🏷️ Basic Information

Make:

Mercedes-Benz

Model:

350 SD Turbo

Year:

1991

Chassis Code:

W140 (S-Class)

Engine Code:

OM603.970

👋 Introduction

Market Position:

The Mercedes-Benz W140 chassis represented the pinnacle of luxury automotive engineering in its time, competing with top-tier offerings from BMW (E32 7 Series) and Lexus (LS 400). The 350 SD Turbo was positioned as a refined, powerful, and fuel-efficient diesel variant within the S-Class lineup.

Competitive Analysis:

At its introduction, the W140 S-Class, including the 350 SD Turbo, was praised for its build quality, advanced technology, and quiet cabin. It faced stiff competition from other German luxury marques and the emerging Japanese luxury segment. This manual provides the specific knowledge required to maintain and repair a vehicle that was a benchmark in its class.

Key Features:

The 350 SD Turbo's key features include its robust turbocharged diesel engine, luxurious interior appointments, advanced safety systems for its era, and the sophisticated engineering characteristic of Mercedes-Benz vehicles of this period.

📋 Manual Details

Coverage Summary:

Comprehensive service and repair for all mechanical and electrical systems of the 1991 Mercedes-Benz 350 SD Turbo (W140).

Page Count Estimate:

Approximately 1500-2000 pages (typical for a full workshop manual of this era)

Manual Type:

Workshop Service Repair Manual

Language:

English

Format:

Digital (likely PDF), printable

📖 Manual Coverage

Engine:

Complete engine overhaul, diagnostics, fuel system, cooling system, lubrication, intake and exhaust systems, turbocharger system.

Transmission:

4-speed automatic transmission (722.3xx series) overhaul, diagnostics, fluid services, control systems.

Drivetrain:

Rear-wheel drive, driveshaft, differential, axle shafts.

Suspension:

Front and rear suspension components, shock absorbers, springs, bushings, hydraulic leveling system (if applicable).

Brakes:

Hydraulic brake system, ABS (Anti-lock Braking System), brake fluid, calipers, rotors, pads, lines, parking brake.

Steering:

Power steering system, steering column, steering gear.

Electrical:

Complete electrical schematics, battery, charging system, starter, ignition system, lighting, instrument cluster, power accessories, ECU diagnostics.

Climate Control:

Automatic climate control system (Klimatronik), A/C system, heating, ventilation, refrigerant.

Body And Chassis:

Body panels, doors, windows, locks, exterior trim, interior trim, chassis components, corrosion protection.

Diagnostics:

On-board diagnostics (OBD) information, troubleshooting guides, fault code interpretation, wiring diagrams for diagnosis.

Scheduled Maintenance:

Detailed procedures for routine maintenance, oil changes, filter replacements, fluid checks and top-offs.

Component Repair:

Step-by-step instructions for component removal, disassembly, repair, reassembly, and installation.

Troubleshooting:

Systematic troubleshooting charts and symptom-based diagnostics for identifying and resolving issues.

Rebuilds:

Complete rebuild procedures for major components such as the engine and transmission.

Electrical Diagnosis:

Detailed diagnostic procedures for electrical circuits and components, including troubleshooting wiring faults.

Depth:

Expert level: Suitable for professional technicians and advanced DIY mechanics. Covers detailed internal component specifications and repair.

📊 Manual Specifications

File Format:

PDF

Print Quality:

High-resolution scans, ensuring clarity of diagrams and text for printing.

Binding Type:

N/A (Digital format)

Paper Type:

N/A (Digital format)

Searchability:

Searchable text within the PDF document for quick access to information.

⚙️ Engine Specifications

Engine Type:

Inline 6-cylinder Diesel with Turbocharger

Engine Displacement:

3.5 Liters (3449 cc)

Engine Code:

OM603.970

Bore X Stroke:

94.0 mm x 92.4 mm

Compression Ratio:

20.2:1

Valvetrain:

SOHC (Single Overhead Camshaft), 12 valves (2 per cylinder)

Horsepower:

136 hp (101 kW) at 4000 rpm

Torque:

243 lb-ft (329 Nm) at 2000 rpm

Fuel Injection:

Bosch VE (Variable Emission) Mechanical Injection Pump with Diesel Injectors

Fuel Type:

Diesel Fuel

Cooling Type:

Liquid-Cooled

Coolant Capacity:

Approximately 10.5 Liters (2.77 US Gallons)

Oil Capacity:

Approximately 7.5 Liters (7.9 US Quarts)

Recommended Oil Type:

API CD or CE rated diesel engine oil (e.g., Mobil Delvac 1300 Super 15W-40, Shell Rotella T 15W-40).

