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A star city HFX6900QG city bus with CA6102N2 type natural gas engine and QJ505 transmission. When the vehicle was less than 8000 km, it was repaired by the company because of the abnormal noise of the transmission. After taking the car: dismantling the transmission inspection, it is found that the clutch drive disk drives will damage, the clutch separation casing is worn seriously on the upper part, the one axle bearing is damaged, the bearing hole in one axis has a large number of pitting, the two axis front bearing is damaged, and the front part of the two shaft diameter is seriously worn. Because the car is still in the "three package" period, the gearbox manufacturers send accessories, after the replacement of the damaged parts, the above failure phenomenon, so the car continued to put into operation. After the car was put into operation only more than 20 days (itinerary is about 4000km), the car is also repaired because of the abnormal transmission. Again, the transmission is removed and checked. It is found that the fault damage is the same as that of the first time. The bad parts left the car out of the factory, but after more than 20 days, the same trouble happened again. It is suspected that there is a problem with the gearbox housing. Under the strong demand of the driver, the transmission manufacturer changed a transmission assembly. However, the new transmission assembly was installed for nearly 1 months (5000 km). The above fault appeared again, because the transmission had been repaired two times, and the transmission assembly was replaced, but the same failure appeared. It seems that the root of the fault should not be in the transmission. Please FAW service station technicians to assist in the diagnosis. The station technician checked the engine and said that the engine was running free. Now the transmission factory and the engine manufacturer all say that their products have no problem, but the fault does exist. It is estimated that the replacement of the damaged parts in the transmission can not solve the problem thoroughly. It seems that we can only find a way to find out the real cause of the failure. According to the failure phenomenon, I suspect that the error of the one axis of the transmission and the coaxiality of the engine crankshaft is too large, but the measurement of the coaxiality of these two needs special measuring instruments. But the author does not have this instrument, and how to measure the coaxiality of the crankshaft and the transmission axis by using the existing instruments in the company. What about the error? The author consulted a lot of data and consulted a lot of colleagues, and finally came up with the following measure by the dial meter: fixing the magnetic meter seat on the engine flywheel. The measuring head of the dial is counterbalanced to the clutch cover slot of the flywheel shell, turning the crankshaft, and measuring the error of the coaxiality between the crankshaft and the flywheel shell to 6.5 m M, find a normal vehicle for the same model. The coaxiality of the crankshaft and the flywheel shell is only about 0.4 mm. Is there a problem in the engine cylinder or the problem of the flywheel shell? The CA6100N2 engine is improved on the basis of the CA6100 type engine. The shape of the flywheel shell has a great change, but the internal and fixed bolts basically do not change, so a good CA6100 type engine flywheel shell is found again on the engine of the fault car, and the coaxiality error of the flywheel shell and the crankshaft is 0.35 mm. The root of all the faults is in the flywheel shell. Then please come to the service station of FAW. In the face of accurate measurement and detailed data, they finally admit that the engine has problems, and all the losses are borne by them. Troubleshooting: after changing the flywheel shell and the damaged transmission parts, the car has been running for half a year, the above failure has not been reappeared, indicating that the fault is completely eliminated. Fault summary: through the diagnosis and elimination of this case, I get some inspiration: first, to eliminate the failure to root for the source, to find out the real cause of the fault, not the headache, the foot pain medical foot; two is to use the existing equipment and instruments, to take full advantage of the existing equipment, to solve practical problems, and sometimes not to solve the problems. We must rely on professional equipment and instruments. Three, we must have a basis for judging faults and speak on data, which is convincing.