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A car with a mileage of only 6000km and a 2013 speed 1.2L car equipped with a manual 5 speed transmission. User response: when the vehicle is running, the speedometer pointer does not show. When the vehicle is started, the speedometer pointer is placed two times, and then the odometer is not shown. Fault diagnosis: confirm the fault phenomenon and verify it. Because the instrument failure light was not lit, the maintenance technician did not see the trouble code. Directly check the speed sensor (Holzer type), pull the plug and ignition switch ON, the C voltage of the multimeter voltage shift sensor is 12V, B voltage is 11.8V, A voltage is 0, the test results are all normal. The resistance value between A and B is 42.58k Omega with a multimeter resistance gauge. The test results are not normal; the resistance between A and C is 6 m Omega, the test results are normal, the resistance between B and C is infinity, and the test results are normal. There is a 1.99m between normal A and B of the same type of vehicle odometer sensor. Resistance, according to the principle of this sensor for Holzer type, signal line B and A resistance should be infinite big right, so it seems normal car sensor is not very ideal. Through the above measurement, the fault of the speed sensor can be judged and need to be replaced. After replacing the speedometer sensor, the vehicle is normal. Less than three weeks later, the car came into service again, and all the Yellow indicators of the vehicle meter were flashing. The RDS+MDI diagnostic apparatus has multiple fault codes, including engine, speedometer and airbag, all of which are historical fault codes. Remove the trouble code, remove the speedometer sensor, measure the resistance of A and B to 3.56k ohm, and the test result is not normal. Both A and C, B and C were all normal. The speed sensor is replaced again, and the test car is normal. The fault occurred again after three days. The resistance between B and A measured after entering the station is 84K ohm, and the test result is not normal. The speed sensor is replaced once again. Why is this vehicle always fault of speed sensor? Is it interfered? Therefore, the remote-controlled door lock is cancelled, the generator is measured, and the waveform of the ignition coil is checked to see if there is any interference. No interference was found in the measurement, so the car was delivered to the customer. One hour later, the main driver was on the phone. The car broke down again, or the speedometer didn't go away. After returning to the station, the measurement of B and A is 11K Omega, and it is bad. This time, when a vehicle speed sensor with H and A resistance is removed, the fault is completely eliminated. Maintenance summary: the automobile fault diagnosis must be analyzed according to the principle requirements. Because Holzer sensor circuit board has many electronic components, it is impossible to measure resistance. If you use multimeter, you can only measure infinity. It seems that the quality of the parts is too important. Secondly, when diagnosing the circuit fault, we should first dismantle and install the components. Although this case has nothing to do with the later installation, this step is necessary. Comment: after reading this case, it feels that the troubleshooting technician is a very patient maintenance worker, and also sympathizes with his poor luck (to change to a good speed sensor for the fourth time). It is worth our maintenance personnel in the maintenance process should be emphasized that the technician in the inspection process to consider whether there is interference.