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A BMW 530i car with a mileage of about 190 thousand km, equipped with a M54 engine and a 6HP19 automatic transmission. The BMW sedan car is equipped with a diesel engine. User reflection: the car can't start. Inspection and analysis: after the maintenance personnel arrived at the scene, they simply inquired about the situation. The user reflected that the car was parked as usual and could not start in second days. So the battery was detected, and it was found that the storage capacity was too low. With the aid of the power supply of the rescue vehicle, the start is successful. It is preliminarily determined that there is a leakage phenomenon in the car. After entering the plant, the battery is first charged. After the charging is completed, the ammeter is used to measure the dormant current. Because the BMW vehicle system is too long to switch from the ignition switch to the dormancy, it needs mandatory sleep. Connect the ISID diagnostic instrument, enter the service function - body voltage supply activate static state - send power off command. So the vehicle can get into the dormant state quickly. After the above operation, the dormant current of the faulty vehicle is measured to be 40 mA, normal. However, after 30 min, the engine control unit is awakened, and the main relay is in working state. Measuring the dormancy current at 4A is too large, which is the problem of the vehicle. Re connect ISID for diagnosis, select operation function structure - body power supply diagnosis. After reading the data, ISID automatically draws the conclusion that the vehicle is repeatedly waken up. From the analysis results, the reason that the control system was aroused was data transmission on K-CAN or PT CAN. During the wake-up process, the bus terminal KL.30g is sometimes turned off, so the wakeup source must not be powered by the bus terminal KL.30g. The examination shows that the vehicle can enter the dormancy normally, but it always wakes up unusually. The next task is to find the control unit that enables the bus to produce data in the dormancy state. Measure the 26 leg voltage of the engine control unit connector X60004, which is the wake-up signal voltage provided by the convenient pedaling control unit CAS. The measurement found that when the engine control unit was awakened, the voltage was 0.13 V, indicating that the engine control unit was not awakened by CAS. Exit the 35 and 36 pins (K -CAN) from the gateway control unit connector X13581, then test it. This time, the engine control unit is no longer waken up, indicating that the problem lies in the LAN where K-CAN is located. The problem is simple. As long as the control unit in the K-C A N network is pulled out of the engine control unit, the engine control unit is dormant when the control unit is pulled out, and the problem is on the control unit. According to the above scheme, the abnormal wake-up phenomenon disappeared when the instrument control unit was removed, indicating that the problem was here. Troubleshooting: replacing instrument control unit and troubleshooting.