|At the Frankfurt motor show IAA, in September 1981, types 380_SEC and 500_SEC were presented. They were coupe-variants of the series 126, succeeding the previous SLC-models. In concept, the new coupe-generation once again was based on the S-class limousines and not, as their direct predecessor, on an SL-model. A six-cylinder variant analogous to the 280_SLC was not available. The V8=engines, already know in principal from the limousines and the SLC-models, had been thoroughly redesigned within the ,Mercedes-Benz energy concept" in order to reduce consumption and pollutants.
Next to an increase of compression, a camshaft with changed timing, air-circulated injection valves and an electronic idle-speed control device stood on the list of improvements. Due to the changed cam shift adaptation the torque maximum could be shifted towards lower numbers of revolution. In case of the 3,8_ltr.-engine it was even increased. This aggregate was thoroughly changed. In order to obtain a more favourable volume-surface ratio the bore was reduced and the stroke increased. Thus the modified 3,8_ltr.=V8 showed a slightly increased displacement. In both eight cylinders slight power reductions had to be taken into account, compensated, however, by a considerably improved economic efficiency. In both cases the rear axle transmission ratio was adapted to the changed characteristics of the engine. Thanks to all these measures the fuel consumption of the SEC-coupes were drastically reduced in comparison to their preceding types.
Except for a few details the chassis corresponded to the limousines on which the vehicle was based. Like those the coupes were also provided with a twin-control arm front axle as well as a semi-trailing arm rear axle with anti-squat control. For the first time the front wheels of the SEC-coupes were equipped with floating calliper disk brakes. Thus larger brake disks could be used and the brake cylinder was located at the better cooled interior side of the wheel.
In principal the platform, too, came from the limousine. It was, however, reduced by 85_mm. Thus the wheel base was 30_mm higher than in the preceding model. The passenger cabin was also much broader as in the SLC-types. The body had been constructed according to the newest insights of safety research and the construction principles, already applied at the limousines, were reinforced by further measures. In order to compensate for the missing centre pillar the roof frame constructions had been improved and the A-pillars provided with welded-in high-strength pipes. Thus the coupes also corresponded to the high safety standards of the S-class limousines. Electrically operated belt hand-overs were an interesting detail of equipment. They belonged to the standard version of the SEC-types and had the task to bring the seat-belt at all four seats into the field of vision and to be easily obtainable for driver and passengers alike. On demand, an air bag for the driver and a seat-belt pretensioning device for the front passenger was available.
The design of the new coupes was also based on the four-door cars. A relict of the SLC-episode was the horizontal front grill typical for the SL. From the very beginning, the most elegant and harmonic shape was a great success . It made the SEC-coupe one of the most beautiful cars after the war. The two bumpers as well as the side protection strips were shaped analogous to the limousines. The front apron, however, was pulled further down, In it fog lamps were integrated. A remarkable construction detail were the aerodynamically optimised door handle recesses which prevented them from getting dirty.
Four years after the presentation of the SEC coupes, a comprehensive model improvement package was applied. In September 1985, once again, a completely redesigned type program of the S-class limousines was presented at the motor show IAA in Frankfurt. Besides a discreet face lifting, concerning primarily the bumpers, the side protection and the wheels, a restructuring of the range of engines was important.
New in the program was a 4,2_ltr.-V8-engine, produced by boring the 3,8_ltr.-aggregate. The latter was not only replaced in the SEC-coupes, but also in the S-class limousines and in the SL. The 5,0_ltr.-engine had also been modified. It was now equipped with an electronic ignition system and an electronically mechanically controlled injection system Bosch KE-Jetronic and mobilised a power of 245_PS.
The most spectacular novelty of the range of engines was a 5,6_ltr.-eight cylinder, developed from the 5,0_ltr.-V8 by prolonging the stroke. It mobilised 272_PS. On demand an even higher compressed version was available. It proudly developed 300_PS. However, it could not be combined with a regulated emission control system. Yet, even without a catalyst, this so-called ,ECE-version" fulfilled the exhaust gas limits fixed by the European Economic Commission. Types 560_SEC and 560_SEL, which were equipped with this engine variant, were at the time of their appearance on the market the most powerful standard cars ever built by Mercedes Benz to that day.
For all variants of the redesigned range of models a regulated emission control system with a lambda-probe equipped catalyst was available - except for types 560_SEC and 560_SEL in ECE-version. The standard equipment was the respective so-called ,R?F-version", in which the vehicle was delivered without catalyst and lambda probe, but with a multifunctional mixture preparation and ignition system. Retrofitting a ,back fitting vehicle" with regulated catalyst was always possible and without problems. This handling allowed the client the highest possible flexibility as to the time of refitting. Since, at that time, unleaded petrol was not available everywhere, this was a considerable advantage. Since 1989 the regulated catalyst has been part of the standard equipment of all Mercedes Benz passenger car models with Otto engine. Back fitting models_ continued to be available on demand till August 1989 with a respective reduction in price_.
As expected, the chassis of the model improved types were not changed very much. However, a few details of the rear axle construction had been modified in order to further improve driving comfort and smooth driving. In addition, all coupes and limousines of series 126 were now equipped with 15_inch-wheels and respectively larger brakes. The design of the all-aluminium wheels, available on demand and standard equipment only in the 560_SEC and 560_SEL, had been actualised and adapted to the compact and medium size series.
All further stylistic changes which had been of benefit to the improved models of series 126 had not only served to actualise the design, but had also been developed for very down to earth technical reasons. By pulling down the aprons the front lift could be decreased further and the down stream at the tail improved. Thus driving stability in speeding could be increased once more. This aspect was of special importance for the driving performance of the new top models 560_SEC and 560_SEL.
The protective strips at the side were now smooth and had no grooves any more. Like the bumper system, they now came further down and comprised additional panelling of the frame side rails. Since the standard 5,6_ltr.-types - in contrast to their less powerful sister models - had been equipped with broader tires of dimension 215/65_VR15, the shape of the front apron and the wing bead had been changed in order to achieve the necessary free travel at the side.
In September 1987, power-increased variants of all V8-engines were introduced. In all cases, the compression was heightened to ten and with the help of other measures the power could be further increased by 6 to 10_% according to the model. Even more drastic was the effect on the catalyst variants. By optimising the emission control system, the power decrease, caused by the catalyst, could be considerably reduced. The ECE-version of the 5,6_ltr.-V8 could be dropped without replacement because after the ,power cure" the R?F-version was able to mobilise 300_PS as well.
In September/October 1991, almost ten years after introducing the SEC-coupes to the market, the production was stopped. The total number of cars built was 74.060. It shows how highly popular this model family was. In comparison to the preceding models of the SLC-type range this meant a plus of more than 11.000 vehicles, even though there was no more low-priced model for beginners analogous to the 280_SLC. The by far rarest variant was the 420_SEC with only 3.680 cars built.