🏷️ Basic Information

Make:

Mercedes-Benz

Model:

350 SD Turbo

Model Year:

1991

Chassis Code:

W140 (though this specific designation is for S-Class; the 350 SD Turbo in this context typically refers to the W126 chassis variant that was phased out around this time, or the early W140 diesels. Given the 'SD Turbo' designation, it strongly suggests the W126 generation's diesel models, specifically the W126 300SD Turbo, which was succeeded by the W140 S350 turbodiesel. For clarity, the manual likely covers the W126 S-Class variant, possibly bridging to early W140 diesel engines if applicable to the 1991 model year transition.)

Engine Code Typical:

OM603.970 (for W126 300SD Turbo, predecessor/related to W140 diesel engines)

👋 Introduction

Positioning:

The Mercedes-Benz 350 SD Turbo (likely referring to the diesel variant of the W126 or early W140 generation) represented the pinnacle of luxury and diesel technology for its era, targeting discerning buyers prioritizing refinement, durability, and fuel efficiency in a premium sedan.

Competitors:

Competed against other high-end luxury sedans with diesel options, such as select BMW models and potentially some American luxury brands with diesel powertrains, although Mercedes-Benz was a dominant force in the premium diesel segment.

Features:

Renowned for robust construction, comfortable ride, advanced diesel engine technology for the time (turbocharging, indirect injection), sophisticated suspension, and luxurious interior appointments. The 'SD Turbo' designation specifically highlights its turbodiesel powertrain, offering improved performance over naturally aspirated diesel engines.

📋 Manual Details

Coverage:

All service and repair operations for the Mercedes-Benz 350 SD Turbo (likely referring to the W126 generation 300SD Turbo or early W140 diesel models).

Specific Coverage:

Includes detailed schematics, torque values, diagnostic procedures, and part numbers for repairs and maintenance specific to the 1991 Mercedes-Benz 350 SD Turbo.

Page Count:

Estimated 1500+ pages (typical for comprehensive service manuals of this era covering all aspects).

Type:

Factory Authorized Service Manual / Repair Manual.

Language:

English.

Format:

Downloadable Digital Format (PDF).

📖 Manual Coverage

Covered Systems:

Engine (diesel, turbocharging), Fuel System (injection, glow plugs), Cooling System, Lubrication System, Exhaust System, Drivetrain (transmission, differentials), Suspension (front, rear, self-leveling if applicable), Steering System, Brakes (ABS), Electrical System (wiring diagrams, components), Climate Control (AC, heating), Bodywork (interior, exterior trim, doors, glass), Diagnostic procedures, Maintenance schedules, Troubleshooting guides, Specifications and tolerances.

Cooling System:

Detailed procedures for radiator, water pump, thermostat, hoses, expansion tank, and coolant flushing/replacement. Includes diagrams and troubleshooting for overheating and leaks.

Lubrication System:

Engine oil, oil filter, oil pan, oil pump, oil cooler (if equipped), and crankcase ventilation system maintenance and repair. Includes oil pressure testing.

Exhaust System:

Catalytic converter (if applicable for diesel market), muffler, exhaust pipes, hangers, and heat shields. Includes inspection for leaks and damage.

Suspension System:

Front and rear suspension components (control arms, ball joints, bushings, shock absorbers/struts), springs, anti-roll bars, and any self-leveling hydraulic systems. Includes alignment specifications.

Electrical System:

Complete wiring diagrams for all circuits, component testing procedures, fuse/relay identification and function, battery charging system, starter motor, lighting systems, instrumentation, and auxiliary systems.

Climate Control:

Manual and automatic climate control systems, HVAC blower motor, heater core, evaporator core, AC compressor, refrigerant charging, climate control panel diagnostics, and troubleshooting for temperature regulation and airflow issues.

Bodywork Interior:

Door panel removal, seat adjustment/removal, dashboard disassembly, headliner replacement, interior trim repair, and body panel alignment.

Bodywork Exterior:

Fender, hood, trunk lid, bumper, grille, headlight, and taillight removal and installation. Glass replacement (windshield, side, rear). Door and lock mechanism service.

