In March 1991 the new S-Class generation (designated internally as the 140 series) made its debut at the Geneva Motor Show. The body design incorporated the typical traditional Mercedes-Benz stylistic elements, enabling it to fit in seamlessly with the visual appearance of the company's other passenger car models. As had already been the case with the SL models in the 129 series, the trademark radiator grill of the new S-Class was given a new stylistic interpretation while retaining the traditional basic shape. This variation on a classic theme was designated the 'integrated radiator' and with its much narrower chrome frame the radiator shell was organically integrated into the engine cover. For the first time, the Mercedes star was positioned not on top of the radiator grill, but slightly to the rear on the bonnet. The overall aim of the design concept of the new S-Class generation was to achieve a high degree of aerodynamic quality while at the same time ensuring maximum everyday practicality.
As with the predecessor models of the 126 series and generations of Mercedes-Benz premium-class series before them, the standard version was accompanied by a long-wheelbase variant, in which the additional 100 millimetres once again served exclusively to increase legroom in the rear. In terms of the engines, four units were initially available on the domestic market, of which only the 5.0-litre V8 four-valve M 119 was an old and familiar friend. As with the 500 E, the engine used here was the Einheitsdeckmotor (standard-deck engine), whose fully electronic LH Jetronic injection system from Bosch was controlled via a hot-wire air mass sensor. The other three engines were newly developed. Like the 5.0-litre unit, the 4.2-litre four-valve V8 was based on the trusty 4.2-litre two-valve engine, and the six-cylinder in-line engine with 3.2-litre displacement was based on the 3.0-litre four-valve unit introduced two years earlier. An interesting detail to note here is that the model designation of the 3.2-litre and 4.2-litre models did not reflect exactly the displacement as had always been the case in the past. Instead, for the sake of consistency, the designations 300 SE/SEL and 400 SE/SEL were chosen.
The 6.0-litre V12 M 120 engine was an entirely new design. Not only was it the first twelve-cylinder Mercedes-Benz made for a production passenger car, but it was also the most powerful Mercedes-Benz car engine of its day, with a rated power output of 300 kW (408 hp). With a peak torque of 580 Newton metres, it reached the 500 Newton metres mark at 1600 rpm. As with the six-cylinder and the two V8 engines, the twelve-cylinder was also equipped with four-valve technology, variable intake camshaft and an electronic injection system with hot-wire air mass sensor. With all engines a high priority was placed on minimising exhaust emissions and reducing fuel consumption. The new fully electronic ignition system calculated the optimum ignition point from 300 ignition maps, tuned for each cylinder individually and to the knock limit in each case. The M 120 was the only twelve-cylinder engine in the world to feature this cylinder-selective anti-knock control. Without this it would not have been possible to achieve the high compression ratio of 10:1, necessary for optimum use of fuel.
The engine management and drive management were also completely new. All their control modules communicated with one another via a common data channel, meaning the control units could all be active at the same time. This was used for rapidly warming up the catalytic converters when cold-starting the engine, as well as for acceleration skid control and for the new engine friction torque control, which maintained handling stability during power-off situations on slippery road surfaces.
The V12 offered the world's largest catalytic converter unit for passenger cars. Its seven-litre volume avoided any excess fuel consumption on account of the catalytic converter and ensured a high degree of long-term stability. Thanks to an innovative concept involving a double-walled and triple-insulated exhaust manifold, as well as double-walled pipes, the ceramic catalytic converters – embedded in insulating expandable matting – reached the optimum operating temperature in a very short time. As well as reducing exhaust emissions and making them less harmful, the new S-Class featured a number of other details that made it a pioneer for environmentally responsible automotive manufacturing. It heralded the age of the CFC-free car and set new standards in recycling. The plastic components used were not only recyclable and clearly identifiable, they were also to a large extent manufactured using regranulated plastics. In 1992, the S-Class was presented with the Stratospheric Ozone Protection Award by the US Environmental Protection Agency.
Not only did the developers of the new S-Class lower fuel consumption and improve environmental compatibility, they also raised comfort and safety to new levels of perfection. The meticulous design and coordination of the running gear played a particular role in this, alongside a number of other factors. A newly developed double wishbone front axle, with the main point of load application mounted on a subframe, provided front suspension – a system designed to isolate the body from audible and perceptible vibrations. Rear suspension was derived from the multi-link independent suspension of the other passenger car ranges, although radically revised in terms of wheel location and modified in line with the special requirements of the S-Class. To take account of the significantly greater longitudinal and transverse forces, the link geometry was also redesigned. Of particular interest was the crossed design of the upper links, which allowed axle space to be kept to a minimum in spite of the long control arms.
