🏷️ Basic Information

Make:

Mercedes-Benz

Model:

300 CD

Model Designation:

W123 Coupe Diesel

Year:

1985

Chassis Code:

C123

Engine Code:

OM617.912

👋 Introduction

Era Positioning:

Represented Mercedes-Benz's commitment to durable, luxury diesel coupes in the mid-1980s. Positioned as a refined, fuel-efficient, yet powerful personal luxury car.

Competitors:

Competed with other European luxury coupes and performance sedans of the era, offering a unique blend of diesel efficiency and German engineering.

Vehicle Strengths:

Exceptional diesel engine longevity, comfortable ride, robust build quality, stylish coupe design, excellent highway cruising capabilities.

Value Proposition:

Provides factory-level diagnostic and repair information essential for maintaining the longevity and performance of the iconic W123 chassis, particularly the sought-after diesel variants.

📋 Manual Details

Primary Focus:

Repair and Service Procedures

Target Audience:

Professional Technicians and Advanced DIYers

Estimated Pages:

High (typically 800-1200 pages for comprehensive factory manuals)

Type:

Factory Repair Service Manual

Language:

English

Format:

PDF Download

📖 Manual Coverage

Engine:

3.0L Inline 5-Cylinder Diesel (OM617.912) - Diagnosis, Overhaul, Tuning, Emissions

Fuel System:

Bosch Diesel Injection Pump, Injectors, Glow Plug System

Transmission:

4-Speed Automatic (722.3 series) - Diagnosis, Repair, Fluid Service

Drivetrain:

Rear Wheel Drive, Driveshafts, Differential - Maintenance and Repair

Suspension:

Independent Front Suspension, Independent Rear Suspension, Anti-Roll Bars, Shock Absorbers, Springs

Steering:

Recirculating Ball Power Steering System - Components, Service, Adjustment

Brakes:

Front Disc Brakes, Rear Disc Brakes, Hydraulic System, Parking Brake - Diagnosis and Repair

Electrical System:

Charging System (Alternator, Regulator), Starting System (Starter), Battery, Ignition System (if applicable, though diesel), Lighting, Gauges, Accessories

Climate Control:

HVAC System (Heater, Air Conditioning) - Components, Diagnosis, Refrigerant Handling (if applicable)

Body:

Bodywork Repair, Door Mechanisms, Glass, Trim, Upholstery

Chassis:

Frame Components, Rust Prevention, Underbody Coatings

Maintenance:

Scheduled Maintenance, Oil Changes, Filter Replacements, Belt Adjustments, Lubrication Points

Troubleshooting:

System-Specific Diagnostic Procedures, Symptom-Based Troubleshooting Charts

Repair:

Component Removal, Installation, Rebuilding, and Adjustment Procedures

Diagnostics:

Electrical Circuit Diagrams, Component Testing Procedures, Diagnostic Codes (if applicable)

Technical Specifications:

Torque Values, Clearances, Timing Specifications, Fluid Capacities, Electrical Schematics

Depth:

Comprehensive - Covers everything from basic fluid changes to complete engine and transmission overhauls, detailed electrical troubleshooting.

📊 Manual Specifications

Digital Format:

PDF

Resolution:

High-Resolution Scans, Clear Text and Diagrams

Digital Format:

Not Applicable (Digital Download)

Digital Format:

Not Applicable (Digital Download)

⚙️ Engine Specifications

Engine Type:

Naturally Aspirated Diesel

Engine Configuration:

Inline 5-Cylinder

Displacement:

3.0 Liters (2998 cc)

Bore X Stroke:

87.0 mm x 88.0 mm

Compression Ratio:

21.0:1

Valvetrain:

SOHC (Single Overhead Camshaft), 2 Valves per Cylinder

Horsepower:

80-84 HP (approx. at 4000 rpm, depending on specific market tuning)

Torque:

145-150 lb-ft (approx. at 2400 rpm, depending on specific market tuning)

Fuel Injection:

Bosch Mechanical Indirect Injection

Glow Plug System:

Pre-glow system for cold starts

Engine Oil Type:

API CD or higher, 15W-40 Diesel Engine Oil recommended

Engine Oil Capacity:

7.5 Liters (8.0 US Quarts) with filter change

Coolant Type:

MB Approved Antifreeze/Coolant (e.g., G05 specification or equivalent)

Coolant Capacity:

9.5 Liters (10.0 US Quarts)

🔧 Transmission

Type:

4-Speed Automatic (722.3 Series)

Type:

5-Speed Manual (Less Common on 300 CD)

Automatic Features:

Hydraulic torque converter, planetary gear sets, lock-up clutch (on later versions) for improved efficiency.

