🏷️ Basic Information

Make:

Mercedes-Benz

Model:

350 SD Turbo

Year:

1991

Chassis Code:

W140 (S-Class generation)

Engine Code:

OM603.970

👋 Introduction

Vehicle Segment:

Full-size luxury sedan, representing the pinnacle of Mercedes-Benz engineering and comfort for its era.

Target Audience:

Affluent consumers seeking unparalleled refinement, advanced technology, and robust build quality.

Key Competitors:

BMW 7 Series, Lexus LS 400, Cadillac Fleetwood, Jaguar XJ.

Drivetrain Emphasis:

The 'SD' designation signifies the turbodiesel engine, offering a balance of torque, fuel efficiency (for its class), and long-distance cruising capability.

Comfort And Luxury:

Known for its exceptionally quiet cabin, plush ride quality, high-quality interior materials, and sophisticated features for the time.

Owner Benefit:

Empowers owners to perform complex maintenance and repairs themselves, saving significant costs associated with dealer servicing and ensuring the longevity of a classic luxury vehicle.

📋 Manual Details

Title:

Mercedes 350 SD Turbo 1991 Full Service Repair Manual

Coverage Scope:

Full Service, Repair, Maintenance, Rebuild, Refurbish, Restoration

Estimated Page Count:

Extensive (Typically 1000+ pages for a full service manual of this era)

Type:

Service and Repair Manual

Language:

English

Format:

Likely digital (PDF) given typical online sales, but could be printed

📖 Manual Coverage

Engine Mechanical:

Detailed disassembly, assembly, inspection, and repair of the 3.5L turbocharged inline-six diesel engine (OM603.970), including cylinder head, block, pistons, crankshaft, camshafts, timing system, and lubrication.

Fuel System:

Comprehensive coverage of the diesel fuel injection system, including the injection pump (Bosch VE type), injectors, glow plug system, fuel lines, filters, and troubleshooting of diesel fuel delivery issues.

Turbocharger System:

Detailed information on the turbocharger unit, its lubrication, cooling, wastegate operation, intake and exhaust plumbing, and diagnosis of turbo-related performance problems.

Cooling System:

Maintenance and repair of the radiator, coolant hoses, thermostat, water pump, cooling fan (likely viscous or electric), and heater core. Coolant specifications and flushing procedures.

Exhaust System:

Information on exhaust manifolds, catalytic converter (if applicable), muffler, and piping. Diagnosis of exhaust leaks and backpressure issues.

Transmission Automatic:

Detailed procedures for the automatic transmission (likely a 722.4 or similar), including fluid changes, filter replacement, valve body inspection, torque converter service, and common failure modes. Shift linkage adjustments.

Drivetrain:

Coverage of the rear-wheel-drive system, including driveshaft, universal joints, differential, axle shafts, and wheel bearings. Lubricant specifications and maintenance.

Suspension:

Front and rear suspension components, including control arms, ball joints, tie rod ends, shock absorbers/struts, springs, and bushings. Alignment specifications and procedures.

Steering:

Power steering system, including pump, reservoir, steering gear/rack, and hydraulic lines. Fluid specifications and bleeding procedures. Steering wheel and column.

Brakes Hydraulic:

Hydraulic brake system components, including master cylinder, brake lines, flexible hoses, brake calipers (front and rear), and wheel cylinders. Brake fluid specifications and bleeding procedures.

Brakes Parking:

Operation and adjustment of the parking brake system.

Electrical System Basic:

12V electrical system, including battery, charging system (alternator), starter motor, ignition switch, fuses, and relays. Wiring diagrams for basic circuits.

Lighting System:

Headlights, taillights, brake lights, turn signals, interior lighting, and dashboard illumination. Bulb types and replacement.

Instrumentation Gauges:

Speedometer, tachometer, fuel gauge, coolant temperature gauge, oil pressure light/gauge, and warning lights. Troubleshooting of gauge and indicator malfunctions.

Body Interior:

Door mechanisms, window regulators (manual and power), seat adjustment, interior trim panels, carpeting, and headliner. Removal and installation procedures.

