A BMW E46330i sedan, the user reflects the car in the normal driving time DSC occasionally alarm, and dashboard multi-function information display will display "ENGINESEATSAFEPROG (engine emergency operation mode)" message, then accelerate the pedal failure. If you turn off the engine and then restart the vehicle, the vehicle will return to normal. Carefully asked the user that the failure in the last six months has appeared five times.
    After receiving the car, connect the fault diagnosis instrument GT1 to detect the vehicle, in the dynamic stability control (DSC) system control unit stored in the meaning of "engine torque interface failure, failure reason is not DSC" fault code; stored in the engine control unit The meaning of the "idle control valve fuck card" fault code. The two fault codes are accidental faults, and the equipment indicates that the possible cause of the fault is divided into: ① the idle control valve is mechanically stuck; ② the idle control valve line fault; ③ the DME control unit (engine control unit) is damaged.
    According to the prompts of the diagnostic equipment and combined with the principle of simplicity and simplicity, we decided to dismantle the inspection idling control valve. After careful examination, did not find the idle control valve there is any jam phenomenon. According to our maintenance experience, the car idler control valve harness connector cable is often damaged, for which we have replaced the idle control valve. As the frequency of the car appears too low, we let the user first drive away their own test. The car from the factory 2 days after about 350km, the failure again. After the factory to connect the fault diagnosis instrument to detect the vehicle, the equipment diagnosis results and the previous test results are exactly the same. As the last time has replaced the idle control valve, this time we decided to carefully check the relevant lines. According to the relevant circuit diagram, the dismantling inspection found that each plug is connected well, did not find any unusual circumstances. After informing the user of the inspection result of the vehicle, the user requests replacement of the DME control unit and the line. As a maintenance technician, we feel that this way of maintenance is too ignorant. But did not capture the real failure, the reality does not allow their own test and long test, we can only rely on the equipment prompts for maintenance, so temporarily so, but always feel that this maintenance is not reliable.
    Unsurprisingly, the car in the factory in April about 5300km after the return to the factory, the user said the fault is more frequent than before, this time has appeared seven or eight times. After consultation with the user, the user decided to leave the car to let us take a long time to test the fault. After a period of trial, the fault finally appeared. We use the fault diagnosis instrument to observe the engine control system dynamic data flow found that the control ratio of the idle control valve timely adjustment, in the air conditioning control ratio increased significantly, while the idle control valve has not implemented the adjustment action. According to the circuit diagram, we use the oscilloscope to observe the idle speed of the control valve 2 and 1,3 feet does not change the pulse width, that is, due to DME control unit internal control problems, the control unit issued instructions and the implementation of the part is not in accordance with the control unit instructions. Why is it so? DME control unit has been replaced, do two DME control unit there are the same problem? To this end, we decided to analyze the DME control unit can cause abnormal reasons. In general, the reasons that can cause the DME control unit to work abnormally include: DME control unit power failure, DME control unit electromagnetic compatibility and peripheral circuit electrical failure.
    After that, we use the oscilloscope to the DME control unit of the supply voltage was detected, no anomalies were found. After careful analysis, we suspect that the accidental power supply caused by failure, it was decided from the waste audio amplifier on the demolition of the power purification circuit, and added to the DME control unit power supply circuit test. But the vehicle just out of the factory failure appeared, it seems that this approach did not work. So is the DME control unit electromagnetic compatibility is poor? As we all know, the car's largest interference system is the generator and ignition system. As the power test has been done in front and there is no problem, we decided to check the ignition system. Tested, the ignition of each cylinder primary, secondary waveform and the ignition line on the suppression of resistance are normal. According to the above test results, combined with the specific characteristics of the car engine, we believe that the idle control valve for the slide valve and the power is large, so the idle control valve action will produce a high self-inductance voltage. We suspect that the ignition system and idle control valve line interference, it decided to try to ignition system and idle control valve line alone shield. To this end we use aluminum foil to the ignition and idling control valve line bandaged, and lead the DME control unit with the tap point. After the connection of the line, after a long test failure has not yet appeared.
    After the car delivered to the user, the vehicle driving distance of nearly 20,000 km, the telephone return to the user, the user does not reflect the failure.
    In the maintenance of the car after the failure, we have to repair the five or six of the same fault vehicles, are used this method successfully resolved. Based on the commonality of these faults, we conclude that the ignition coil of the M54 type electronic control engine ignition system is independent, but the drive module is designed in the DME control unit so that the ignition line transmits a large current when the engine is running To drive the ignition coil, and have a high frequency, which will interfere