and the ground (GND) pin returns to the PCM. The change in resistance alters the voltage, which is then sent back to the PCM.
• PCM Calculation: The PCM uses the returned voltage to calculate the coolant temperature.
• Application: The calculated temperature is used in the engine management system to switch on the cooling fan or activate other emission controls as needed.
• Examples:
  • GM coolant sensor: 10000 Ohms resistance at 32°F, 200 Ohms resistance at 200°F
  • Ford coolant sensor: 95000 Ohms resistance at 32°F, 2300 Ohms resistance at 200°F

Advantages and Disadvantages of Coolant Sensors

Following are the advantages of Coolant Sensors:

• Simple two-wire resistance measurement (in ohms).
• Fast response time.
• High sensitivity, providing a large output change based on input temperature.
• More accurate compared to RTDs and thermocouples.
• Inexpensive and available in small sizes.

Following are the disadvantages of Coolant Sensors:

• Limited temperature range.
• Requires a current source for operation.
• Fragile.
• Non-linear output function.
• Subject to self-heating. To limit self-heating errors, it’s crucial to limit the sensing current to a lower value.