Fault phenomenon: A BMW 335L sedan with approximately 120,000km on the clock and an N55 engine, the car would not start after being parked overnight.
Diagnostic analysis: Maintenance personnel test found that the car in the lock after the ignition switch lights have not been extinguished, and the eccentric shaft servo motor will issue every once in a while “click” sound, like opening the ignition switch after the system self-test sound. Measuring the hibernation current waveform, it was found that when the servo motor was working, the current reached 10A (Fig. 1). 1h later the vehicle was still in the wake-up state.
Considering that the servo motors have been repeating the self-test action, this should fall under the control vein mess. So disconnect the engine control unit to observe, and find that the vehicle still can not enter the dormant state. Pull out the fuses one by one to observe, and finally find that as long as the convenient pedal car control unit is working, the vehicle can not enter the dormant state.
To further isolate the point of failure, the PT CAN and F CAN buses on the Junction Box Control Unit JBE were disconnected to measure the signal waveform of the K-CAN bus alone. It was found that the K CAN bus signal waveform remained unchanged, while the signal quickly changed to DC after locking the car in a normal car. Disconnect the control units on the K CAN one by one, and found that when disconnecting the roof control unit FZD (as shown below), the system finally went into hibernation.
However, the FZD is not the final point of failure. A closer look at the wiring diagram revealed that the FZD is also connected to the car’s anti-theft monitoring sensor via K CAN (below). After disconnecting this sensor, the system was able to go to sleep normally, so this is where the real problem lies.
Troubleshooting: Replace the anti-theft monitoring sensor in the car, and test drive to confirm troubleshooting.
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