The Mercedes-Benz W123 Series, produced from 1977 to 1985, represents a significant era in automotive engineering, renowned for its exceptional build quality, durability, and reliability. This generation, encompassing the 300D, 300CD, and 300TD models, including their turbo variants, was designed with robust construction and a focus on longevity. The W123 is celebrated for its solid chassis, dependable inline 5-cylinder diesel engines (OM617 series), and comfortable ride, making it a favored choice for long-distance travel and everyday use. Its reputation for enduring performance and relatively simple mechanicals has solidified its status as a classic vehicle. This comprehensive service repair manual provides in-depth guidance for the Mercedes-Benz W123 Series, covering models from 1977 through 1985. It details essential maintenance procedures, including scheduled service, fluid checks, and filter replacements, alongside step-by-step repair instructions for all major systems. The manual addresses engine mechanicals, fuel systems, transmission, suspension, steering, brakes, bodywork, and electrical systems, as well as climate control and auxiliary systems. With detailed illustrations and dealer-level information, this resource is invaluable for DIY enthusiasts, independent mechanics, and technicians aiming to maintain, diagnose, and repair these iconic vehicles effectively.