Identification and Detection of Thermistor-Based Water Temperature Sensors 1

1) Identification of the water temperature sensor

The engine coolant temperature sensor, also known as the water temperature sensor, is mostly made of a negative temperature coefficient thermistor, which has a negative temperature coefficient. When the water temperature is low, the resistance value is high, and when the water temperature is high, the resistance value is low. The structure and characteristics of the water temperature sensor are shown in Figure 1. It is typically installed in the water jacket of the engine block or cylinder head, or inside the thermostat, and extends into the water jacket, as illustrated in Figure 2. The water temperature sensor connector has two terminals connected to the ECU, with one being the signal line. The output voltage varies with the thermistor's resistance value, and the ECU measures the engine's water temperature based on the voltage changes. The other terminal serves as the ground.  diode

 Figure 1 Structure and Characteristics of Thermistor-type Engine Coolant Temperature Sensor  

Figure 2 Installation Location of Engine Coolant Temperature Sensor in Beijing Cherokee Automobile

Figure 3 shows the connection circuit between the thermistor-type coolant temperature sensor connector terminals of the Toyota Crown 3.0 sedan and the Beijing Jeep Cherokees and the ECU. The circuit characteristics are illustrated in Figure 3(b). The ECU applies a 5V voltage through a 10kΩ resistor and a parallel circuit consisting of a 10kΩ resistor and a 1kΩ resistor. When the 10kΩ and 1kΩ resistors are connected in parallel, the resulting resistance is 909Ω, approximately 1kΩ. When the engine coolant temperature is below 51.6°C, the 5V voltage passes only through the 10kΩ resistor, ensuring accurate data measurement by the sensor. As the engine coolant temperature rises, the higher resistance of the 10kΩ resistor and the lower resistance of the sensor's thermistor cause the sensor's measured data to become inaccurate. When the engine coolant temperature reaches 51.6°C, the ECU controls the transistor to turn on, allowing the 5V voltage to pass through the parallel resistors to the sensor. This ensures the coolant temperature sensor provides accurate measurements even at high temperatures. The coolant temperature signal is input to the ECU, providing a precise basis for adjusting fuel injection quantity and determining the injection timing.diode

Figure 3 Engine Coolant Temperature Sensor and ECU Connection Circuit

(2) Detection of the water temperature sensor

The performance of the water temperature sensor significantly affects the fuel injection volume of the engine, thereby influencing its combustion efficiency. When the air-fuel mixture is too rich or too lean, the engine's combustion deteriorates, leading to difficulty in starting and unstable operation. In such cases, the water temperature sensor should be inspected. The inspection items are as follows.diode