This manual provides comprehensive service and repair instructions for the Doosan DE12T, DE12TI, DE12TIA, and DE12TIS diesel engines. It covers detailed procedures for diagnosing, troubleshooting, and repairing these engines, including the fuel system, cooling system, lubrication system, and electrical components. The manual offers step-by-step guidance for disassembly, inspection, reassembly, and adjustment of engine components. Additionally, it includes technical specifications, maintenance schedules, and safety precautions. This document is essential for mechanics and technicians to ensure proper maintenance and repair, maximizing engine performance and longevity.