A 2002 - year POLO1.4 hatchback car, model BCC, is equipped with manual transmission and mileage of 100 thousand km. Due to the accident, the front end of the car was hit hard, but after several repair works, the vehicle could not start. According to users, the engine has changed the engine control unit and several sensors, and the cylinder head and transmission cracks have been repaired by welding. After receiving the car, the personnel conducted routine checks on the vehicle, and found that many of the pipelines in the engine room went wrong, and no abnormalities were found in other aspects. The fault diagnosis instrument V.A.G1552 was connected to the vehicle to detect the fault, and the fault code was found to be "no signal from the data bus of the air bag control unit." After examination, no air bags were installed in the steering wheel, which was not directly related to engine failure. Start the engine at this time, there is no sign of starting. Because the engine uses an independent ignition system, the barrel type ignition coil is inserted into the spark plug to make it iron, the high pressure fire is checked, the spark is found weak, the injector is pulled out and the oil is ejected. After the cylinder pressure was measured only 750kPa, some of the oil was injected into the spark plug holes, and the cylinder pressure was up to 1050kPa. When the engine is started again, only the "flapping" echo of the intake pipe is heard, and there is an avalanche and avalanche sound in the exhaust pipe. According to experience, it is like the symptoms of ignition disorder caused by the ignition timing. The control unit can not accurately confirm the fault of the first cylinder upper stop point and the engine speed. The POLO car has been upgraded in the electronic control technology. Even if the crankshaft position sensor is pulled out, the engine control unit J448 will enter the emergency operation state, but the control unit J448 will still calculate the engine speed and determine the position of the convex axis according to the signal provided by the camshaft position G40 sensor. Start the engine. At this point, it was proposed to pull out the crankshaft position sensor to test the car. After pulling out the sensor plug, the engine started smoothly. But why can't the crankshaft position sensor be activated? It seems that only the transmission can be removed to find the answer. After dismantling the transmission, it is found that the trigger wheel of the engine speed G28 sensor is integrated in the seal flange after the crankshaft, and the G28 sensor is fixed on the flange with the internal six angle bolts, but now there are six angle tool breakers in the hole, which is just flat with the plane of the bolt, and can not be dismantled. It can be determined to be artificial dismantling. It is the result of bad. The seal flange is a polytetrafluoroethylene (PTFE) seal ring, the sensor wheel has 58 teeth, of which 2 teeth have a gap size of 2 teeth in width, used as a reference mark. The sensor trigger wheel is pressed on the crankshaft in a precise position. When the crankshaft is rotating, the trigger wheel is driven by the engine speed sensor G28 to perceive the signal, and the engine speed and the precise position signal of the crankshaft are provided to the engine control unit. When manual clockwise rotation of the engine to the first cylinder stop point position, it is found that the reference mark on the sensor trigger wheel has 28 teeth in the vertical position of the sensor, and the normal case should be 14 teeth. Again, the mounting hole on the trigger wheel is exactly aligned with the sensor. It is inferred that the former is mistaken for the installation of the mounting hole when the sensor wheel is installed. As a result, the engine control unit can not determine the accurate signal, causing engine ignition disorder, unable to drive. In the correct case, the engine control unit should calculate the 14 teeth of the sensor trigger wheel after the reference mark, and calculate the position of the crankshaft accordingly. After replacing 1 new sealing flanges and crankshaft position sensors, the positioning holes on the trigger wheel are aligned with the arrow mark on the sealed flange (lower right corner), and the tools are slowly pressed into the sealing flange, and the sealing flange positioning bolt is loaded, and the other accessories and the transmission are reloaded, and the test car is cut in normal. In eliminating this fault, a problem is found. When measuring the position sensor of the crankshaft, it is found that there is a 5V voltage between the 1 and 3 terminals. It appears that the sensor should be a Holzer sensor and belong to an active sensor. Many data are called magnetoelectric sensors, which belong to passive sensors. At the beginning of the diagnosis, the data is accurate. It is mistaken for a magnetoelectric sensor with a measured resistance value of 4.9M Omega, and the normal resistance of the magnetoelectric speed sensor should be 700~1100 Omega. Therefore, we should not blindly believe in some materials. We should develop the habit of making materials in practice, so as to avoid taking detours and wasting time.