Body Exterior:

Body panel removal and installation (hood, fenders, doors, trunk lid), door hinges, latches, weatherstripping, bumpers, and grilles. Paint repair guidance (basic).

HVAC System:

Climate control system, including A/C compressor, condenser, evaporator, heater core, blower motor, and control modules. Refrigerant R12 specifications and service (if applicable), or system conversions.

Power Accessories:

Power windows, power seats, power door locks, power mirrors, sunroof operation, and cruise control. Troubleshooting of electric motors and control switches.

Safety Restraints:

Seat belts, pretensioners (if equipped), and airbag system (basic description and precautions, detailed diagnostics may be limited).

Chassis Electrical:

Central locking system, alarm system (if equipped), and related sensors and control units.

Scheduled Maintenance:

Detailed maintenance schedules for various mileage and time intervals, including oil changes, filter replacements, fluid checks and top-offs, belt inspections, and chassis lubrication.

Troubleshooting Diagnostics:

Systematic troubleshooting charts and diagnostic procedures for common and uncommon problems across all major vehicle systems, often linking symptoms to probable causes and corrective actions.

Component Replacement:

Step-by-step instructions for removing, installing, and adjusting virtually every serviceable component on the vehicle.

Rebuilding Overhauling:

Comprehensive guidance on the complete disassembly, inspection, measurement, machining, and reassembly of major mechanical components like the engine and transmission.

Electrical Diagnostics:

Detailed diagnostic procedures for electrical faults, including the use of multimeters, continuity testers, and interpretation of wiring diagrams.

Specifications Data:

Torque specifications for all fasteners, alignment specifications, fluid capacities, electrical specifications, engine and transmission data, and critical measurement tolerances.

Special Tools:

Identification and usage instructions for specialized tools required for specific repair and maintenance procedures.

Depth:

Expert-level coverage suitable for professional mechanics and advanced DIY enthusiasts, encompassing everything from routine maintenance to complete engine and transmission overhauls. Includes detailed technical explanations and illustrations.

📊 Manual Specifications

Digital Format:

PDF (Searchable text is highly desirable for ease of use)

Quality:

High-resolution images and clear text reproduction are essential for effective use.

Binding:

N/A (if digital), or durable spiral-bound/stitched for printed versions to lay flat during use.

Paper Quality:

N/A (if digital), or heavy-duty, glossy paper for printed versions to withstand workshop conditions.

⚙️ Engine Specifications

Engine Type:

Inline-6 Cylinder, Turbocharged Diesel

Displacement:

3.5 Liters (3449 cc)

Bore X Stroke:

94.0 mm x 83.6 mm (approximate)

Compression Ratio:

22.0:1 (typical for OM603 series)

Valvetrain:

Single Overhead Camshaft (SOHC), 2 valves per cylinder

Horsepower:

134 hp @ 4000 rpm (approximate, variations may exist)

Torque:

235 lb-ft @ 2400 rpm (approximate, variations may exist)

Injection Type:

Bosch VE Rotary Injection Pump with Mechanical Governor

Fuel Type:

Diesel Fuel

Glow Plug System:

Direct glow plug activation for cold starts, with an indicator light and automatic shut-off timer.

Oil Capacity:

7.5 Liters (8.0 US quarts) with filter (typical)

Oil Type Specification:

API CD or ACEA E1 specification diesel engine oil, SAE 15W-40 or 10W-40 depending on climate. Consult manual for exact MB approval numbers.

Coolant Capacity:

12.5 Liters (13.2 US quarts) (approximate, consult manual)

Coolant Type:

Proprietary Mercedes-Benz blue coolant (G48 specification) or equivalent silicate-free coolant, mixed 50/50 with distilled water.

🔧 Transmission

Type:

4-Speed Automatic (722.4xx series)

Availability:

Not available with manual transmission.

Shift Modes:

Comfort (C) and Sport (S) modes, affecting shift points.

Fluid Type:

Mercedes-Benz ATF (Automatic Transmission Fluid) Specification 236.3 or Dexron III compatible fluid. Consult manual for exact MB approval.

Capacity:

Approximately 8 Liters (8.5 US quarts) for a full drain and refill (consult manual).

