The engine front-end accessory drive (FEAD) system, as a dynamic auxiliary power transmission subsystem in internal combustion and hybrid powertrains, plays a role beyond that of a simple motion transmission mechanism. The dynamic behavior of the FEAD system under accessory loads and thermal variations has become a significant challenge in ensuring the stable operation of engine accessories. In this study, under partial combustion load conditions, the effect of engine accessory loads on the dynamic behavior of the belt and idler pulley was investigated using the commercial software GT-SUITE through the development of a one-dimensional multibody dynamic model coupled with accessory loads. The model incorporates the main components of the FEAD system, including pulley inertias, the elastic and damping characteristics of the belt, and the contact forces between the belt and pulleys. The dynamic analysis results include the idler support reaction force located between the alternator pulley and the air-conditioning tensioner, angular velocity fluctuations, and belt slip. Furthermore, good agreement was observed between the simulation results and the experimental data. This demonstrates the capability of the proposed model to accurately predict the real dynamic behavior of the FEAD system under partial combustion load conditions and confirms its applicability for the analysis and optimization of such systems.