Auto repair oscilloscope measurement and analysis of automobile engine cooling fan signal

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Auto repair oscilloscope measurement and analysis of automobile engine cooling fan signal [Copy link]

The cooling fan of a car engine is an important part of the vehicle cooling system. If the fan fails, it will lead to insufficient or excessive cooling of the engine, causing the engine working environment to deteriorate, thus affecting the engine's performance and service life. The performance of the fan directly affects the heat dissipation effect of the engine, thereby affecting the engine's performance.

The engine computer generates a duty cycle signal (PWM) according to the target temperature to control the fan speed. The higher the duty cycle signal, the faster the fan speed will be. The fan is mainly powered by the battery directly to the fan controller through a blown fuse.

Let's see how to use an oscilloscope to measure the signal of a car engine cooling fan:

Take a BNC to banana plug cable and connect it to channel 1 of the oscilloscope. Connect the red banana plug to a needle and the black banana plug to an alligator clip for grounding. Insert the red needle into the signal wire on the fan plug (usually the thinnest of the three wires, the other two are one power wire and one ground wire)

Set the oscilloscope's channel attenuation ratio to 1X, the vertical scale to 5V, and the time base to about 10-20ms. To avoid observation interference, you can also turn on the low-pass 30K function to filter out part of the signal.

Some oscilloscopes have built-in car package setting software, which can set various settings of the oscilloscope with one click.

The system changes the fan speed by changing the signal duty cycle. When the fan is stationary, the duty cycle is about 10%; at maximum speed, the duty cycle is about 90%. The fan has a constant voltage power line, and its ground loop is switched/on and off by the car's electronic control unit (ECM). As shown in the figure below, the power supply voltage of the car cooling fan is 14V and the frequency is 100Hz. The car's electronic control unit ECM adjusts the fan speed by changing the pulse width of the square wave.

The cooling fan is controlled by the engine computer receiving signals from various sensors (including intake temperature sensor, accelerator pedal position sensor, throttle position sensor, crankshaft position sensor, engine water temperature sensor, radiator outlet temperature sensor, knock sensor, etc.), and then sending out corresponding duty cycle signals based on the data obtained to control the electronic fan controller.

When all components are working properly, the fan control duty cycle is about 10% - 90%.

The following situations may cause the electronic fan to run at high speed: when the radiator outlet temperature sensor fails, the electronic fan control duty cycle signal is about 90%; insufficient voltage supply in some Audi vehicles can also cause the electronic fan to run at high speed.

The following situations may cause the electronic fan to run at a low speed for a long time: when the throttle valve fails or is too dirty and the throttle valve is open too much; when the accelerator pedal position sensor fails; when the air intake sensor deviates but is still within a reasonable range, creating the illusion of a heavy load.

When the intake air temperature sensor fails or the coolant temperature sensor is open, the fan control duty cycle signal is approximately 77.3%, and the electronic fan runs at a higher speed.

Jacktang

It has built-in car package setting software,,, very powerful

zxopenljx

Thanks for sharing