Included Procedures:

Routine maintenance (oil changes, filter replacements, fluid checks), Engine overhaul and repair (cylinder head, pistons, crankshaft, timing), Fuel injection system diagnostics and repair, Turbocharger system inspection and repair, Transmission service and repair (automatic transmission overhaul), Brake system bleeding and component replacement, Suspension component replacement and alignment, Electrical system troubleshooting, Body panel repair and adjustment, Diagnostic trouble code (DTC) interpretation and resolution, Component testing procedures, Speculative fault finding.

Engine Diagnostics:

Compression testing, cylinder leak-down testing, fuel pressure testing, boost pressure testing, diagnostic code reading (if applicable for the era), live data interpretation (if electronic diagnostics are supported), and troubleshooting engine performance issues like rough running or lack of power.

Transmission Diagnostics:

Fluid level and condition checks, shifting diagnostics, hydraulic circuit testing, valve body troubleshooting, and torque converter inspection. Repair procedures for seals, clutches, and bands.

Brake System Repair:

Brake pad and rotor replacement, caliper overhaul, brake line replacement, master cylinder service, brake fluid bleeding, and ABS sensor testing/replacement.

Steering System Service:

Power steering pump and rack service, tie rod end replacement, steering column adjustments, and alignment procedures.

Electrical Troubleshooting:

Use of wiring diagrams to trace faults, continuity testing, voltage drop testing, troubleshooting power and ground issues, diagnosing sensor and actuator failures, and locating shorts and open circuits.

Diagnostic Procedures:

Step-by-step diagnostic flowcharts for common problems, symptom-based troubleshooting, and detailed explanations of how various vehicle systems function.

Depth:

Comprehensive. Covers everything from basic preventative maintenance to complete engine and transmission rebuilds, including detailed electrical schematics and diagnostic trees. Suitable for professional technicians and advanced DIY enthusiasts.

Rebuild Procedures:

Detailed instructions for the complete disassembly, inspection, repair, and reassembly of major components like the engine, cylinder head, turbocharger, fuel injection pump, and automatic transmission.

Specifications Tolerances:

Includes critical torque specifications for fasteners, engine component clearances, bearing clearances, piston ring gaps, valve lash (if applicable), and wheel alignment angles.

Component Identification:

Clear identification and location of all major and minor components within the vehicle, aiding in diagnosis and repair.

📊 Manual Specifications

File Format:

PDF.

Print Quality:

High resolution, enabling clear reproduction of diagrams, schematics, and text. Searchable text functionality is expected for digital formats.

Binding Type:

N/A (Digital Download).

Paper Type:

N/A (Digital Download).

⚙️ Engine Specifications

Engine Details:

Inline 6-cylinder, turbocharged diesel engine. Likely the OM603.970 (3.0L) or a closely related variant. These engines were known for their torque, durability, and relatively good fuel economy for a large luxury vehicle.

Bore Stroke:

87.0 mm x 84.0 mm (for OM603.970).

Compression Ratio:

21.0:1 (for OM603.970).

Engine Cooling Type:

Liquid-cooled.

Horsepower:

Approximately 148-150 hp (for OM603.970).

Torque:

Approximately 230-240 lb-ft (for OM603.970).

0 To 60 Mph Acceleration:

Approx. 10-12 seconds (model dependent).

Fuel Injection:

Bosch inline mechanical injection pump with turbocharger boost pressure compensation. Glow plug system for cold starting.

Fuel Type:

Diesel Fuel.

Turbocharger Model:

Garrett AiResearch or similar, with wastegate control.

🔧 Transmission

Transmission Type:

4-speed Automatic Transmission (e.g., 722.4 or early 722.6 variants).

Optional Transmission:

Manual transmission options were rare or non-existent for diesel models in this segment during this period, especially in North America. Primarily automatic.

Transmission Features:

Hydraulic torque converter, electronic control (KÜGELAUGE or similar early systems), torque converter lock-up for improved fuel efficiency.

📐 Dimensions

Length:

W126: ~197 inches; W140: ~200-203 inches (model dependent).

Width:

W126: ~70.5 inches; W140: ~73.6 inches (model dependent).

Height:

W126: ~56 inches; W140: ~57-58 inches (model dependent).

Trunk Volume:

Approximately 17-18 cubic feet (W126, W140).

Curb Weight:

Approximately 3800-4200 lbs (1724-1905 kg).

Gross Vehicle Weight Rating:

Approx. 5000-5300 lbs (2268-2404 kg).

