Gear transmission is widely used in mechanical transmission system and acts an important role in automotive industry. Manufacturing errors, assembly looseness, gear wear issues may result in gear backlash, noise and fatigue damage seriously affecting efficiency and service life of gear transmission. For gear transmission assembled, it is important to monitor the conditions of gear meshing and prevent the occurrence of dangerous situations. How to define the issues of gear tooth wear, misaligned bearing, gear eccentricity, backlash, and how to find faulty planetary gear sets and specific issues existing in gear transmission are meaningful and significant to ensure the quality of product. This paper starts from the analysis on gearing mechanism. Based on the behaviors represented by the issues, gear tooth wear, misaligned bearing, gear eccentricity and backlash are demonstrated and explained in detail. The results illustrate that gear wear can be expressed by excitation of natural frequency along with 1×rpm side-bands of defective gear, and gear misalignment can excite second order or higher harmonics with side-bands of running speed and gear meshing frequency (GMF for short) with fairly high amplitude indicates eccentricity, backlash and non-parallel shafts. In addition, gear meshing and order tracking analysis are conducted in the transfer case. The results reveal that there exist two problems covering shaft looseness and misalignment of the planetary gear set. The premier noise sources relate to the looseness of input and output shafts at high gear range, while the planetary gear set with eccentricity has shown significant contribution to the noise at low gear range. Thus, the novel conclusion proposed provides theoretical basis and data support for the further study on gear noise.