Chery's SQR7150 car with a mileage of about 183 thousand km. Users reflect: the vehicle is difficult to shift in or out of the 3 gear. Fault diagnosis: after detailed understanding, the user reflected that the car broke the gearbox shell, changed the gearbox shell in a Chery service station in the province, found the 3 gear shift difficult on the way back to Shanxi, and after a friend introduced to a repair factory to repair it, the failure was still. The user experienced the failure of the first two repairs and drove the car to the Chery service station. After the next task, the maintenance personnel made a careful examination, found that the fault has the following characteristics, no matter the engine starting or not starting, it is difficult to hang on the 3 gear; and other gear, especially the 4 gear, and the 3 gear shared a set of synchronizer and a fork, but can be easy to hang. The 3 gear can be a smooth gear case: the engine is not running when the engine is not running, if it can be relaxed once and once again, then repeat 3 gears again and again easily and easily, and change other gears to replace the 3 gear is very heavy; in the engine operation, the 3 gears also occasionally appear smooth. However, the maintenance personnel of the maintenance station failed to find the cause of the breakdown for the first time. After being invited, I listened to the description of the maintenance personnel and decided to decompose the transmission again, and concentrated the fault parts to the 3-4 gear synchronizer slider (teeth set) and the synchronous ring. After checking, it is found that the tooth ends of the 3-4 gear synchronizer slider inner gear are chamfered at the same time, the ends become sharp corners, and one side is not chamfered, the ends are flat. Without chamfering, it is meshing with the third gear, and the cause of the failure is finally clear. The reason is that, with a chamfering side (4 gear), in the process of shifting, the slip sleeve is synchronized with the circumferential speed of the gear, because the axial force of the synchronous sliding sleeve is shifted to the synchronous ring and the reverse force of the corresponding gear, the contact of the two sharp angle can break up part of the axial force of the gear shift into two opposite directions. The rotation force makes the synchronizer sliding sleeve rotate relative to the meshing gear, so that the sliding sleeve and gear can mesh smoothly and easily. While the sliding sleeve without chamfering side (3 gear) is in the process of shifting, the plane of the inner tooth of the sliding sleeve is in contact with the sharp angle of the 3 gear gear. When the sharp corner of the 3 gear is in the plane of the sliding sleeve, the axial force of the gear shift can not be decomposed into the opposite force in the two directions, and the slide sleeve is not opposite to the gear, so it is difficult to achieve the gear gear. Only when the gear between the sliding sleeve and the gear is exactly right, can the 3 gear be smoothly linked. However, users do not see this phenomenon before changing the transmission housing. There is no doubt that the ends of the inner teeth of the 3-4 synchronizer slider are chamfered. With this problem further observed, it is found that the profile and geometry of the 5 gear synchronous sliding sleeve and the 3-4 gear synchronizer are approximately the same. Is it wrong to change the installation error? But my maintenance staff do not agree that changing the transmission housing does not need to decompose the components mentioned above. Where does the unqualified 3-4 gear slipper come from? According to the general shape of the geometry of the 3-4 and 5 gear synchronizer, it is decided to decompose the five gear synchronizer and compare the two synchronizer slips. It is found that not only the geometric shape is consistent, but also the geometric size is the same, but one is only a chamfering end at one end. According to this feature, two sliding sleeves are exchanged, reassembled and tested, and trouble shooting is eliminated. When the vehicle is connected, the user can easily and conveniently shift gears in the road test, and is satisfied with the maintenance result. Maintenance summary: Chery SQR7150 sedan's manual transmission of the 3-4 gear and 5 gear synchronizer can not be interchangeable. Why does the transmission factory do not convert the 3-4 gear to the 5 gear synchronizer? The reason is that the 5 gear synchronizer slider only uses one end, and the other end does not use it, so it no longer processes chamfering, so as to reduce working procedure and reduce processing cost. As a skilled repairman, it is necessary to fully understand the characteristics, properties and working principles of various mechanical structures. In this way, we can make accurate judgments of the various abnormal failures in the repair work, and do not "repair" new failures under unknowable circumstances. This case diagnosis method is clear, and the force analysis of the inner surface of the sliding sleeve and the outer tooth end of the gear is correct. However, as the Chery special service station, the maintenance manual is not read throughout the maintenance process. It is a shortage of this diagnosis. Most of the places in the manual are easy to make mistakes in the installation. If the gear shift is difficult, we should suspect that the clutch is not separated. If a gear shift is difficult, it should be doubted that the synchronizer of this gear has problems, including the synchronizer lock, the positioning slider, the slider spring, and the chamfering of the inner teeth of the teeth and the end of the gear. No wear. The author quickly found that when the last disassembly transmission was the last time, the 3-4 and 5 gear sets were mistaken, and the fault was removed after the correct installation. In order to avoid the wrong parts of the assembly transmission, for the car maintenance personnel, it is necessary to take the principle of how to remove the installation. It is best to note in this paper, and can also use the gear bar to tie the paper, so that the score is better and can avoid unintentional drop. However, after the decomposition of the inspection should also carefully check the previous installation is wrong, not the last maintenance personnel to continue the mistakes to themselves, who is the main major who is responsible. It is necessary to understand the structure and working principle of each piece of machine for the maintenance personnel who have long gone through the sand field, and every part should be fully justified so that it will not be wrong and the previous errors can be found.