The world premiere of the new 129 series SL in March 1989 belonged to the main attractions of the Geneva Motor Show. The new types 300 SL, 300 SL-24 and 500 SL showed no common features with their preceding models except for their overall concept. Just like 18 years before, when the 350 SL demonstrated a great number of innovative construction details in automotive progress, the 129 series SL now also was a completely new car.
The new SL generation convinced by a harmonic and very successful stylistic shape. It expressed dynamics, sportiness, but also comfort and elegance. On the one hand, the design presented itself traditional, on the other hand, its new solutions of many details looked far into the future. Thus, for instance, the radiator grill, the legendary trade mark of the SL models, had been newly interpreted on the basis of remaining traditional shapes. In contrast to the preceding types, where the broad chrome frame and the star were dominant, the radiator cowling was now organically integrated into the engine bonnet. The star, now the only chrome decoration of the car, had been completed by horizontal lamellas of anodized aluminium.
Considerable improvements as to passive safety could be achieved in comparison to the preceding models. With high efforts in development the exemplary safety standard of the saloons and coupés could also be reached in the SL. Since there was no normal roof unit, a basically different body structure and therefore an intensive development work was demanded. The envisioned impact-resistant passenger compartment was achieved by combining several structural elements. Included were, first of all, a specially stiff body platform with high-strength metal sheets, large supporting elements as well as flexural resistant transitions from the A-pillars and the rear side parts to the frame side rails. The doors played a decisive role. By a well-devised bead of the door shell inner panel they functioned as a pressure resistant link between the front wall pillar and the centre pillar. Thus they contributed to preserving the survival space in case of a collision.
A further point of development was the realisation and development of the forward structure, already well-proven in the saloon range. It was based on the fact that front-end impacts are often offset. By reinforcing the front end cross-ties, the side farther away from the front crash is now able to absorb part of the impact energy.
An integral part of the safety concept was the automatic rollover bar. It had been realised in car manufacturing for the first time. Its task was to secure the survival space of the passengers in case of a rollover. In order not to spoil the joy of driving in an open car by a fixed and inflexible rollover bar, a flexible solution was realised. Now the rollover bar is only activated in case of need. During the rest state the safety bar, consisting of a U-shaped high-strength steel pipe clad with PU-foam, was positioned in front of the folding-top compartment. It formed the end of the tail and was in one line with the folding-top compartment lid. In case of an imminent rollover the rollover bar was released by an electromagnatic sensor system. It folded up within 0.3 seconds by the force of prestressed springs and was secured by ratchets. The high-strength centre pillars served as basis for fixing and support. They were extensively linked with the rear frame side rails. In addition to the automatic „crash-triggering“ in case of need, there was also a manual operation. If desired by the driver, it could be activated by a switch. For this „comfort operation“ the folding up and the retracting was regulated by a hydraulic element.
But the survival space in case of a rollover was not only secured by the rollover bar. It was also supported by a special construction of the A-pillars. They consisted of high-strength metal sheets and were reinforced with high-strength tubes at the area of the upper door edges. Together with a two-tray front roof frame and a windscreen bonded with the body a very high stability was reached even if force was applied on one side only.
Due to the rollover bar, the coupé hardtop had no more decisive function for the stability of the passenger compartment any more. Therefore, it now consisted of all-aluminium and weighed 10 kg less than the hard top of the preceding model, even though it had a larger window area. This was of great advantage for mounting and removing the top. Altogether the reduction of weight was an important factor. By consistent light-weight construction and the extensive use of high-strength sheets a body shell structure weight of 405 kg could be reached. In spite of a significantly improved structure safety this was only 20 kg more than that of the preceding model.
A further important feature of SL-specific occupant protection were the newly developed integral seats, which were designed to provide extra protection in the case of side impact. The seat frame, made of five extremely stiff magnesium castings, made it possible to integrate into the seat the seat belt system including tensioner and belt height adjustment with automatic head restraint adjustment, as well as the all-electric seat adjustment functions.
Not only safety was important, but also comfort, the foundations for which had been laid in the design of the bodywork. Additional diagonal members had been included to reduce the vibration and torsional movement typical of an open sports car. The front axle sub-frame was connected to the door sills and at the rear the spare wheel well was also connected to the door sills. This meant that torsional stiffness was 30 % better than with its predecessor and total stiffness was similar to that of a saloon car.
The new design of electro-hydraulic roof, standard equipment on the SL, was particularly convenient. By operating a single push-button the roof could be opened and stowed within 30 seconds. Closing the roof was just as fast. While opening or closing the roof the side windows and roll-over bar were automatically lowered and then raised again. The process was micro-processor controlled, involved the use of 17 end-position switches, 15 pressure cylinders and 11 magnetic valves.
