2017-2020 Mercedes-Benz S-class (W222 facelift ) S 400d (340 Hp) G-TRONIC

The Mercedes-Benz S 400d (W222 facelift, produced from 2017 to 2020) represents a significant iteration of the flagship S-Class sedan, positioned as a high-performance diesel variant within the broader W222 generation. This model, part of the post-2017 facelift, aimed to deliver a blend of luxury, performance, and efficiency, appealing to buyers seeking a refined grand tourer experience with lower running costs than its gasoline counterparts. The S 400d utilized a newly developed inline-six diesel engine, marking a departure from the previous V6 diesel offerings, and paired it exclusively with a 9-speed automatic transmission (G-TRONIC). It occupied a middle-to-upper position in the S-Class lineup, bridging the gap between the entry-level S 350d and the more powerful S 560 models.

Powertrain & Engine Architecture

The heart of the S 400d is the OM 656.929, a 2.9-liter inline-six cylinder diesel engine. This engine represented a substantial advancement in Mercedes-Benz diesel technology, featuring a number of innovations designed to improve efficiency and reduce emissions. Key features include a steel piston, a cam-in-head valvetrain with variable valve timing (Camtronic), and a two-stage turbocharging system. The two-stage turbocharging utilizes a smaller turbocharger for immediate response at low engine speeds, supplemented by a larger turbocharger for increased power at higher RPMs. This configuration minimizes turbo lag and provides a broad torque curve.

Fuel is delivered via a commonrail direct injection system operating at extremely high pressures (2,700 bar), ensuring precise fuel atomization and efficient combustion. The engine also incorporates a diesel particulate filter (DPF) and selective catalytic reduction (SCR) system with AdBlue injection to meet stringent Euro 6 emission standards. The G-TRONIC 9-speed automatic transmission is a torque-converter automatic, known for its smooth shifting and wide gear ratio spread. It allows the engine to operate within its optimal efficiency range more frequently, contributing to improved fuel economy. The transmission features a wet multi-plate clutch and fully automatic electronic control.

Engine Specifications

The OM 656.929 engine produces 340 horsepower at 3,600-4,400 rpm and a substantial 700 Nm (516.29 lb-ft) of torque between 1,200-3,200 rpm. This high torque output, available across a wide rev range, provides effortless acceleration and strong in-gear performance. The engine’s compression ratio is a high 15.5:1, contributing to its thermal efficiency. The engine oil capacity is 8.5 liters (8.98 US qt / 7.48 UK qt) and requires a specific Mercedes-Benz approved oil specification.

Driving Characteristics

The S 400d delivers a driving experience characterized by smooth, refined power delivery and exceptional comfort. The high torque output of the diesel engine provides strong acceleration from a standstill and effortless overtaking capability. The 9-speed automatic transmission contributes to this by keeping the engine within its peak torque band. While not as overtly sporty as the gasoline-powered S 560, the S 400d offers a more than adequate level of performance for most drivers. The 0-60 mph acceleration time is approximately 5.1 seconds, and the car is electronically limited to a top speed of 155.34 mph (250 km/h). The inline-six engine is remarkably quiet and refined for a diesel, minimizing the typical diesel clatter. The suspension, typically utilizing Mercedes’ AIRMATIC air suspension system, provides a comfortable and controlled ride, absorbing bumps and imperfections in the road surface effectively.

Equipment & Trim Levels

The S 400d, as a flagship model, came standard with a comprehensive array of luxury features. Standard equipment typically included leather upholstery, heated and ventilated front seats with massage function, a premium Burmester surround sound system, a large infotainment display with Mercedes’ MBUX system, and a suite of advanced driver-assistance systems such as adaptive cruise control, lane-keeping assist, and automatic emergency braking. The interior trim options included a variety of wood veneers, aluminum accents, and leather colors. Optional extras included rear-seat entertainment systems, a panoramic sunroof, and upgraded interior lighting packages. The S 400d was generally offered in both standard and long-wheelbase configurations, with the long-wheelbase version providing significantly more rear legroom.

Chassis & Braking

The S 400d utilizes a fully independent suspension system, typically featuring Mercedes’ AIRMATIC air suspension with adaptive damping. This system allows the ride height to be adjusted, providing a balance between comfort and handling. The front suspension employs a multi-link design, while the rear suspension also utilizes a multi-link setup. The braking system consists of ventilated discs on all four wheels, providing ample stopping power. Anti-lock braking system (ABS), electronic stability control (ESC), and brake assist are standard features. The steering system is a rack-and-pinion type with electric power assistance, offering precise and responsive steering feel. Standard tire size is 245/50 R18, mounted on 18-inch alloy wheels.

Market Reception & Comparison

The S 400d was generally well-received by automotive critics, who praised its blend of performance, efficiency, and luxury. The inline-six diesel engine was lauded for its refinement and power delivery. Fuel economy figures ranged from 5.2-5.4 l/100 km (45.2-43.6 US mpg, 54.3-52.3 UK mpg), making it significantly more fuel-efficient than the gasoline-powered S 560. Compared to the S 350d, the S 400d offered a substantial performance upgrade, while remaining more affordable than the top-of-the-line S 560. Reliability has generally been good, although diesel particulate filters can require periodic maintenance or replacement, particularly with frequent short trips. The S 400d offered a compelling alternative for buyers who prioritized fuel efficiency and torque without sacrificing luxury or performance.

Technical Specifications

Legacy

The OM 656 engine family, introduced with the S 400d, has proven to be a robust and reliable powertrain. The S 400d, while discontinued in 2020, remains a desirable used car option, offering a compelling combination of luxury, performance, and fuel efficiency. The inline-six diesel engine is known for its longevity and relatively low maintenance costs. However, potential buyers should be aware of the potential for DPF issues, particularly if the vehicle has been primarily used for short trips. The S 400d represents a high point in Mercedes-Benz diesel technology, offering a refined and capable grand touring experience that continues to appeal to discerning buyers in the used car market.