A mileage of about 30 thousand km, equipped with 2.0L engine, automatic transmission of the KIA remote GL sedan. User reflection: when the car accelerates, the speed is jitter when the speed is 60 to 70km/h. Three months ago, the control unit of the automatic transmission is replaced by the 2 gear change 3 gear impact. The user thinks that the control unit has a fault again, and 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.