Daimler’s development of torque converter automatic transmissions spans more than 50 years, beginning with the four-speed K4A025 that was brought to the market in 1961. That was the start of a development timeline that has seen big changes in automatic technologies, most noticeably the number of speeds a transmission has.
In 2013 Daimler brought its latest torque converter to the market, the nine-speed W9A700 system. The transmission has been used in the firm’s Mercedes-Benz E-Class. The first vehicle to receive it was the 185kW/620Nm V6 diesel version of the sedan, and it helped to cut fuel consumption and CO2 emissions from 5.5 litres/100km and 144g/km to 5.3 litres/100km and 138g/km respectively.
The transmission has now been adapted to be used in SUVs too, as well as in über luxury sedans from the firm’s Mercedes-Maybach brand, allowing it to be mated to rear-wheel and all-wheel drive platforms. The first application was in the Mercedes-Benz GLE.
Dr Christoph Dörr, senior manager for automatic transmission projects at Daimler, said: “The modular components and subassemblies introduced in the torque converter meant a simple adaptation became possible for different vehicles and with diesel and gasoline engines.”
The transmission was developed with Tier One supplier Schaeffler. It is an improved version of the previous-generation W7C700 seven-speed unit, and it retains the wide-angle double torsional damper system and centrifugal pendulum absorber for improved NVH.
The reasoning behind the transmission’s design was to optimise start-up comfort, suitability for larger vehicles, and to accommodate torque levels of up to 1,000Nm.
As with the two-wheel drive variant, the ratio spread has risen from 6.00 on the older seven-speed unit to 9.15 to improve driveability, refinement and fuel efficiency.
The gear layout of the transmission consists of four single planetary gear sets and six shift elements. There are three clutches and three brakes.
Gear sets one and two use cast aluminium carriers, while three and four use sheet steel. To reduce weight, the torque converter housing is aluminium and the transmission casing is magnesium. The sump is a plastic moulding. One of the most difficult components to make is the input shaft. At 548.9mm, it’s pretty long, and features three deep, concentric bores.
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