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Fault phenomenon, a Ruifeng automatic commercial vehicle, mileage of 65 thousand km. The maximum speed of the vehicle can only reach 120km/h, and the shifting feeling is not obvious. According to the status of the owner, the road test vehicle is the same as the owner of the car. When the vehicle runs to 120km/h, the vehicle continues to step on the accelerator. The speed of the car does not increase; the engine speed rises suddenly when the throttle is stepped on. Enter the engine system with the diagnostic instrument to check the engine trouble free code, and the data flow is normal. Enter the automatic transmission system, the same no trouble code. Entering the data flow to observe the gearshift lifting condition, it is normal to move to the 4 gear in the 1 block. When the speed is 80km/h, the transmission does not enter the torque converter locking state. By consulting the maintenance manual, it is found that the transmission can not be locked in the following four states: 1. at idle state; 2. when the brake switch is open; the 3. transmission gear switch is opened at L, 2; the 4. water temperature switch is opened. The above items are excluded respectively. Checking the transmission data flow indicates that the brake switch has been in the open state. There is no change in the brake pedal signal repeatedly, but the rear brake light is normal. Check the brake light switch with the multimeter. Check the circuit diagram to learn that the brake signal enters TCU 12V voltage signal, check the TCU connector M18-31 terminal voltage, the voltage 12.8V when the brake light switch on, and when disconnect 7.54V, how will the voltage exist after disconnection? Again check the brake light circuit, there is a signal to the ABS computer, the ABS computer plug terminal pull down and then measure the TCU connector M18-31 terminal voltage, 12.8V, when the connection is 12.8V, when the break is 0. At this point, the transmission data flow also returned to normal, with the brake pedal down and lifting, the data flow signal changes in ON and OFF. In order to further confirm the fault, the ABS computer plug the line after pulling out the test, the transmission low speed rising gear normal, in the 80km/h transmission lock control, the engine speed from 2500r/min to 2200r/min, continue to accelerate, speed to 150km/h. The reason for the increase of the potential of ABS may be caused by poor ABS computer grounding. At this point, the ABS system works normally. After the G07 is reassembled, the failure of the assembly can not be eliminated, and a ABS control assembly is replaced. The road test vehicle runs normally. The reason for the breakdown is that the internal wiring of the ABS computer pulls up the potential of the brake light signal, which is a virtual voltage and does not light the brake light. Through the process of troubleshooting, the maintenance data and correct detection and judgment are the root of the correct and fast handling of the problem, and the correct way of thinking is to be cultivated.