Vehicle: remote GL vehicle, equipped with 2.0L engine, automatic transmission. Driving mileage: 31082km. Failure phenomenon: when users reflect this car acceleration, the speed is 60 to 70km / h, the feeling of jitter, three months ago because of the 2 gear to change the 3 gear impact, changed the automatic transmission control unit, the user thinks the break unit has a failure, the automatic transmission control unit is required to be replaced free of charge. Fault diagnosis: the maintenance personnel found that after the trial run, the fault phenomenon, as described by the user, disappeared when the speed exceeded 80km / h. It was found by diagnostic instrument that there was no trouble code between engine and transmission. According to the analysis of the fault phenomenon, the causes of the fault are torque converter fault and engine ignition system failure. The test found that when the feel jitter, the transmission has entered the 4 gear, the locking clutch has been worked, and the turbine is rigid connection with the pump wheel. There is no fluctuation in the engine tachometer. The operation of the locking clutch is controlled by the transmission control unit. The control unit controls the engagement and separation of the locking clutch by controlling the solenoid clutch solenoid valve according to the set control program. It plays a role of increasing torsion when separating, meeting the working requirements of the automatic transmission and ensuring the driving capacity of the vehicle. When jointed, the turbine is rigid connection with the pump wheel, and the transmission efficiency is 100%, which can minimize fuel consumption. Because the locking clutch does not work after increasing the torque, the engine resistance increases, and the mixture is not easy to ignite and break off the fire. Check engine ignition system. It is found that 1 cylinder spark plug and cylinder line are damaged, and there are signs of leakage. Troubleshooting after replacement. Troubleshooting: replace the spark plug and the high pressure line. Fault summary: there are two reasons for this failure, one is torque converter failure, the other is engine system failure. Which is the difference, mainly depends on whether the tachometer is fluctuating when the fault occurs. If the fluctuation is torque converter fault, no fluctuation will cause engine system failure.