The Mercedes-Benz C123 300 CD Automatic, produced between May 1977 and September 1979, represents a unique and relatively rare variant within the broader W123 series. Positioned as a luxury coupe, the 300 CD distinguished itself with a diesel powertrain, offering a blend of fuel efficiency and comfortable cruising. This model occupied a niche position, appealing to buyers seeking the elegance of a coupe with the economy of a diesel engine, a somewhat uncommon combination for the era. It was built on the 123 platform, sharing many components with the sedan and wagon versions but offering a more sporting and exclusive profile.
Technical Specifications
| Brand | Mercedes-Benz |
| Model | W123 |
| Generation | C123 |
| Type (Engine) | 300 CD (78 Hp) Automatic |
| Start of production | May, 1977 |
| End of production | September, 1979 |
| Powertrain Architecture | Internal Combustion engine |
| Body type | Coupe |
| Seats | 4 |
| Doors | 2 |
| Fuel consumption (combined) | 11.5 l/100 km |
| Fuel consumption (US mpg) | 20.5 US mpg |
| Fuel consumption (UK mpg) | 24.6 UK mpg |
| Fuel consumption (km/l) | 8.7 km/l |
| Fuel Type | Diesel |
| Acceleration 0 – 100 km/h | 20.8 sec |
| Acceleration 0 – 62 mph | 20.8 sec |
| Acceleration 0 – 60 mph | 19.8 sec |
| Maximum speed | 143 km/h (88.86 mph) |
| Weight-to-power ratio | 19.9 kg/Hp |
| Weight-to-torque ratio | 9.9 kg/Nm |
| Power | 78 Hp @ 4400 rpm |
| Power per litre | 26 Hp/l |
| Torque | 156 Nm @ 2400 rpm (115.06 lb.-ft. @ 2400 rpm) |
| Engine layout | Front, Longitudinal |
| Engine Model/Code | OM 617 D 30 / 617.912 |
| Engine displacement | 3005 cm3 (183.38 cu. in.) |
| Number of cylinders | 5 |
| Engine configuration | Inline |
| Cylinder Bore | 91 mm (3.58 in.) |
| Piston Stroke | 92.4 mm (3.64 in.) |
| Compression ratio | 21.5:1 |
| Number of valves per cylinder | 2 |
| Fuel injection system | Multi-port manifold injection |
| Engine aspiration | Naturally aspirated |
| Valvetrain | SOHC |
| Engine oil capacity | 7.5 l (7.93 US qt | 6.6 UK qt) |
| Coolant capacity | 11 l (11.62 US qt | 9.68 UK qt) |
| Kerb Weight | 1550 kg (3417.17 lbs.) |
| Max. weight | 2015 kg (4442.31 lbs.) |
| Max load | 465 kg (1025.15 lbs.) |
| Fuel tank capacity | 80 l (21.13 US gal | 17.6 UK gal) |
| Length | 4763 mm (187.52 in.) |
| Width | 1786 mm (70.31 in.) |
| Height | 1395 mm (54.92 in.) |
| Wheelbase | 2710 mm (106.69 in.) |
| Front track | 1488 mm (58.58 in.) |
| Rear track | 1446 mm (56.93 in.) |
| Drivetrain Architecture | Rear wheel drive |
| Number of gears | 4 |
| Gearbox type | Automatic transmission |
| Front suspension | Double wishbone |
| Rear suspension | Trailing arm |
| Front brakes | Disc |
| Rear brakes | Disc |
| Steering type | Cone worm with recirculation balls |
| Power steering | Hydraulic |
| Tires size | 195/70 R14 |
| Wheel rims size | 6J x 14 |
Powertrain & Engine Architecture
The heart of the C123 300 CD was the OM 617 D 30 engine, a 3.0-liter inline-five cylinder diesel. This engine, designated with the code 617.912, was a robust and over-engineered design, known for its longevity. It featured a naturally aspirated configuration and utilized a multi-port manifold injection system, a relatively advanced feature for a diesel engine of that period. The single overhead camshaft (SOHC) valvetrain operated two valves per cylinder. The engine’s high compression ratio of 21.5:1 was typical for diesels, contributing to its fuel efficiency but also requiring high-quality fuel. The four-speed automatic transmission was a crucial component, designed to deliver smooth shifts and complement the diesel engine’s torque characteristics. This transmission was not known for its sportiness, prioritizing comfort and ease of use over rapid acceleration.
Driving Characteristics
The 300 CD Automatic offered a distinctly different driving experience compared to its gasoline-powered counterparts or even manual diesel versions. With only 78 horsepower, acceleration was leisurely, taking approximately 20.8 seconds to reach 62 mph. However, the 156 Nm (115 lb-ft) of torque available at 2400 rpm provided adequate pulling power for highway cruising and overtaking, albeit without any sense of urgency. The automatic transmission, while smooth, contributed to a somewhat detached feel, with noticeable gear changes and a lack of engine braking. The gear ratios were optimized for fuel economy and relaxed driving, rather than performance. Compared to the more powerful 300 CE (gasoline) or even the later 300 D Turbo, the 300 CD felt significantly slower and less responsive.
Equipment & Trim Levels
The C123 300 CD Automatic was positioned as a well-equipped luxury coupe. Standard features typically included power steering, disc brakes on all four wheels, and comfortable seating for four passengers. Interior upholstery was generally cloth, though leather was available as an option. The dashboard layout was typical of Mercedes-Benz of the era, featuring wood trim and a comprehensive array of gauges. Optional extras included air conditioning, a sunroof, and upgraded audio systems. The trim level was generally consistent across the 300 CD models, with options being the primary way to personalize the vehicle.
Chassis & Braking
The C123 chassis featured independent suspension all around, with a double wishbone setup at the front and a trailing arm configuration at the rear. This suspension design provided a comfortable ride, absorbing bumps and irregularities in the road surface effectively. The braking system utilized disc brakes on all four wheels, offering good stopping power for a vehicle of its weight class. The steering was a cone worm with recirculation balls, providing a relatively heavy but precise feel. Stabilizer bars were fitted to both the front and rear axles to reduce body roll during cornering.
Market Reception & Comparison
The C123 300 CD Automatic received mixed reviews. Critics praised its fuel efficiency and comfortable ride, but many found its performance underwhelming. Compared to the gasoline-powered C123 coupes, the 300 CD was significantly slower. However, it offered a substantial improvement in fuel economy, making it an attractive option for drivers who prioritized efficiency over outright speed. The 300 CD also offered a different proposition than the more common W123 sedan, appealing to buyers seeking a more stylish and exclusive vehicle. Reliability was generally good, typical of Mercedes-Benz diesels of the period, but maintenance could be costly due to the complexity of the fuel injection system.
Legacy
The Mercedes-Benz C123 300 CD Automatic, while not a high-volume seller, has gained a following among enthusiasts and collectors. The OM 617 engine is renowned for its durability and longevity, and well-maintained examples can still provide reliable service today. The C123 coupe body style is also highly regarded for its elegant design. In the used car market, these vehicles command a premium compared to the more common W123 sedans, particularly those in good condition with low mileage. The 300 CD represents a unique chapter in Mercedes-Benz history, showcasing the brand’s commitment to both luxury and fuel efficiency during a period of rising fuel prices and increasing environmental awareness.
