How to detect the signal of the car water temperature sensor

The inside of the car water temperature sensor is the car water temperature sensor. The lower the temperature, the greater the resistance; conversely, the smaller the resistance. It is installed on the water jacket of the engine cylinder block or cylinder head and is in direct contact with the cooling water. Thus, the temperature of the engine cooling water can be measured. A positive temperature coefficient sensor such as PT1000 can also be used.

Its function is to provide an analog signal of temperature change to the engine control unit. Its power supply voltage is 5V power provided by the control unit, and the signal returned to the control unit is a linear change signal of 1.3V-3.8V. Its main function is to tell the engine control unit what the current temperature is.

The water temperature sensor is connected to the AD acquisition port in the VCU with a pull-up resistor. The resistance value of the variable resistor and the temperature sensor can be obtained through the resistor divider voltage at the signal end, and the corresponding temperature can be obtained by looking up the table.

The water temperature sensor is also called the coolant temperature sensor. It is generally a 2-wire system. Its main uses are:

Provide engine coolant temperature parameters to the engine management system controller (ECM). This temperature parameter can control the fan connector to control the engine's cooling fan.

The water temperature signal is an important parameter for calculating calibration settings such as air/fuel ratio (air-fuel ratio), ignition advance angle (ignition timing), etc.

Cooling water, intake air, exhaust air, air inside and outside the vehicle, engine oil

How is the signal from the water temperature sensor detected?

The water temperature sensor is mainly a thermistor, which has two types of temperature coefficients: positive and negative. The positive temperature coefficient means that the higher the water temperature, the greater the resistance, while the negative temperature coefficient means that the resistance of the water temperature sensor decreases when the water temperature rises. The water temperature sensor used in cars has a negative temperature coefficient.

In practical applications, the water temperature sensor is a two-wire system, one 5V voltage comes out from the computer, and the other is grounded through the computer. When testing, there will be a pull-up resistor inside the computer, connected to a 5V voltage, and the water temperature sensor will form a series circuit with the pull-up resistor. This circuit has the function of constant current and voltage division. In other words, the total current of this circuit is constant, and the sum of the voltages on the two resistors is equal to the total voltage.

Therefore, once the water temperature sensor is plugged in, the thermistor and the pull-up resistor will distribute the 5V voltage according to the resistance values ​​on both sides. Because the resistance on the pull-up resistor is basically constant, and the water temperature sensor is a thermistor, when the water temperature changes, the resistance on the sensor will change, so the water temperature signal is displayed below the pull-up resistor.

The following figure is a PT1000 temperature sensor:

Measuring temperature: -50~150℃

Accuracy: ±1%FS

Wiring definition: 1-Signal output, 2.3-Null, 4-Signal output

Supplement: Pull-up resistor

Pull-up is to clamp an uncertain signal at a high level through a resistor, and the resistor also acts as a current limiter. Pull-up is the input current of the device. When a pull-up resistor is added to the IO port of the microcontroller, the voltage can be clamped at a high level. If a variable resistor is added to the output input end of the signal to form a voltage divider circuit, the resistance changes, and the voltage at the signal end will also change accordingly.

Reasons why the temperature sensor needs to be connected to a pull-up resistor:

This resistor is usually relatively large, such as 5K. Its function is to pull up the temperature sensor to a high level if it is open or not connected, thereby protecting the subsequent circuits.

The data pin of the sensor does not correspond to the pin level of the microcontroller, so a pull-up resistor is connected to increase the pin input and output level capabilities.