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Failure phenomenon: a 2010 production of the Passat 2 car, the driving mileage of about 60 thousand km, according to the driver reflected that the car speed meter pointer sloshing serious, sometimes will suddenly drop to zero, no matter how slow the response. Fault diagnosis: after commissioning, it is found that the fault phenomenon is consistent with the driver's description. Check the circuit diagram, and combine the failure phenomenon to deduce the possible cause of failure: instrument fault, line fault between instrument and speed sensor, speed sensor fault. According to the principle of first simplex and rear propagation, the speed sensor wire connector on the transmission is removed first, a new speed sensor is attached, then the ignition switch is connected, the magnet is sloshing on the surface of the sensor continuously, and the speed meter pointer can be deflected up and down, but the fault is still in the car after changing the speed sensor. The wire connector T32a on the left side of the instrument and the speed sensor wire connector are used to measure the resistance of the side terminal 28 of the wire connector T32a wire and the brown / red wire between the traverse side terminal 2 (speed signal terminal) of the speed sensor wire connector with a multimeter, which is about 400 N, indicating that the conductance of the conductor is undesirable. It is considered that the middle of the conductor is connected through the wire connector T10b terminal 10. Then remove the traverse connector T10b. Remove the engine control unit, and find the black wire connector T10b on the left side of the lower part of the engine. Check its terminal 10, and find the oxidation phenomenon, and the cause of the failure becomes clear. Troubleshooting: handling the oxidized terminals and connecting all the broken wire connectors. After trial run, the speedometer indicator is normal. Failure analysis: because of the poor speed of signal to the meter, the speedometer is not working well. The vehicle speed sensor is a dry reed tube. When it is under strong magnetic field, the contacts inside the tube are closed; when the magnetic field is weakened, the contacts in the tube are broken. In the process of alternating strong and weak magnetic field, the internal contact of the tube is broken and the frequency of magnetic field alternately determines the frequency and strength of the pulse signal, thus causing the change of the deflection amplitude of the pointer of the speed meter. In addition. When such faults are eliminated, a conductor can be drawn from the speed signal input terminal (T32a / 28) on the meter, and then switch on the ignition switch. The lead wire is repeatedly touched with the body of the body, which can simulate the output signal of the speed sensor when the speed is low. If the speed meter deflects, the instrument can be considered to work normally.