A Audi A6L car with a mileage of about 170 thousand km, equipped with a BDW 2. 4L engine and an automatic transmission, is available in 2006. Owner's response: the brake warning lamp on the dashboard of the car has been flashing and accompanied by buzzing alarm sound. Fault diagnosis: the maintenance technician first verified the fault phenomenon, opened the ignition switch, the brake alarm lamp on the dashboard and the indicator light on the electronic parking brake switch flicker together quickly, which indicates that the system is stored in the current fault code. So the connection fault diagnosis instrument VAS5052 enters the 53 parking brake system, and finds the fault code of the "right rear wheel brake motor electrical fault", but the fault code can be cleared. After removing the fault code, press the electronic parking brake switch to hear the sound of the left rear wheel brake motor work, but at the same time the brake alarm lights on the dashboard start to flicker again, remove the fault code again, press the electronic parking brake switch, listen carefully to the working sound of the right rear wheel brake motor, and can be used at the same time as the switch. To hear a "click" sound on the motor of the right rear wheel brake. The electronic parking brake system of Audi A6L has the protection function, that is, when the system has failed to detect the fault, the system will store the fault code. If the current fault still exists, the system will perform the protection function, that is, the system is no longer working, and the fault code must be cleared to execute the power of the electronic vehicle. Through the previous inspection, the maintenance technician suspects that the line to the right rear wheel parking motor or the motor is defective. So it is decided to check the line first, when the plug of the right rear wheel motor is checked, the plug connection is perfect, the plug is put on the test lamp. At this time, the electronic parking brake switch is pressed, and the working sound of the left rear wheel motor can be heard, but the test lamp on the right rear wheel motor plug is not lit at the end of the light. When the test lamp is not bright, the circuit of the electronic parking brake control unit to the right rear wheel motor should be checked, and the electronic parking brake control unit J5409 is dismantled in the rear right corner of the luggage compartment, and the conductance of the wiring harness is measured in accordance with the circuit diagram. The result is good. Since there is no problem with the line, why is the test lamp not lit? Is it the failure of the control unit J540? At this point, the maintenance technician suddenly remembered that whether the J540 identified the incorrect load and implemented the power failure protection function. In order to verify the conjecture, the maintenance technician took an alternative method to connect the plug line of the right rear wheel brake motor to the left rear wheel brake motor, and the same reason also connected the plug line of the left rear wheel brake motor to the right rear wheel brake motor. After the connection is finished, the fault code is cleared and the electronic parking brake switch is pressed. It is found that the left rear wheel brake motor can work normally, while the right rear wheel brake motor is still not working. It seems that the right rear wheel brake motor is really damaged. Replace the motor of the right rear wheel brake, thoroughly remove the test trouble. Fault summary: through the troubleshooting, it shows that the method used to detect the test lamp in the past is not applicable when repairing a lot of new electronic components, and it must be clear about the working principle and the function of load identification and protection to solve this kind of fault. It is hoped that the general car maintenance personnel should pay great attention to this point.