Symptom: a 2007 annual production of modern Tucson 2.0L manual block SUV, mileage of 20,000 km. According to the user to reflect the car in the normal driving sometimes there will be "self-braking" phenomenon, serious downhill road conditions can not even slip skid, the fault is usually after the hot car.
    Inspection and analysis: The car is equipped with ABS system and TCS traction control system, the first use of fault diagnosis instrument for testing, the system without fault code storage, the engine control system is also normal, according to the preliminary fault in the mechanical or hydraulic parts, do not rule out the circuit Cause the possibility of ABS system malfunction. Then the basic inspection of the brake system, brake calipers, brake master cylinder and brake cylinder action is flexible, no brake after the release of hysteresis, the replacement of brake fluid and exhaust as required, the road test occasionally There will be self-braking phenomenon. As the mechanical and oil part of the structure is relatively simple, check to confirm no problem, so the focus should be on the search ABS system to find the cause of malfunction, especially the wheel speed signal false trigger, which should be from the ABS system data stream Found abnormal.
    Lift the vehicle with the lift, hang the wheel to rotate, use the fault diagnosis instrument to observe the ABS system data flow. Repeated commissioning until the fault occurs, the diagnostic speed of the display device suddenly becomes 0,4 wheel speed signal also becomes zero, the yaw rate signal is normal, then the engine sound suddenly changes, the wheel stops rotating. It is judged that the vehicle speed signal of the input ABS control unit is faulty. For further confirmation, the control circuit diagram (with photos) to enter the ABS control unit speed signal line to check, in the inspection found that when shaking the right front A column under the plug, the vehicle speed data sometimes without failure also appeared, I think Due to the plug in the vehicle speed sensor line caused by failure caused by failure. The control circuit will pick out the corresponding plug and found that the plug contact with the shrapnel is flattened to lose flexibility, and sometimes jitter will cause poor contact, speed signal interruption.
    Troubleshooting: the plug shrapnel up to stir up, re-handle plug the plug after the test, shake the plug when the fault is no longer appear, the problem is ruled out.
    Recall summary: As the input speed signal of ABS control unit is not normal, the input signal is 0km / h when the contact is bad, the wheel speed signal is greater than the vehicle speed signal, the slip rate exceeds the set value, the control unit judges the vehicle is in the slip state, causing the control unit Start TCS work. TCS work, according to the slip rate control wheel braking force, control to reduce the wheel speed, so that the tire to form a greater adhesion, thereby increasing the traction torque, so that the formation of a certain braking force. As the control process without logic and parts of the poor, and the failure process is very short, did not meet the conditions of the fault code storage, which also to repair the work difficult to determine.