Abstract: A new method based on the signed directed graph (SDG) model was provided for fault diagnosis of clutch engagement in heavy off-road vehicle equipped with AMT, aiming to solve the problem of fault-prone and difficult diagnosis. A most complicated process of clutch engagement-starting engagement was taken as an example to build its SDG model of working process, to find the source of the faults by the real measurement and analysis of nodes and directed edge in the model,then the function of fault diagnosis was verified. The results show that it is feasible to use SDG model for fault diagnosis of clutch engagement in AMT system.