Automatic Transmission Fluid Type:

MB Specification 236.1 (e.g., Dexron II or equivalent)

Automatic Transmission Fluid Capacity:

Approximately 8.0 Liters (8.5 US Quarts) for a complete fluid change and torque converter fill.

📐 Dimensions

Body Style:

2-Door Coupe

Length:

Approx. 4730 mm (186.2 in)

Width:

Approx. 1790 mm (70.5 in)

Height:

Approx. 1390 mm (54.7 in)

Wheelbase:

Approx. 2710 mm (106.7 in)

Curb Weight:

Approx. 1500-1600 kg (3300-3520 lbs)

🛡️ Safety Features

Crash Testing Era:

As per 1980s standards, not directly comparable to modern NHTSA/Euro NCAP ratings. Focus on passive safety features.

Braking System:

Four-wheel disc brakes, power assisted.

Suspension Stability:

Designed for high-speed stability and handling.

Body Construction:

Reinforced safety cage, crumple zones (front and rear), side impact beams.

Restraints:

3-point front seat belts, optional rear seat belts.

Steering Column:

Collapsible steering column.

💡 Technology Features

Original System:

Analog radio, cassette player (optional), mechanical climate controls.

Features Of Era:

ABS (Anti-lock Braking System) was an optional feature on some later W123 models, but not standard on all 1985 300 CD units. Cruise control was also optional.

Features Of Era:

None (pre-digital era).

🔧 Maintenance

Oil Change Interval:

Typically every 7,500 - 10,000 miles (12,000 - 15,000 km) or 12 months, whichever comes first, using quality diesel engine oil.

Fuel Filter Replacement:

Every 15,000 - 20,000 miles (24,000 - 32,000 km) or annually, crucial for diesel longevity.

Air Filter Replacement:

Every 15,000 - 20,000 miles (24,000 - 32,000 km) or as needed based on conditions.

Transmission Service Interval:

Fluid and filter change recommended every 30,000 - 40,000 miles (48,000 - 64,000 km).

Power Steering Fluid:

MB Specification 236.3 (e.g., Pentosin CHF 11S or equivalent)

Brake Fluid:

DOT 3 or DOT 4

Diesel Engine:

Vacuum leaks, injector pump timing issues, wear on vacuum pump and shifter bushings, potential for injector seal leaks.

Body And Chassis:

Rust proliferation (especially wheel arches, sills, undercarriage), rubber seal degradation (window, door).

Electrical:

Older wiring harnesses can become brittle, dashboard illumination, vacuum-actuated door locks can fail.

Transmission:

Hydraulic leaks, worn clutch packs in automatic transmission, potential for transmission mount wear.

Suspension:

Worn ball joints, tie rod ends, shock absorbers, and rubber bushings (especially in the rear axle).

📚 Vehicle History

Mercedes Benz Legacy:

Mercedes-Benz has a long-standing reputation for engineering excellence, durability, and safety, dating back to the invention of the automobile.

W123 Generation:

The W123 chassis (1976-1986) is renowned for its bulletproof reliability and over-engineering, often considered one of Mercedes-Benz's most durable generations. The C123 coupe variant offered a sportier, more luxurious take on the W123 platform.

Production Years:

W123 Coupe production: 1977-1985. The 1985 model year is towards the end of this robust generation's lifecycle.

Manufacturing Location:

West Germany

🏆 Awards & Recognition

Industry Acclaim:

The W123 generation as a whole is consistently recognized in automotive media and by enthusiasts as one of the most reliable and durable Mercedes-Benz models ever produced, often cited for its build quality and longevity.