A further innovation, used for the first time on the 129 series SL, was the so-called „windbreak“ consisting of a fine mesh material with a metal frame which could be fitted to the roll-over bar. When raised, the windbreak considerably increased comfort while driving open by reducing wind and wind noise to a minimum. All new SL versions were fitted with electric windows and central locking as standard equipment. The 500 SL also had electric steering column adjustment for height and distance of the steering wheel. The SL’s technical equipment was a combination of innovation and tried-and-tested technology. Its suspension was fundamentally the same as that of the 201 and 124 series. Thus the new SL models also had spring-damper struts at the front and multi-link suspension at the rear, guaranteeing outstanding ride and roadholding. Many components had to be adapted for the SL and axle geometry was also specially tuned.
All 129 series models had 16-inch light-alloy wheels fitted with 225/55 ZR 16 tires as standard equipment. These larger wheels meant that larger brakes could also be used than on the predecessor. All three models had anti-locking brake systems (ABS). The range of engines was also a combination of old and new. All three engines were fitted with three-way catalytic converters. The 3-liter six-cylinder engine of the 300 SL had not only been used in the previous SL but also in the 124 and 126 series. The most important improvements for this application were a redesigned combustion chamber, giving reduced emissions of unburnt hydro-carbons, and a modified and improved catalyst and exhaust system, which boosted nominal output of the M 103 engine from 180 to 190 hp.
The four-valve version of the 3-liter engine as used in the 300 SL-24 was a new development based on the M 103. The 4-valve cylinder head and the camshaft adjustment were new. Combined with higher compression and electronic ignition with anti-knock control the 4-valve engine (M 104) had a power output 40 hp higher than that of the M 103. This meant that the 300 SL-24 had a significantly better performance than the 300 SL with almost identical fuel consumption, however, the price of the car was almost DM 10,000 higher.
The main attraction at the Geneva Motor Show was the 500 SL, not only the most powerful SL with its 326 hp 5-liter 4-valve V8-engine, but also the most powerful standard Mercedes-Benz car. The design of the 4-valve V8 with code M 119 was based on the 5-liter light-alloy M 117, which had been produced for ten years. To improve performance the crankcase, crankshaft and connecting rods had been extensively improved. The 4-valve cylinder head was a new development; like that of the six-cylinder engine it had adjustable inlet camshafts. As with both six-cylinder engines, the exhaust and catalyst system had been thoroughly revised. The sheer power of the 5-liter 4-valve engine gave the 500 SL breathtaking performance far in excess of that of its predecessor: 0-100 kph was achieved in 6.2 seconds and the top speed was electronically limited to 250 kph.
Unlike its predecessor, for capacity reasons the new SL was not produced in Sindelfingen, but in Bremen. Production started there in March 1989. The enthusiasm with which the 129 series SL was immediately received can be seen from the fact that annual production of 20,000 was insufficient to meet demand and shortly after its introduction there was a waiting list of several years. An unpleasant side-effect of the high demand was that some speculators were able to re-sell their new SL cars for a profit well in excess of DM 10,000.
From October 1992, three and a half years after its presentation in Geneva, the SL was also available with the 400 hp 6-liter V12-engine, which had already been used in S-Class saloons and coupés. The fuel injection and throttle systems were modified to reduce emissions further resulting in an output of „only“ 394 hp, 14 hp less than in the original version. The V12-engine gave the 600 SL extremely high performance, although the figures were not much better than for the V8: 0-100 kph in 6.1 seconds and the same electronically limited top speed of 250 kph. The difference in price of over DM 60,000 meant that the 600 SL was a matter of prestige. Of course the 600 SL was comprehensively equipped, including many features that were options on the 500 SL such as adaptive damping with body-height control on both axles, air-conditioning, leather upholstery, heated seats etc.
At the same time as the new top model appeared the 500 SL was modified slightly. The original version of the 4-valve V8-engine with Bosch KE-Jetronic fuel injection was replaced by the one used in the 500 E and the S-Class saloons and coupés with Bosch LH-Jetronic fuel injection with electronic air-mass sensoring and an altered crankcase. The richer fuel mixture during full throttle was eliminated in the interest of lower emissions, as with all other V8- and V12-models, resulting in a slight loss of output of 6 hp which made hardly any difference to performance.
In June 1993 the new model designation system, introduced with the presentation of the C-Class, was also applied to the SL models. The traditional „SL“ now came before a three-digit figure which represented cubic capacity; thus the 500 SL became the SL 500. Some technical changes also took place: both 6-cylinder models got new engines; the two-valve and four-valve versions of the old 3-liter engine were replaced by new 4-valve engines with 2.8 and 3.2 liters. These engines belonged to the M 104 series, like the old 3-liter 4-valve, and had already been used in other series.
The new 2.8-liter engine of the SL 280 had been used in the 124 series and the S-Class since October 1992. It gave better power and torque than the old 2-valve 3-liter of the M 103 family without using more fuel. The 3.2-liter engine of the SL 320, which had been used in the S-Class since March 1991, had the same nominal output as the old 3-liter 4-valve, but this output was reached 900 rpm earlier. Maximum torque was increased and higher torque was available at all engine speeds. The new six-cylinder versions were significantly more powerful and smoother than the old models. Only the SL 280 was still available with manual 5-speed transmission, all other versions had automatic transmission as standard.