🛡️ Safety Features

Crash Test Ratings:

While official NHTSA/IIHS ratings as we know them today were less standardized, Mercedes-Benz vehicles of this era were built to rigorous internal safety standards, emphasizing structural integrity and occupant protection.

Active Safety:

Anti-lock Braking System (ABS) was standard. Power-assisted steering. Possibly traction control on later models/variants.

Passive Safety:

Reinforced chassis and body structure, energy-absorbing crumple zones, driver and passenger airbags (becoming standard on W140, optional on late W126), seatbelt pre-tensioners, padded interior surfaces.

💡 Technology Features

Infotainment System:

Factory original systems would have included an AM/FM/Cassette player, possibly with a multi-disc CD changer as an option. Analog controls for climate and audio. Some later W140 models might have had early navigation systems.

Display Technology:

Analog gauges for speedometer, tachometer, fuel, and temperature. Warning lights for various systems. LCD displays for odometer and trip computer on some models.

Assistance Features:

ABS, power steering, cruise control. Advanced electronic systems were present but not as pervasive as modern vehicles.

Connectivity Features:

Minimal by modern standards. Primarily focused on internal vehicle systems. May have had provisions for external antenna and basic diagnostic communication.

🔧 Maintenance

Oil Change Interval:

Typically every 5,000-7,500 miles or 6-12 months, depending on driving conditions and oil type used. Refer to manual for exact recommendation.

Fuel Filter Interval:

Every 15,000-30,000 miles or as per manual recommendation, particularly important for diesel engines.

Air Filter Interval:

Every 15,000-30,000 miles or as needed based on inspection, crucial for turbo-diesel performance.

Engine Oil Specification:

API CD or higher; SAE 15W-40 or 10W-40 (check manual for specific climate recommendations). Mobil 1 Turbo Diesel Truck 5W-40 is a good modern choice.

Coolant Specification:

Mercedes-Benz specified coolant or an equivalent silicate-free, ethylene glycol-based coolant. Typically a 50/50 mix with distilled water. MB Part # Q1030000 or equivalent.

Transmission Fluid Specification:

Dexron III or equivalent. MB Part # Q13300005 or equivalent.

Brake Fluid Specification:

DOT 3 or DOT 4 (check manual). MB Part # Q1300000001040 or equivalent.

Transfer Case Fluid:

N/A (Rear-wheel drive or AWD typically did not use separate transfer case fluid in this configuration).

Differential Fluid:

GL-5 specification gear oil, SAE 90 or 85W-90. MB Part # Q104000001020 or equivalent.

Common Problems:

Head gasket failures (especially on OM603 due to heat/age), turbocharger wear, glow plug issues, vacuum leak problems affecting central locking and engine performance, aging fuel lines and injector seals, potential for wiring harness degradation, ABS system sensor issues, suspension component wear (especially self-leveling rear suspension if equipped).

Electrical Quirks:

SBC (Sensotronic Brake Control) issues on later W140 models were not present in 1991. More common are issues with the vacuum system controlling central locking, door locks, and potentially idle control. Age-related degradation of wiring insulation, especially in the engine bay, is a common concern.

📚 Vehicle History

Heritage:

Mercedes-Benz has a long-standing legacy in automotive engineering excellence, particularly with diesel engines, dating back to the early 20th century. They pioneered many advancements in automotive technology and safety.

Model Lineage:

The 'SD Turbo' designation typically refers to the diesel variants of the S-Class. In 1991, Mercedes-Benz was transitioning from the W126 S-Class to the new W140 S-Class. The 350 SD Turbo would be a successor to the 300SD Turbo (W126) and a precursor to the S350 Turbodiesel (W140). The OM603 engine family was a key diesel offering in this period.

Chassis Generations:

The 350 SD Turbo designation implies the W140 chassis, which replaced the W126 in 1991. However, it's possible the manual also covers late W126 models if the transition occurred mid-year or early in the model year. The W140 was significantly larger and more technologically advanced than the W126.

Production Years:

The OM603 turbodiesel engine was produced throughout the late 1980s and into the 1990s, powering various Mercedes-Benz models including the W124, W126, and early W140. For the 350 SD Turbo specifically, this timeframe aligns with the W140 introduction.

Manufacturing Locations:

Primarily manufactured in Germany.