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Failure phenomenon: a 2007 Beijing modern tuco 2.0L manually block SUV car, the cumulative driving about 20 thousand km, in normal running, sometimes the phenomenon of automatic braking, serious downhill road conditions can not even slip, this fault is usually easy to appear in the state of the heat engine. Inspection and analysis: according to the data, the car is equipped with ABS and TCS (traction control system). First, the fault detector is used to detect, and no fault codes are stored in each system. Based on this, it is preliminarily judged that faults in mechanical or hydraulic parts do not exclude the possibility of ABS misoperation caused by circuit. Next, the brake system is basically checked. The brake caliper, the brake master cylinder and the brake wheel cylinder are flexible, and there is no delay phenomenon after brake release. Replacement of brake fluid and exhaust test after required, found that occasionally there will be automatic braking phenomenon. Because the mechanical and oil road parts are relatively simple, the inspection confirms no problem, so the key point should be to find out the reason for the misoperation of ABS, especially to check the false trigger of the wheel speed signals, which should be found from the ABS data flow. Lift the car up with the lifting gear, turn the wheel around, and observe the ABS data flow with the fault diagnosis instrument. When the fault appears repeatedly until the failure occurs, the speed data of the fault detector suddenly changes to 0, the 4 wheel speed signals become 0, the partial pendulum rate is normal, the engine sound changes suddenly, and the wheel stops turning. The above fault is caused by poor speed signal input to the ABS control unit. In order to further confirm, the control circuit diagram checks the speed signal line of the input ABS control unit. In the inspection, when the wire connector under the right front A column is rocked, the speed data is sometimes no and the fault occurs simultaneously. I suspect that it may be due to the bad contact of the speed sensor line in the connector. And the cause. In the control circuit diagram, the corresponding connector terminals are checked out. It is found that the connector terminal contact shrapnel is flattened and flattened, and sometimes the jitter will cause bad contact and result in the interruption of the speed signal. Troubleshooting: to pick up the bullet on the wire connector terminal, rehandle the connector after the connector and test the car, shake the connector, the above fault will no longer appear, the above fault is completely eliminated. Fault summary: as the input speed signal of ABS control unit is abnormal, the input signal is o km/h when the speed signal wire connector is in bad contact, the wheel speed signal is larger than the speed signal, the slip rate exceeds the set value, and the ABS control unit determines that the vehicle is in the skid state, so the TCS work is started. When TCS works, the wheel braking force is controlled according to the slip rate, and the wheel speed is controlled so that the tire can form greater adhesion, thus increasing the traction torque, thus forming a certain braking force. Because there is no logic error and bad parts in the control process, and the failure process is very short, the condition of failure code storage is not reached, which also makes it difficult for the maintenance and judgment work.