Positioning:
The Mercedes-Benz W126 chassis, particularly the 300SD, represented the pinnacle of luxury diesel sedans in its era, known for robust engineering, durability, and comfort. This manual caters to owners and independent workshops seeking to maintain and repair these sophisticated vehicles.
Competitors:
Competed with other luxury sedans of the era, but stood out with its diesel engine's fuel efficiency and torque, a unique selling proposition in the luxury segment. Competitors included BMW's larger sedans, and high-end American marques.
Distinguishing Features:
The 300SD was distinguished by its reliable OM617 turbodiesel engine, exceptional build quality, advanced safety features for its time (like optional ABS), and a comfortable, luxurious interior. Its longevity and build quality have made it a sought-after classic.
Engine:
Detailed diagnostics, repair, overhaul of the OM617.951 inline-five turbodiesel engine.
Fuel System:
Fuel injection system (Bosch inline injection pump), glow plug system, fuel lines, filters, and tank.
Transmission:
Automatic transmission (e.g., 4-speed 722.3) service, repair, and adjustments.
Drivetrain:
Driveshaft, differential, axles, and wheel bearings.
Suspension:
Front and rear suspension components, shock absorbers, springs, control arms, bushings.
Steering:
Power steering system, steering gearbox, tie rods, steering column.
Brakes:
Hydraulic brake system, ABS (Anti-lock Braking System) if equipped, calipers, rotors, pads, lines, master cylinder, parking brake.
Electrical System:
Battery, charging system (alternator), starting system, ignition system (though less complex on diesel), lighting, wipers, horn.
HVAC:
Heating, Ventilation, and Air Conditioning (HVAC) system, climate control, blower motor, refrigerant system.
Body:
Body panels, doors, windows, locks, bumpers, interior trim, sunroof (if equipped).
Chassis:
Frame, subframes, and mounting points.
Maintenance:
Oil changes, filter replacements (oil, fuel, air), coolant flushes, fluid checks, lubrication points.
Troubleshooting:
Diagnostic procedures for common and uncommon faults, symptom-based troubleshooting guides.
Repair:
Step-by-step instructions for component removal, overhaul, and reinstallation.
Overhaul:
Complete engine rebuild procedures, transmission overhauls.
Adjustment:
Valve adjustments, shift point adjustments, parking brake adjustments.
Installation:
Installation procedures for replacement parts and components.
Diagnostics:
Detailed diagnostic charts, wiring diagrams, and component testing procedures.
Depth:
Comprehensive from basic scheduled maintenance to complex engine and transmission rebuilds and electrical system diagnostics.
Crash Testing Era:
While official NHTSA/IIHS ratings as we know them today were less standardized, the W126 was considered a very safe vehicle for its time, featuring a robust safety cage and crumple zones.
ABS:
Optional Anti-lock Braking System (ABS) available on some models.
Traction Control:
Not typically equipped with electronic traction control systems of the modern era.
Airbags:
Driver's side airbag became standard during the W126's production run, but might not be present on the earliest 1981 models. Passenger airbags were not common until later generations.
Seatbelts:
3-point inertia-reel seatbelts front and rear.
Construction:
Reinforced passenger compartment, energy-absorbing steering column, impact-resistant side-door beams.
Oil Change Interval:
Approximately every 5,000 to 7,500 miles (8,000 to 12,000 km) or 6-12 months, depending on driving conditions and oil quality. Consult the manual for precise recommendations.
Fuel Filter Replacement:
Typically every 15,000 to 30,000 miles (24,000 to 48,000 km). Crucial for diesel fuel system health.
Air Filter Replacement:
Inspect every oil change, replace as needed, typically every 15,000 to 30,000 miles (24,000 to 48,000 km).
Coolant Flush:
Every 2-3 years or as recommended by the manual.
Engine Oil:
API CD or higher, SAE 15W-40 or 10W-40 diesel engine oil recommended (check manual for ambient temperature suitability).
Engine Oil Capacity:
Approximately 7.0-7.5 liters (7.4-7.9 US quarts) with filter change.
Transmission Fluid:
MB Approval 236.2 or DEXRON II (check manual for exact specification).
Coolant:
Ethylene glycol based antifreeze, typically a 50/50 mix with distilled water. MB Approval 325.0/325.5.
Power Steering Fluid:
MB Approval 236.3 (ATF Dexron II compatible).
Common Problems:
Vacuum system leaks (affecting door locks, HVAC, cruise control), glow plug issues, injector pump calibration, potential for rust in certain climates, wear in suspension bushings, and aging rubber hoses.
Brand Focus:
Mercedes-Benz has a long-standing reputation for engineering excellence, safety, luxury, and durability, particularly with its diesel engines.
W126 Introduction:
The W126 chassis replaced the W116 S-Class and was produced from 1979 to 1991. It featured significant aerodynamic improvements and modern styling.
300Sd Introduction:
The 300SD variant, powered by the OM617 turbodiesel, was introduced to the US market and was a key model in the W126 lineup, known for its fuel economy and long-distance cruising ability.
Production Years Covered:
1981, 1982, 1983, 1984, 1985
Manufacturing Location:
Sindelfingen, West Germany
