Year model: 2020
Use a diagnostic instrument to detect the fault code in the PSM control unit (commonly known as ABS anti-lock braking system): 011D03 Mechanical failure of the right front wheel speed sensor, other control unit fault codes are the accompanying fault codes caused by the PSM system fault, and other faults when the instrument is on, the light is caused by a malfunction of the PSM.
The technician replaced the right front wheel speed sensor based on the previous maintenance experience, cleared the fault code, and tested the car. The customer drove the car away soon and returned to the shop to say that the fault light was off, but the braking system was still faulty. During the road test with the customer, it was found that the vehicle occasionally had a phenomenon when braking. It felt that the brake was automatically released when the vehicle was about to stop, and then braked again after 1 second. The customer said that there was no such feeling before, and it was only after the fault light was turned on. Connect the diagnostic instrument to the road test to see the data flow and find that the value of the right front wheel speed sensor is occasionally slower than the other wheels, but not all the time. Since the right front wheel speed sensor is a new original accessory, the sensor will not have quality problems. The point of problem detection is to find out what is causing the speed value to be occasionally slow. Use an oscilloscope to see the front left and front right sensor waveforms, as shown in Figure 1.
In Figure 1, the blue waveform is for the right front wheel, and the red waveform is for the left front wheel. From the waveform alone, the voltages of the two waveforms are basically the same, and there is no abnormality in the waveforms, but I feel that the cause of the fault is the signal panel of the sensor. Because there is a problem with the PSM control unit and the circuit, it should be able to report a fault code, and the waveform is also there. Based on the above analysis, it is decided to remove the signal panel of the right front wheel bearing to check the sensor. The cause of the problem was found just after the right front wheel bearing was removed, such as shown in Figure 2.
The reason for the failure is that part of the signal plate has aged and fallen off, because the signal plate is glued to the bearing with a layer of rubber, and now the signal will be slower or weaker when the speed is slow. The new original bearings are shown in Figure 3.
The signal plate of the new bearing is normal and complete. After the installation is completed, the failure phenomenon is gone, and the failure has been solved so far. Next, use an oscilloscope to measure whether the waveform of the right front wheel speed sensor signal panel is different, as shown in Figure 4.
The waveform in Figure 4 is similar to the waveform in Figure 3, but its voltage amplitude is higher than the waveform measured by the old signal panel. The old voltage is about 2V, and the new voltage is 3.2V.
Fault summary: It is the first time that the signal panel has fallen off after repairing a car. This car is still superimposed by two faults, which makes maintenance a little more difficult. However, if you analyze it carefully, you can still find the crux of the problem.