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The car’s anti-lock brake system is abbreviated as ABS. Its function is to automatically control the braking force of the brakes when the vehicle is braked, preventing the wheels from locking up and keeping them in a state of rolling while slipping (with a slip rate of around 20%), thereby ensuring that the traction between the wheels and the ground is at its maximum. In ABS, the role of the wheel speed sensor is to measure the rotation speed of the vehicle’s wheels. The wheel speed sensor detects the frequency signal (rotation speed signal) of each wheel’s rotation, and then transmits this signal to the ABS computer. When the vehicle speed reaches the set value and an emergency brake is applied, the ABS system starts to function. When the ABS computer controls the wheels to brake and release alternately, the wheel speed sensors transmit signals indicating the distance the tires rotate as they go from being fully braked to rotating, thereby allowing the ABS to regulate braking to achieve the optimal braking distance. The common types of wheel speed sensors are magnetoelectric wheel speed sensors and Hall effect wheel speed sensors. Magnetoelectric wheel speed sensors are designed based on the principle of electromagnetic induction. It has a simple structure, low cost, is resistant to dirt, does not require power supply, and is widely used in ABS anti-lock braking systems. However, it also has some disadvantages, such as a low frequency response; when the vehicle speed is too high, the sensor’s frequency response cannot keep up, leading to erroneous signals. Another issue is the poor ability to resist electromagnetic interference. The waveform measured by the oscilloscope is sinusoidal in shape, and the higher the wheel speed, the greater the amplitude of the output signal voltage. Hall effect wheel speed sensors are constructed based on the principle of the Hall effect; the amplitude of their output signal voltage is not affected by the rotation speed, they have a high frequency response, and they are resistant to electromagnetic interference. However, they require a power supply. We use this sensor as an example for measurement: First, connect a BNC to banana plug cable to one channel of the oscilloscope; the red end is connected to the probe as the positive pole, and it is linked to the sensor’s signal line, while the black end can be connected to a alligator clip or a probe as the negative pole for grounding. Open the oscilloscope channel menu, set the channel attenuation to 1X, adjust the vertical scaling to 1V/div, and set the time base to around 10ms; adjustments can be made later based on actual conditions. Since the Hall effect wheel speed sensor is powered, a DC voltage of around 11-12V should be present in the waveform observed at this time. Lift the vehicle with a jack, rotate the wheel, and use an automotive oscilloscope to measure the waveform of the sensor signal output. A Hall effect sensor consists of a nearly closed magnetic circuit formed by a permanent magnet or magnetic pole. A soft magnetic impeller rotates through the gap between the magnet and the pole; when the openings on the impeller allow the magnetic field to pass through unimpeded to the Hall effect sensor, the magnetic field is interrupted (as the blades serve as the medium for conducting the magnetic field to the sensor). As the impeller opens and closes these openings, it allows the magnetic field to pass through or blocks it, causing the Hall effect sensor to turn on and off like a switch. This is why some automobile manufacturers refer to Hall effect sensors and other similar electronic devices as Hall switches – this device is essentially a switching device. Therefore, the signal waveform of a Hall effect sensor is actually a series of pulses, that is, square waves. The faster the wheel rotates, the higher the frequency of the signal waveform, but its AC voltage value remains unchanged, staying at 0V to 1V. By slowing down the speed of rotation of the wheel, it can be seen that the frequency of the signal waveform also decreases. If this digital ABS sensor continuously outputs only 0 volts, it is necessary to first check whether it is receiving power supply. Then check whether the signal frequency of the sensor matches the wheel speed closely; otherwise, it indicates a fault.