A Beijing modern car with a mileage of about 20 thousand km. User response: the vehicle lights up after the machine starts. Normally, the instrument lights should be on the bright side after starting, and the lights will slightly darken when the outline lights are turned on. After the car: the test car found the following phenomena: the engine tachometer floating back and forth, but the engine runs very smoothly, the instrument lamp sometimes flicker, when the engine speed exceeds 3000 r/min or pull up the vehicle brake instrument lamp is normal, using fault detection instrument detection, found engine speed normal, The system has no fault code from the diagnosis. Upon enquiry, the customer was informed that the car was inspecting at other repair shops, reflecting the failure of the generator. So the output voltage of the generator under different working conditions was checked at about 14V. According to the fault analysis, the causes of the trouble are as follows: electrical interference, instrument failure, poor grounding. First, the modified xenon headlamp is removed from the test, and the fault remains unchanged. Then refer to the circuit diagram to remove the engine speed signal line of the instrument at the instrument, check the power line and the iron line related to the instrument, and have not found the abnormal. Then the target is transferred to the inside of the instrument, and the same type of instrument debugging is found. It is found that the fault phenomenon is not changed, and it is in trouble at this time. To reorganize the mind to reorganize the train of thought, analyze the electric interference of the car. The components that can produce interference when starting the machine have the ignition coil to produce high voltage, and the capacitance to prevent the high pressure reflux. So try to replace the ignition coil to eliminate the fault. After the replacement of the 1 cylinder ignition coil test, the fault is excluded. After examination, the coil resistance is 69.3 M ohm, and the normal coil resistance is 6.68 K ohm.