A Citroen Elysee car with mileage of about 180 thousand km, equipped with 8V engine and A14 automatic transmission. The user reflects: the car has an obvious shift impact when the automatic transmission is switched from low speed gear to high speed gear in the condition of cold car, and the gear shift is easy to appear in the shift, and the automatic transmission fault lights flicker in the dashboard. When the engine is shut down and the vehicle is restarted, the fault will temporarily disappear. After receiving the vehicle, the engine system and the automatic transmission system are self diagnosed by the diagnostic instrument. The engine system has no fault information, and the automatic transmission system has the fault information about the pressure regulation. Check the automatic transmission oil at idle speed, the oil quantity is normal, but the oil quality is very dirty. Replacement of automatic transmission oil, trial run, failure continues. The terminal voltage of the automatic transmission control unit is measured in three cases where the ignition switch is closed, open, and engine idle speed, including power supply, iron lap, multi-function switch, oil pressure sensor, input speed sensor, output speed sensor, oil temperature sensor, torque converter lock solenoid valve, main oil pressure regulating solenoid valve, flow Adjusting the voltage of the terminal of the solenoid valve and comparing the measured value to the standard value, all of them conform to the requirements. Thus, the control unit and the line of the automatic transmission are normal. Connecting the diagnostic instrument for the road test and reading the data flow, it is found that the transmission ratio of the automatic transmission has an obvious impact when the transmission is from 1 to 2, and the 2 to 3 gear will appear when the main oil pressure suddenly changes from 400kPa to 1200kPa, and the automatic transmission fault lights flicker. It is preliminarily determined that the fault is related to the main oil pressure regulating solenoid valve in the hydraulic control box. Replacement of the solenoid valve, road test, the phenomenon of disappearing, but sometimes there will be gear impact failure. At this point, read the fault information of the automatic transmission system, and there is no fault information. Continue to replace the torque converter locking solenoid valve, road test, shift impact phenomenon disappeared, so that the fault completely eliminated. The fault is caused by the stagnation of the main oil pressure regulating solenoid valve and the torque converter lock solenoid valve. The instability of the main oil pressure and the intermittent movement of the locking clutch will result in the impact of the gear shift and shift.