Oscilloscope measurement and analysis of vehicle speed sensor signal

Speed ​​sensors are widely used in automobiles. They are needed in the control systems of automobile engines, chassis, and body. There are four main types of speed sensors: vehicle speed sensors, wheel speed sensors, deceleration sensors, and yaw rate sensors. The speed sensor detects the speed of the electronically controlled vehicle. The control module uses this input signal to control the engine idle speed, the torque converter lock of the automatic transmission, the automatic transmission shift, the opening and closing of the engine cooling fan, and other functions such as cruise control. The speed sensor of some automatic transmission models is also called the transmission output shaft speed sensor. The speed sensor of some vehicles uses the wheel speed sensor of the ABS control system to obtain the vehicle speed signal through conversion. The sensors mainly include reed switch type, electromagnetic induction type, photoelectric type, Hall effect type, and magnetoresistive element type.

The vehicle speed sensor is usually installed in the drive axle housing or transmission housing, and the vehicle speed sensor signal line is usually installed in a shielded jacket. This is to eliminate the electromagnetic and radio frequency interference generated by high-voltage wires and car phones or other electronic devices, to ensure that electronic communications are not interrupted, and to prevent driving performance deterioration or other problems.

Let's see how to use an oscilloscope to measure the car speed sensor signal. First, place the jack on a stable and level ground, lift the vehicle's drive wheel, and remove the wheel.

Connect a BNC to banana cable to any channel of the oscilloscope, and connect a needle to each of the red and black banana plugs. You can see that there is a wire harness on the wheel, and the wire going up is connected to the sensor. Insert the red needle into the signal wire of the sensor, and the black needle into the ground wire of the sensor.

The oscilloscope's channel coupling can be changed to AC coupling to remove DC signals, and the channel attenuation ratio should be adjusted to 1X. The signal of the speed sensor is generally a pulse signal one after another, so the trigger mode of the oscilloscope should be set to edge rising edge trigger. A ground wire that is too long will generally introduce interference, so it is best to turn on the low-pass 30KHz filter function for the oscilloscope. Since different vehicle speeds will cause changes in signal frequency, the oscilloscope's time base and vertical gear can be adjusted according to actual conditions after the signal is input.

Start the engine, shift to any gear, and when the wheels start to turn, the oscilloscope will show a series of square wave signals, the amplitude of which is usually equal to the supply voltage of the sensor, and the pulse interval and shape are consistent. As the vehicle speed increases, the frequency of the signal will become faster, and the number of pulses on the oscilloscope screen will increase, but the duty cycle and amplitude of the signal will not change.

In addition to ensuring that the frequency of the waveform is synchronized with the vehicle speed and that the duty cycle does not change, observe the consistency of the waveform, check the consistency of the top and bottom corners of the waveform and the amplitude, and the waveform height should be equal. This is because the power supply voltage to the sensor is constant. Some examples show that there are gaps or irregularities at the bottom or top of the waveform. The key is that the stability of the waveform remains unchanged. If the waveform is too high to the ground potential, it means that the resistance is too large or the sensor is poorly grounded. Also observe the waveform anomalies caused by the occurrence of driving performance problems and the appearance of fault codes, so that signal problems directly related to the root cause of the reflected fault or driving performance failure can be determined.