📐 Dimensions

Length:

5113 mm (199.5 in)

Width:

1886 mm (74.3 in)

Height:

1474 mm (58.0 in)

Wheelbase:

3040 mm (119.7 in)

Curb Weight:

Approximately 1870 kg (4123 lbs)

Gross Vehicle Weight Rating:

Approximately 2400 kg (5291 lbs)

🛡️ Safety Features

NHTSA Ratings:

Not directly comparable to modern standards; likely performed well for its era in crashworthiness.

ABS:

Anti-lock Braking System (ABS) standard.

Traction Control:

ASR (Anti-Slip Regulation) traction control system available or standard on some models.

Airbags:

Driver's airbag standard. Passenger airbag likely an option or standard on later models within the W140 series.

Seat Belts:

Three-point inertia-reel seat belts with pre-tensioners for front occupants.

Structure:

Reinforced passenger cell, crumple zones, and side-impact protection beams.

💡 Technology Features

Audio System:

Premium sound systems were available, often with cassette players and optional CD changers. Basic radio and speaker configurations were standard.

Cruise Control:

Standard cruise control system.

Climate Control:

Automatic climate control with dual-zone or multi-zone functionality, capable of maintaining set temperatures. Vacuum operated flaps were common.

Connectivity Features:

Limited by modern standards. Primarily mechanical and electrical connections; no integrated Bluetooth or advanced digital connectivity.

🔧 Maintenance

Oil Change:

Every 10,000 - 15,000 km (6,000 - 9,000 miles) or 12 months, whichever comes first. Refer to manual for specific interval based on driving conditions.

Transmission Service:

Every 40,000 - 60,000 km (25,000 - 37,000 miles) or as recommended by the manual, including fluid and filter replacement.

Fuel Filter Replacement:

Every 20,000 - 30,000 km (12,000 - 18,000 miles) or as specified.

Air Filter Replacement:

Every 20,000 - 30,000 km (12,000 - 18,000 miles) or annually.

Coolant Flush:

Every 2-3 years or 40,000 - 60,000 km (25,000 - 37,000 miles).

Brake Fluid Type:

DOT 3 or DOT 4 brake fluid (consult manual for specific MB approval).

Power Steering Fluid:

Mercedes-Benz Power Steering Fluid (MB Spec 343.0) or Pentosin CHF 11 S. Consult manual for correct specification.

Diesel Engine Specific:

Glow plug system issues, vacuum line leaks affecting engine performance and door locks, potential for injector pump wear, injector seal leaks, and turbocharger actuator problems.

Transmission Common:

Valve body issues leading to harsh shifting or slipping, torque converter seal leaks, and potential for transmission control unit (if electronically controlled) problems.

Electrical Complexities:

Complex electrical systems of the W140 era can be prone to harness degradation, faulty sensors, and control module failures. The vacuum system controlling central locking, climate control flaps, and more is a common source of issues.

Suspension And Steering:

Wear in control arm bushings, ball joints, and tie rod ends is common. Power steering leaks and component wear can occur.

Body And Interior:

Corrosion can be an issue if not properly maintained. Headliner sagging, window regulator failures, and door seal degradation are also noted.

📚 Vehicle History

Mercedes Benz Philosophy:

Renowned for engineering excellence, durability, safety, and comfort, often setting industry benchmarks.

W140 Introduction:

The W140 generation S-Class was introduced in 1991, replacing the W126 S-Class. It was a significant technological leap forward.

Sd Variant Significance:

The 350 SD Turbo was one of the diesel variants offered in the W140 chassis, emphasizing efficiency and torque for long-distance luxury travel.

Production Period:

1991-1998 (W140 Generation). The 350 SD designation was specific to earlier models within this generation, often succeeded by the S350 Turbodiesel (inline-six).

Manufacturing Location:

Germany

🏆 Awards & Recognition

Industry Acclaim:

The W140 generation was widely recognized for its engineering prowess, safety advancements, and unparalleled comfort, establishing a benchmark for luxury sedans.

Automotive Journalism:

Often praised for its build quality and quietness, though some criticized its size and weight compared to predecessors.