Turbocharger Model:

Garrett (specific model number may vary, typically GT32 or similar for this application)

Boost Pressure:

Approximately 0.9-1.0 bar (13-14.5 psi) nominal

🔧 Transmission

Transmission Type:

4-Speed Automatic

Transmission Model:

722.370 (part of the robust 722.3xx series)

Transmission Features:

Lock-up torque converter, electronically controlled clutch pressure, winter/economy mode selection.

Transmission Fluid Type:

MB 236.1 (e.g., Mercedes-Benz ATF Dexron II or equivalent)

Transmission Fluid Capacity:

Approximately 8.0 Liters (8.5 US Quarts) for a dry fill, less for a fluid change.

📐 Dimensions

Length:

Approx. 5113 mm (201.3 in)

Width:

Approx. 1886 mm (74.3 in)

Height:

Approx. 1480 mm (58.3 in)

Curb Weight:

Approx. 1960 kg (4321 lbs)

Gross Vehicle Weight Rating:

Approx. 2500 kg (5511 lbs)

🛡️ Safety Features

Safety Ratings Era:

As an early 1990s luxury vehicle, specific NHTSA/Euro NCAP ratings as we know them today were not standardized. However, it was designed to meet rigorous safety standards of the era.

Active Safety Features:

Anti-lock Braking System (ABS), Traction Control System (ASR - Automatic Slip Regulation) was often optional or standard on higher trims, advanced suspension geometry for stability.

Passive Safety Features:

Reinforced body structure, dual front airbags, crumple zones, side-impact protection beams, seatbelt pre-tensioners.

💡 Technology Features

Infotainment System:

Basic for the era: AM/FM stereo with cassette player, potentially optional CD changer. Emphasis was on sound quality and integration.

Driver Assistance Features:

ABS, ASR (optional), cruise control, power-adjustable seats with memory.

Connectivity:

Limited by modern standards. Primarily for internal vehicle systems communication.

Convenience Features:

Power windows, power door locks, power mirrors, automatic climate control, heated seats (optional), self-leveling suspension (optional).

🔧 Maintenance

Oil Change Interval:

Typically every 7,500-10,000 miles (12,000-15,000 km) or 12 months, depending on driving conditions and oil used.

Transmission Service Interval:

Fluid and filter change every 30,000-50,000 miles (48,000-80,000 km).

Fuel Filter Replacement:

Recommended every 20,000-30,000 miles (32,000-48,000 km) for diesel.

Air Filter Replacement:

Every 15,000-30,000 miles (24,000-48,000 km) or as needed based on inspection.

Coolant Type:

Mercedes-Benz Specification 325.0 (Antifreeze/Coolant)

Brake Fluid Type:

DOT 3 or DOT 4

Power Steering Fluid Type:

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

Known Issues:

Early W140s are known for potential 'dying battery' issues due to complex electronics draining power if not addressed. Biodegradable wiring insulation was used in some models, leading to degradation. Vacuum line leaks can affect central locking and other systems. Overheating due to radiator or coolant issues. EGR valve and turbocharger maintenance is critical for diesel longevity.

📚 Vehicle History

Brand Heritage:

Mercedes-Benz has a long-standing reputation for engineering excellence, luxury, and safety, dating back to the invention of the automobile itself. The S-Class line has always represented the flagship model, showcasing the latest in automotive technology and comfort.

Model Evolution:

The W140 S-Class replaced the W126 S-Class and was produced from 1991 to 1998. It was a significant departure from its predecessor, featuring a larger, more imposing presence, and a vastly expanded array of technological features. The diesel variants, like the 350 SD Turbo, were popular in markets where diesel fuel was economical and emissions regulations allowed for their performance.

Production Years:

1991-1998 (W140 Chassis)

Manufacturing Location:

Germany

🏆 Awards & Recognition

Recognitions:

The W140 S-Class was widely recognized for its technological advancements and luxury appointments, often lauded by automotive journalists for setting new benchmarks in its segment upon release.