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Fault diagnosis and repair example of Audi A6 ABS
  • Automotive

A Audi A62.8 car reflects the ABS fault warning light on the dashboard of the vehicle. After checking the vehicle, it is found that the fault is true as the user described. After trial confirmation, the function of the ABS system is invalid, and the 4 wheels are locked in emergency braking. It is obvious that the braking of 4 wheels is equivalent to that of the 4 wheels. It is possible to confirm that the braking force of the two wheels is more balanced, so there is little possibility of leakage in the hydraulic system. The ABS system is detected by the connection fault diagnosis instrument V.A.G1552, and 2 fault meanings are found to be the power supply voltage failure of the ABS pump, the right rear wheel speed sensor circuit breaker or the fault code to the positive pole short circuit. According to the hint of the fault code, the author decided to confirm the performance of the executive element, and then used the diagnostic instrument to perform the operation of component diagnosis. During the performance test of the hydraulic pump, the ABS hydraulic pump V39 does not act, and the pedal has no vibration. According to this phenomenon, the author analyzes 3 possible reasons for failure: hydraulic pump V39 damage, relay problem, or hydraulic control unit damage. After that, the author carried out tests on other components. Because of the little difference in braking force between the 4 wheels during the trial run, we skipped it quickly. After that, I was going to read the relevant data to see if I could find anything, so I entered the data stream of the ABS system. The vehicle is lifted, the wheel is rotated with the hand, and the 001 sets of data are observed. The result of the diagnostic instrument shows that the right rear wheel speed is zero. It appears that the wheel speed signal is not received or recognized by the ABS control unit. There are 3 possibilities for such a phenomenon: no signal generation, signal line problem, or control unit damage. To this end, we have carried out the following steps. (1) detection of the right rear wheel speed signal. The wheel speed sensor of the right rear wheel is directly measured by an oscilloscope. The result shows that the amplitude of the voltage increases with the increase of the speed, and the frequency reflects well. (2) detection of the left rear wheel speed signal. Using the oscilloscope to measure the wheel speed sensor of the left rear wheel directly, the results also have signals, but the amplitude of the voltage is not obvious with the increase of the speed, and the frequency reflects well. Due to the failure of the left rear wheel sensor in the control unit of the ABS system, the clearance of the left rear wheel sensor is adjusted, but the waveform is still the same. (3) connect the left rear wheel speed sensor to the signal line of the right rear wheel and read the data with the diagnostic instrument. After connecting, the result shows that the right rear wheel speed is zero. It seems that there is a problem inside the signal line or control unit. For this reason, we decided to test the related wiring harness. After testing, the right rear wheel signal line, the wiring post 15 power supply pin, the battery 30 supply pin and the grounding pin are all normal. According to the above measurement results, the author decides that there is a problem in the V39 relay or hydraulic control unit of the hydraulic pump, but it needs further dismantling. Since the price of BOSCH ABS pump is nearly 10000 yuan, it has decided to dismantle it and try to repair it. So the author opened the hydraulic control unit of the ABS hydraulic pump. After examination, it found that the relay burned and the circuit board was damaged. According to the observed fault phenomenon, the author restored the circuit board with soldering tin to make it turn on, and replaced the damaged internal relay with external relay. V39 is resumed after using the fault diagnosis device to perform the operation of component diagnosis. After that, the ABS control unit was opened. After examination, it was found that the internal pins were connected by tiny wires and attached to 1 pieces of ceramic. After finding the input foot of the right rear wheel, it is found that the very thin wire is broken. To this end, the author uses a wire to weld it well. After using the diagnostic instrument to read the data again, the right rear wheel signal returns to normal, and the problem of signal waveform difference is no longer there.

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