1. Introduction In recent years, with the continuous development of China's automobile industry, the number of vehicle models has been increasing. To meet the needs of users carrying light and bulky items, the chassis of similar vehicles has been lengthened and the wheelbase increased. This makes the original universal joint transmission system, consisting of two driveshafts and one intermediate support, unsuitable. For example, the extended Dongfeng truck uses a three-driveshaft system with two intermediate supports. Because most drivers and mechanics do not pay much attention to the installation of the three-driveshaft system, misdiagnosis of driveshaft faults often occurs in practice, resulting in unnecessary economic losses. 2. Fault Phenomena and Misdiagnosis A Dongfeng extended truck experienced abnormal noise from its transmission system. According to the owner, the noise occurred at high speeds, the vehicle speed did not reach the design standard, and the vehicle body vibrated at high speeds. When asked about the cause, the owner stated that the vehicle had experienced this fault before. After repeated inspections at a private repair shop, the noise was attributed to the main reducer, and the main reducer assembly was replaced, but the fault was not resolved. Clearly, the original fault was a misdiagnosis. 3. Inspection Results and Cause Analysis Based on the user's description, we first conducted a road test. During the test, we found an irregular "whooshing" noise at speeds above 60 km/h, which persisted even when coasting in neutral. The noise became more pronounced and the vibration more intense at higher speeds. To determine the cause, we jacked up the rear axle and operated the vehicle at high speed in a high gear. Even at around 50 km/h, the abnormal noise remained. No obvious bending of the driveshaft was observed at low speeds. Inspecting the driveshaft connection revealed that the universal joint forks at both ends of the first and second driveshaft sections were not in the same plane. Removing the intermediate support revealed slight looseness in the bearing. The driveshaft coaxiality error was within the specified range, but the markings on the expansion joint were blurred. After inspection, we replaced the intermediate support bearing, ensuring the universal joint forks at both ends of the first and second driveshaft sections were in the same plane. The universal joint forks at both ends of the main driveshaft were reinstalled in their original, plane-aligned positions. During test drives, it was found that the abnormal noise lessened at 60 km/h, but persisted at 70 km/h, and the vibration worsened with increasing speed. After returning to the factory, the main driveshaft extension joint was reversed by 180° and reinstalled. The test drive then proceeded without abnormal noise or significant vibration, and the speed could be increased to over 85 km/h, proving the fault had been resolved. Analysis revealed the fault lay in the universal joint drive system, not the main reducer. The universal joint drive system uses non-constant velocity joints. To achieve constant velocity transmission, two conditions must be met: First, the driven fork of universal joint I and the driving fork of universal joint II must be connected by driveshaft III, and the universal joint forks at both ends of the driveshaft must be in the same plane; second, the angles between the input and output shafts and the drive shaft must be equal, i.e., a1 = a2. The second condition is generally determined by the vehicle's structure. In a non-independent rear axle suspension, the relative angle and position of the drive axle input shaft, intermediate shaft, and transmission output shaft can change due to suspension deformation and wear of the intermediate support connection hole. The first condition can be ensured through proper assembly. The two basic conditions mentioned above must be met during installation. If these two conditions are not met during installation, the drive shaft will vibrate and produce abnormal noise due to unequal speed transmission between the input and output shafts. In this vehicle, the installation of the first and second drive shafts did not meet the requirements. Furthermore, for the main drive shaft, mechanics often only pay attention to keeping the universal joint forks at both ends on the same plane during installation, neglecting to mark them during disassembly or removal. Thus, although the universal joint forks at both ends of the drive shaft may be on the same plane, they may still be 180° off from their original balance position, disrupting the original balance and causing vibration and abnormal noise due to centrifugal force during operation. In summary, the main cause of the abnormal noise and vibration in this vehicle's transmission system is incorrect drive shaft installation. 4. Lessons Learned a. Carefully distinguish the characteristics of abnormal noise and avoid blindly replacing parts. The various assemblies in the transmission system are interconnected. A malfunction or vibration in one part will transmit the sound to all related components. Failure to carefully examine and distinguish the patterns of abnormal noise can lead to misdiagnosis and unnecessary economic losses. Therefore, during fault diagnosis, it is necessary to use methods such as segmented disconnection of the power source, depressing or releasing the clutch pedal, shifting gears, accelerating, decelerating, and coasting to differentiate the characteristics of abnormal noise and accurately identify the fault. b. Repair personnel should strive to improve their professional skills and master the basic principles of automobiles. For universal joint transmissions, the transmission characteristics of ordinary universal joints must be clearly understood. Different vehicle models and transmission arrangements must be analyzed specifically; experience in installing two-section driveshafts cannot be used to install three-section driveshafts. When installing three-section driveshafts, the universal joint forks at both ends of each driveshaft must be in the same plane. c. When disassembling and assembling rotating parts of an automobile, attention must be paid to the original vehicle's assembly marks. If no marks are found, marks should be made at the joints to avoid disrupting the original balance of the rotating parts.