This manual covers the Doosan MEGA 300-III Wheel Loader, designed for robust performance in demanding operational environments. The MEGA 300-III is engineered for reliability and efficiency, featuring advanced hydraulic and electrical systems to ensure optimal functionality. Its construction emphasizes durability and operator comfort, making it a dependable choice for heavy-duty tasks. This publication details the specific schematics essential for understanding and maintaining the vehicle's complex operational framework. The purpose of this manual is to provide comprehensive electrical and hydraulic schematics for the Doosan MEGA 300-III Wheel Loader. It includes detailed diagrams for both electrical circuits (publication number HAAE1440) and hydraulic circuits (publication number HAAE1450). This resource is intended to assist technicians and operators in diagnosing issues, performing maintenance, and understanding the intricate systems of the vehicle, thereby ensuring its continued optimal performance and longevity.