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A 2013 modern Beijing mobile 1.6 manual block car with a mileage of about 20 thousand km and a 5 shift manual transmission. The user reflects: the car's speed meter is out of order. Inspection and analysis: the maintenance personnel tried the car and found that the speedometer needle was always pointing to 0 km/h during the vehicle running. Test instrument control unit, no fault code. The speed sensor of the car is a Holzer sensor, which generates a pulse signal through the gear ring in the transmission, and the signal is processed into the speed data after the control unit is processed. Disconnect the connector of the speed sensor and turn on the ignition switch to measure the voltage of its power, signal and terminal. They are 12V, 0V and 0V respectively. The signal terminal of the sensor is connected to the signal processor of the control unit, so 0V is obviously abnormal. Measuring the resistance between the signal end and the grounding iron is found to be a short-circuit. So where is the short-circuit point? In view of the circuit diagram, it is found that the speed sensor signal is sent to several control units, which include the engine control unit, the anti lock brake system control unit, the body control unit, the air conditioning control unit and the sound control unit. In order to find the short-circuit point, breaking the door one by one will take a lot of time and effort. Careful observation revealed that the sound system of the car was modified. Disassembling the sound control unit inspection revealed that one of the wires was misconnected to the speed signal line. Troubleshooting: correctly connect the tie line and troubleshooting.