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Key words: ABS sensor fault phenomenon: an Dongfeng Nissan blue bird 7200 will damage the right front wheel speed sensor when the right front wheel bearing is replaced in a repair plant. After the replacement of the new part, the ABS lamp will alarm, and the ABS does not function, that is, the emergency brake wheel is locked. Analysis and exclusion: after receiving the vehicle, the failure phenomenon is verified first. When the speed is above 30km/h, the wheel is locked when emergency braking, indicating that the fault phenomenon does exist. This vehicle is equipped with BOSCH5.3 four sensor and four channel ABS system. ABS electronic control unit, hydraulic control unit and ABS pump are integrated and have self diagnosis function. When the system fails, the corresponding code will be stored, and the code can be sent out by the diagnostic instrument, so as to provide basis for troubleshooting. The fault code is read by the diagnostic instrument: the left front sensor is short circuited, and the right front sensor is short circuited. In order to verify whether the fault meaning is true, unplug the two sensor plug. The fault meaning is the left front and the right front sensor opening. The plug is inserted back to clear the fault code, and the fault remains the same, indicating that the fault is not accidental. Next, lift the car and turn the two front wheel. At this time, the data is displayed as 0, while the speed of the rear wheel can increase with the increase of wheel speed. This indicates that the fault is on the sensor, control unit or line. Because the probability of the failure of the control unit is very low in www.carxl.com.cn, the sensor and the line are very easy to fail, so the sensor and the line are checked. Disconnect the control unit plug and check the power supply and grounding according to the circuit diagram. Measurement of resistance values of 4 and 5, 6 and 7 is 2.2k ohm, which is the resistance of the front wheel sensor. The resistance measured by 1 and 3, 8 and 9 for rear wheel sensor terminals is 1.5k ohm, while the normal sensor resistance is 1.44 to 1.76k ohm. Obviously, two front wheel sensor is abnormal. In the process of detection, 4, 5, 6, 7 feet have different degrees of oxidation, after processing and replacing two of the front wheel sensors, the ABS lamp is extinguished, read the fault code, and the system shows no fault storage. In the trial run, the speed is mentioned above 30km/h, and the emergency brake is working normally. After several tests, ABS worked well. I thought that when the fault was removed, abnormal conditions were discovered. When the braking speed of the vehicle is below 20km/h, the sound of the pump motor is heard in the engine compartment, while the brake pedal has a slight rebound. When the data is read, when the fault occurs, the left front input valve of the motor relay is shown to open (the opening is the meaning of the electric action), and the four wheel speed of the data is consistent at this time. Each time is the left front input valve action, sometimes when the motor is abnormal rotation when the brake switch is open, the motor will turn, and even when the vehicle is completely stationary, the motor will not stop immediately. In addition, when the motor rotates, it does not feel the deceleration of the vehicle, indicating that the system is not working properly. Some vehicles have also encountered low speed braking ABS action, that is because the sensor dirty or the gap is not normal, so that the voltage of the sensor is reduced, so that the electric control unit thinks that the wheel will be locked to death, and intervention, this should be seen from the data, but the car is not abnormal when the obstacle occurs. For the sake of prudence, four rounds of sensors were inspected, all the signal wheels were free of foreign bodies, and the gap was normal, and the number of gear teeth was the same. Since the sensor input signal is normal (data display), and the ECU control output is abnormal, it is concluded that the ECU is faulty. After the replacement, the failure of the test is still. At this point, calm down and re analyze the work done before. After eliminating the sensor, control unit and circuit, the signal tooth is only available. If the fault point is in front of the left side, if the fault is reached to the right side, the fault location can be determined. In order to verify the judgement of the author, the test failed.