Foreword
Sensors are a crucial component of automotive electronic control systems. With technological advancements, the applications of automotive sensors have become increasingly widespread. Many aspects of a vehicle, including safety, comfort, and fuel economy, rely heavily on sensor operation. To ensure the safe and normal operation of a vehicle, we must be familiar with the working principles and structural components of commonly used automotive sensors. This allows for timely and accurate diagnosis when they malfunction, enabling proper vehicle diagnostics and repair. Below, we will use the automotive coolant temperature sensor as an example to introduce its principles, structure, and detection methods.
The performance of a car's coolant temperature sensor significantly impacts the engine's fuel injection quantity, thereby affecting its combustion performance. When the air-fuel mixture is too rich or too lean, combustion deteriorates, leading to difficulty starting and unstable engine operation. In such cases, the coolant temperature sensor should be checked. To test a car's coolant temperature sensor, it's essential to understand its principles, structure, and components.
1. Principle of automotive water temperature sensor
The engine coolant temperature sensor contains a semiconductor thermistor. The lower the temperature, the higher the resistance; conversely, the higher the temperature, the lower the resistance. It is mounted on the water jacket of the engine block or cylinder head, in direct contact with the coolant. This allows it to measure the engine coolant temperature. The electronic control unit (ECU) uses this temperature change to determine the engine coolant temperature, which is then used as a correction signal for fuel injection and ignition timing. Simply put, we can understand the current operating status of the car—whether it is stopped or moving, or how long it has been moving—by checking the engine coolant temperature.
2. Composition of an automotive coolant temperature sensor
A typical automotive coolant temperature sensor assembly consists of three parts: a gasket, a coolant temperature sensor, and a wire connector, as shown in the figure.
3. Structure of the car coolant temperature sensor
The engine coolant temperature sensor is installed on the water jacket of the engine block or cylinder head, in direct contact with the coolant, and is used to measure the engine coolant temperature. The temperature sensor used in the coolant temperature gauge is a negative temperature coefficient thermistor (NTC), whose resistance decreases as temperature increases. One wire is connected to the electronic control unit (ECU), and the other is grounded, as shown in the figure.
4. The function of the car coolant temperature sensor
When malfunctions such as excessive vehicle load, lack of coolant, incorrect ignition timing, or a malfunctioning fan cause overheating, the engine block temperature will rise, preventing the engine from operating. Therefore, a coolant temperature sensor is designed into the instrument panel. This sensor detects the engine coolant temperature, allowing the driver to visually monitor the coolant temperature under any operating condition and take appropriate action. A coolant temperature sensor is also installed in the electronic control system for fuel injection correction signals.
5. Testing of the car coolant temperature sensor
1. Use a multimeter to test the car's coolant temperature sensor.
(1) In-vehicle inspection. Turn off the ignition switch, disconnect the sensor connector, and use an automotive multimeter set to the Rx1 range to test the resistance between the two terminals of the sensor. Taking the THW and E2 terminals of the Crown 3.0 as an example, the resistance is 4-7kΩ at 0℃; 2-3kΩ at 20℃; 0.9-1.3kΩ at 40℃; 0.4-0.7kΩ at 60℃; and 0.2-0.4kΩ at 80℃. The resistance value of the automotive coolant temperature sensor is inversely proportional to the temperature.
(2) Individual Inspection. Disconnect the car coolant temperature sensor wiring connector, and then remove the sensor from the engine. Place the sensor in a beaker of water and heat the water. As the temperature gradually increases, use a multimeter in resistance mode to measure the sensor's resistance value. Compare the measured value with the standard value. If they do not match, the car coolant temperature sensor should be replaced.
2. Checking the output signal voltage of the car coolant temperature sensor
Install the vehicle coolant temperature sensor and connect the sensor connector. With the ignition switch in the ON position, output voltage should be measured between the "THW" terminal of the connector (Toyota vehicles) or the "THW" terminal of the ECU connector and E2. The measured voltage should change inversely proportional to the coolant temperature. Alternatively, disconnect the coolant temperature sensor wiring harness connector, turn on the ignition switch, and measure the power supply voltage of the coolant temperature sensor; it should be 5V.
3. Check the resistance of the wiring harness connecting the car's coolant temperature sensor and the ECU.
Using a high-impedance multimeter in resistance mode, measure the resistance between the two wiring harnesses connecting the car's coolant temperature sensor and the ECU (the resistance between the sensor signal terminal and the ground terminal and the corresponding two terminals of the ECU). The circuit should be continuous. If the circuit is not continuous or the resistance value is greater than the specified value, it indicates that the sensor wiring harness is open or the connector has poor contact, and further inspection or replacement is required.
Conclusion
There are many types of automotive sensors (automotive sensor applications), which brings certain troubles to car repair. Different sensors have different detection and repair methods, but understanding the principles, structure and detection methods of automotive sensors is very helpful in solving problems. This article takes the automotive water temperature sensor as an example to introduce the principle, structure and detection of the automotive water temperature sensor.
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