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A Mercedes 600SEL car with a mileage of over 200 thousand km, a M120 engine and an automatic transmission, and a chassis model of W140. User response: in recent years, when using vehicles, you need to start several times to get the car. After that, the malfunction became more and more serious until it could not be started. After receiving the car: charge the battery and start it. The starter is running, but the engine is showing signs of no car. The test fuel pressure is 350kPa, normal. The fire test is carried out, and the high pressure fire is normal. Remove the spark plug and find that the spark plug electrode is soaked with fuel. The engine is a V12 engine with a total of 12 spark plugs. Remove these spark plugs and clean them with compressed air. The residual fuel in the cylinder is sucked out, the spark plug is installed, the engine is running, the idle speed and the acceleration are normal. Connect the diagnostic device for self diagnosis without fault code. View data flow, all data in the standard value range. The measurement charge voltage is 13. 9V. The air conditioning system and headlights are opened, and the charging voltage is kept at 13. 5V, indicating that the charging system is normal. Remove 12 injectors and leak test on the test bench, normal. The car was equipped with two fuel pumps. One of the fuel pumps was found to have a lot of noise in the inspection, indicating severe wear. Replacement of the fuel pump and 12 spark plugs, trial run, failure has not been reproduced. A week later the car was rebuilt for the same fault. The battery is exhausted due to repeated starting and the spark plug is wetted again. After dealing with these problems, the vehicle can start smoothly. The dormancy current of vehicle electrical system is measured. The result is less than 30mA, which shows that there is no leakage problem. In order to find the cause of the failure, the same Mercedes Benz was found and the starting contrast test was carried out. It was found that the starter needed a long time to run, and the engine was running, and the engine speed was low during the starting process. When the starter was dismantled for disassembly inspection, it was found that the carbon brush was seriously worn. Replace the starter, and the failure is completely eliminated. The starting speed is an important condition to ensure the success of the starting. Too low moving speed will result in low pressure of the cylinder, and the mixture is difficult to ignite, and the repeated starting will result in the inundation until the battery circuit is exhausted. The basic idea of the maintenance work is correct, but neglecting the basic testing item of the starter, the result has no one-time troubleshooting, which also explains the importance of the standard maintenance procedure.