In terms of active safety the S-Class saloons in the 140 series were known for their exceptionally good straight-line stability even on uneven road surfaces, their low crosswind sensitivity, their precise and responsive steering, and the way that their handling was largely unaffected by the size of payload. The brake system for the eight and twelve-cylinder models was a fundamental innovation. By distributing more braking power to the rear wheels, it was possible to improve the fade resistance of the brake system and reduce wear to the front wheel brakes.
The reduction of tyre noise and vibrations transmitted to the passenger compartment, the minimisation of the pitching motion on moving off and braking, the reduction of the rolling motion on cornering or uneven road surfaces and the fact that jolts transmitted from the road had virtually no impact on the steering all contributed to the excellent ride comfort of the S-Class. A 'parameter steering' system with speed-sensitive steering moment was fitted as standard to the eight and twelve-cylinder variants, reducing the steering effort required by the driver at low speeds, for example when parking.
But it wasn't only ride comfort and active safety where progress was made. Passive safety also saw advances. Safety had already been taken to high standards in the predecessor model series, but here numerous measures achieved further improvements. The new body structure, for example, provided even more safety in all types of accidents. And a series of small design improvements designed to reduce hazards from potential impact points also gave added protection to other road users.
Central locking and electrically operated windows came as standard and ensured a high level of operating comfort. The first soundproofed glass windows to be fitted to a passenger car series also made a significant contribution to improving comfort and incorporated a range of safety and comfort features. The windows were less liable to fog or ice up or form condensation, they gave better heat insulation as well as soundproofing from external noise, they improved external air flow and they eliminated wind noise caused by window seals.
Two further design details – folding exterior mirrors and extendable guide rods to help with reversing – gave drivers additional assistance when manoeuvring in small spaces with poor visibility. The electrically operated exterior mirrors could be folded back to gain extra space when manoeuvring in tight situations by means of a centrally positioned switch on the centre console, the same switch serving also to angle the mirrors. The two extendable guide rods integrated into the rear wings – one on the left and one on the right – helped the driver to judge distances to obstacles when performing reversing manoeuvres. Two seconds after selecting reverse gear, the pneumatically operated 65-millimetre-long chrome rods automatically extended vertically, returning again eight seconds after a new gear had been selected.
At the Paris Motor Show in October 1992, the 300 SE 2.8 and 300 SD models were introduced, adding two cheaper and particularly economical variants to the S-Class range. The 300 SD attracted particular attention – a vehicle that had been exported to the USA since October 1991, but which was now the first diesel model in the S-Class to become available in Germany. The 300 SD was powered by a 3.5-litre six-cylinder engine with exhaust-gas turbocharger, a unit which was in principle the same as the one used in the predecessor model from the 126 series, but now in a revised version delivering 110 kW (150 hp). Like the 300 SE, the second new arrival, the 300 SE 2.8, offered a six-cylinder in-line unit with four-valve technology – also a member of the M 104 family of engines. The newly developed 2.8 litre variant was used from the same point in time in the 124 series as well, and was equipped with a microprocessor-controlled direct injection system in which the hot-wire air mass sensor had been replaced by a hot-film air flow sensor. In addition to the two new models, the eight and twelve-cylinder versions appeared in Paris with revised engines. All three units did away with mixture enrichment under full load, resulting in a minor loss of output but bringing benefits in terms of emissions.
In June 1993 the model designations were changed to come in line with other series in the passenger car range; the 'S' was now placed before the three-figure number, and suffixes such as 'E', 'D' and 'L' were omitted. The 500 SE, for example, became the S 500, and in accordance with the new system of nomenclature the 600 SEL was renamed the S 600 long. Ever since, the plate on the boot lid has documented only the class and engine displacement and not the body variant (normal or long-wheelbase version) – this was entirely apparent to anyone taking a closer look. The most significant changes came in redesignating the 4.2-litre and six-cylinder models. Instead of the figures used up to that point, which had been rounded off to full hundreds for consistency, the figures used corresponded to the actual displacement values. And so the 300 SE, for example, became the S 320 and the 300 SD was now known as the S 350 Turbodiesel. In addition to these purely superficial changes, the two 3.2-litre models also benefited from a number of technical improvements. The engine was replaced with a revised version that had been in service since October 1992 in the 124 series and now also featured a variable-resonance intake manifold and a direct injection system with hot-film air flow sensor. These improvements permitted an increase in torque and meant both maximum output and maximum torque could be achieved at lower engine speeds. Thanks also to an additional reduction in friction losses, fuel consumption was reduced by a total of 7.5 percent and overall performance marginally improved.
In keeping with tradition, the S-Class saloons were also available as armoured versions – with a choice of 5.0-litre V8 or 6.0-litre V12 engine. Production of both these special protection models began in February 1992, one year after main production start-up for the 140 series.