Cooling water temperature sensor structure, characteristics and connection circuit with ECU

The water temperature sensor is installed near the engine thermostat water outlet, and its function is to detect the engine cooling water temperature. Because a certain amount of additional enrichment is required during the engine warm-up process, and the amount of enrichment mainly depends on the temperature, load and speed of the engine, a water temperature sensor is used to transmit the water temperature signal to the ECU.

The structure of the water temperature sensor is shown in Figure a. It consists of a negative temperature coefficient thermistor (NTC resistor) enclosed in a metal box that is very sensitive to temperature changes. It uses changes in resistance value to detect the temperature of the cooling water. The characteristics of the thermistor are shown in Figure b. The lower the cooling water temperature, the greater the resistance value, and the higher the cooling water temperature, the smaller the resistance value. By inputting the signal from this sensor to the ECU, the amount of fuel injection can be controlled based on the cooling water temperature. The connection circuit between the cooling water temperature sensor and the ECU is shown in Figure c.

Cooling water temperature sensor structure, characteristics and connection circuit with ECU

a) Structure of water temperature sensor b) Characteristics of water temperature sensor c) Connection circuit with ECU 1-NTC resistor 2-Shell 3-Wire connector 4-Cooling water temperature sensor 5-Connected to battery terminal 6-Electronic control unit (ECU) 7-Water temperature signal

The 5V power supply voltage in the ECU is added from the terminal THW to the water temperature sensor through the resistor R (the resistor R and the water temperature sensor are connected in series). When the resistance value of the water temperature sensor changes with the cooling water temperature, the potential of the terminal THW also changes. Based on this signal, the ECU increases or decreases the fuel injection amount to improve the cold operation of the engine.

When the engine is started when the external ambient temperature is low, the thermistor resistance of the water temperature sensor is relatively large. At this time, the ECU receives the low-temperature signal and gives the injector more instructions for additional fuel injection, causing the injector to inject more fuel. The injector injects more fuel. When the temperature of the engine cooling water gradually increases, the resistance of the thermistor gradually decreases, so the control unit controls the injector to gradually reduce the additional fuel injection. If the temperature of the engine cooling water reaches above 80°C, the resistance value of the thermistor of the water temperature sensor is about 0.4kΩ. At this time, the ECU controls the injector to perform normal fuel injection without additional fuel injection, and the engine enters normal working status.