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In the Audi car equipped with gasoline engine and automatic transmission, the idle state after cold start, because the engine running resistance is large and the load is larger, if the air conditioning is turned on at the same time, the load will be bigger and the vacuum degree of the intake manifold is reduced, and the brake vacuum booster is not required at this time. The degree of vacuum. If you step on the brake pedal at this time, you need to increase pedal force because of insufficient assistance, otherwise the brake will not be good enough, and the foot pedal force will increase the driving comfort. The vehicle equipped with a manual transmission is also very heavy in the idle state of the engine after cold start, but the wheel drive force is passed to the wheel after lifting the clutch pedal, so there is no need to step down the brake pedal before starting. Therefore, as an integral part of the active safety system, Audi installed an electric vacuum pump to compensate for the lack of brake vacuum due to the above reasons. As shown in Figure 1, when the brake force pressure sensor G294 recognizes that the absolute pressure of the intake pipe increases about 50mbar, the engine control unit J220 will activate the relay J569 of the electric vacuum pump, and the electric vacuum pump begins to operate in order to establish the necessary vacuum degree for the system work; when the vacuum degree in the inlet is reduced to about 30mbar, the electricity is reduced. The moving vacuum pump stops working. When the electric vacuum pump or brake assist pressure sensor fails, the corresponding failure code will be stored in the engine control unit. When testing the brake auxiliary vacuum system in the maintenance work, the "vehicle diagnosis and information system VAS5052" can be used to read the data of the 08 display group of the engine control unit and the 4 display areas are the brake light switch signal, the electric vacuum pump working state (starting and stopping), the brake auxiliary vacuum degree, the system shape. State. When 50mbar, the vacuum pump will be triggered; if the engine control unit has already triggered the vacuum pump and the brake light switch closed, and the absolute pressure of the system setting time (the maximum of about 20s) is still higher than 65 mbar, the fault code "17887, the brake system has a vacuum failure". Note: if the claim for spare parts is involved, do not delete the fault code easily, because sometimes it is difficult to reproduce it in a short time. Case: failure of vacuum pump (1) malfunction: FAW Audi C5 A6 2.4 BDV engine with mileage of 133400km and poor braking after cold start. (2) fault diagnosis: after routine testing of the brake system, the possibility of failure of the brake and parking brake is eliminated. When the engine starts to cool down, the brake pedal is felt heavy, and the braking efficiency is bad. After a few minutes, it will return to normal. The engine control unit detected by VAS5052 stores the fault code: 17887, there is a vacuum fault in the brake system. The test found that the electric vacuum pump did not work. Further examination revealed that there was no power supply at the plug of the electric vacuum pump, and then checked for fuse fuses. After the fuse is replaced, the fuse will be fused again. Then disconnect the plug of the electric vacuum pump and test it with the test lamp instead of the vacuum pump. The fuse does not fuse. The electric vacuum pump was dismantled, and the rotor and bearing of the driving motor were sintered, causing the current in the drive circuit of the motor to be too large to fuse the fuse. Replacement of electric vacuum pump and fuse, troubleshooting.