## 1 Introduction  
A tractor's working conditions are complex, and it often needs more working gears. For example, Foton Lovol's P2654-N tractor and Changfa's CFK2204 tractor have 40 forward gears and 40 reverse gears. Excessive working gears make the transmission structure complex; therefore, CVT (continuously variable transmission) becomes an ideal tractor transmission system [1]. Since Fendt [2] first equipped hydrostatic power split CVT in tractors in 1996, ZF, Claas, John Deere, and other companies have successively developed their own tractor CVTs, such as the S-Matic transmission produced by ZF, the Eccom transmission and Traxion transmission produced by Claas, the AutoPowr transmission produced by John Deere, etc. [3]. However, different from the design of Fendt, these CVTs generally use multi-range speed control technology, which can effectively reduce the energy loss of the hydraulic system and improve the fuel economy of the tractor [4; 5], but the premise is to solve the problem of power shift. The principle of power shift [6; 7; 8] is to ensure the continuous power transmission during the shift by the sliding friction of the wet clutch, which is very important for tractors that often work in heavy load conditions. The hydrostatic power split CVT is a complex mechanic-electronic-hydraulic integrated system, which requires high robustness and reliability for each subsystem [9; 10; 11]. Once the hydraulic system that controls the clutches fails, it may affect the driving comfort and even endanger the driving safety during the shift [12; 13]. In recent years, some enterprises and universities in China have successfully developed their own tractor CVTs [14; 15; 16], but industrialization has never been realized, one of the important reasons being the high failure rate. The research of fault problem needs test data as support. However, in order to obtain the test data when the fault occurs, the usual method is to destroy the research object. For example, [PERSON] et al. [17] used wire cut electric discharge machining to generate the spall and the crack on the gear surface for transmission fault simulation. [PERSON] et al. [18] did a similar work: two types of ringing defect, including outer race and rolling element fault, were manually scratched in the research. Compared with the ordinary transmission, [PERSON] et al. [19] studied the fault diagnosis of the planetary gearbox, and eight bearing and gear faults were manually manufactured. However, the cost of the hydrostatic power split CVT is very high. Damage to the hydraulic system for research purposes can lead to unpredictable consequences, such as permanent damage to the clutch or transmission, which is not acceptable in terms of cost. Therefore, there is little research on the fault problem of the hydrostatic power split CVT, especially its shifting process. In order to evaluate the impact of hydraulic system failure on the shifting quality and driving safety without damaging the tractor CVT, the following method is proposed in this paper: first, the oil pressure data of the transmission clutch under different hydraulic system faults are obtained without starting the engine, and then, the response of the CVT tractor to the fault oil pressure is obtained by computer simulation.