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A Volkswagen Passat B5 car equipped with a 1. 8T engine with a mileage of about 180 thousand km. When the vehicle suddenly fails after driving, it can no longer start the car. In the ignition state, there is no gear display and gear locking electromagnetic valve warning light on the meter. Fault diagnosis: first, V. A. 55051 detected that 01 engine control system has a fault code: 18056, transmission system data bus failure; 02 transmission control system has a fault code: 18262, transmission system data bus failure; 15 airbag control system has an accidental Barrier Code: 01312, transmission system data bus has Failure. At this time, the 19-08-125 group is read, showing that the engine is 0 and the gear position is 0, while in group 08-130, the control unit of the comfort system is 1, the left front door control unit is 1, the right front door control unit is 1. The bus can identify the comfort system bus and can not identify the control unit on the power bus. Two abnormal displays are related to the control unit of the transmission. First, the transmission control unit plug is pulled out. It is found that the vehicle can start the car at this time, and then the 19-08-125 group engine is shown to be 1, entering the engine electronic control unit, inquiring the fault code 18056, and the transmission system data bus has a failure to happen occasionally. At this point, it can be explained that the fault is because the internal circuit fault of the transmission control unit affects the power bus system. The internal resistance between the T88/85 foot of the CAN bus and the T88/86 foot is normal, the new transmission control unit is replaced, the zero part number is 4B0 927159 DA, and the car is still not starting. To check the fault code and the data stream, it is the same as the original fault phenomenon. In order to confirm that the new replacement control unit has no quality problem, the control unit is replaced to the other car of the same type, and all the tests are normal, proving that the transmission control unit is good. When the control unit of the transmission is disconnected, the engine is working normally and the new control unit is replaced, the fault is reproduced. It is inferred that there is only one reason, that is, the failure of the power CAN bus and the interaction of the control unit of the transmission lead to the failure of the CAN bus mode. Unplug the transmission control unit, idle the car, measure the waveform, and find the bus waveform abnormal. The CAN low is 2 to 2.5V, and the CAN high is 2.5 ~ 3V. The location of the 10cm length from the control unit of the transmission is found at last. The insulating peel outside of the CAN-H orange / black line is peeled and the adjacent black / brown line, which is also adjacent to the partial dissection of the outer skin, is also found. And the wiring harness is relatively wet, asking the driver to know that the front passenger seat has recently spilled water, thus it is not difficult to analyze that the two line insulated outer skin partially removed and the conduction of water and electrolytes will inevitably interact. From the circuit diagram, it is found that the black / brown line interacting with the CAN-H line connects the T88/25 foot of the transmission control unit, which is defined as an empty foot on the circuit diagram, and the external power supply voltage of the foot fluctuates between 2.5 and 6V. Due to the short circuit of CAN-H line and black / brown line, the 25 feet of the control unit of the transmission control unit add the high fluctuation voltage to the bus CAN-H, which causes the CAN bus failure. Therefore, the CAN bus quit the normal mode of work, so that the engine can not use the bus to receive and receive all kinds of information and can not work. When the transmission control unit plug is broken, the bus waveform is only characterized by the high and low voltage. Because of the fault tolerance of the gateway CAN signal management, the transmission of the dynamic data can still be carried out, so the engine can work normally. Troubleshooting: repair the transmission line harness, and then measure the waveform when disconnecting the transmission control unit plug, for normal waveform (CAN-L voltage is 1.5 to 2.5V, CAN H voltage is 2.5 to 3.5V). After all fault codes are cleared, the trial run is normal.