Mercedes-Benz first series automatic transmission

This transmission was the first Mercedes-Benz automatic transmission in house developing.[1] Before this, the company used semi-automatic systems like a vacuum-powered shifting for overdrive or the "Hydrak" hydraulic automatic clutch system. Alternatively, they bought automatic transmissions of other vendors, such as BorgWarner.

The automatic transmissions are for engines with longitudinal layout for rear-wheel-drive layout passenger cars. The control of the fully automatic system is fully hydraulic and it uses electrical wire only for the kickdown solenoid valve and the neutral safety switch. Three-speed units use two planetary gearsets and 4-speed units use three planetary gearsets.[2]

Physically, it can be recognized for its pan which uses sixteen bolts.

For the 4-speed models[lower-alpha 1] 8 main components[lower-alpha 2] are used. The 3-speed-models[lower-alpha 3] uses 7 main components,[lower-alpha 4] which shows economic equivalence with the direct competitor.

The K4A 025 (without type designation) is the first of the series, launched in April 1961 for the W111 220SEB, later replaced with the K4C 025 (type 722.2). It is a 4-speed unit and uses fluid coupling (also referred in some manuals as hydraulic/automatic clutch). It is the only exemption which uses only two planetary gearsets for four speeds.

The layout of the K4A 025 turned out as not very reliable. The Mercedes-Benz 600 from 1964, the first post-war "Grand Mercedes", is powered by the Mercedes-Benz M100 engine. The automatic transmission was replaced by a three planetary gearset design to handle the much higher torque of the big block V8 engine and to meet the much higher standards of luxury vehicles.

The K4C 025 (type 722.2) was the core model of the first automatic transmission series from Mercedes-Benz. The layout is based on the K4B 050, which turned out as much more reliable as the previous layout from the K4A 025. With the small block V8 engine M 116 the K4A 040 (type 722.2) as a stronger version of the same design was launched.

As a three-speed unit, the W3A 040 and W3B 050 (type 722.0) is combined with V8 engines, and it uses torque converter instead of fluid coupling.[1][2] The transmission saves one planetary gearset and uses the same housing as the four-speed versions. The free space therefore is used to reinforce the shift elements (brakes and clutches) to handle the higher torque of the V8 engines.

First the W3A 040 was released for the all new M117 V8 engine of the Mercedes-Benz W108 and W109 in 1971. The second in the series is the W3B 050, which was released initially for the W116 450 SE and SEL in 1973. At that time the four-speed transmission for the 350 SE and SEL was replaced by this three-speed model. The reinforced W3B 050 reinforced (type 722.003) is the strongest of the series, able to handle the input of the enlarged version of the Mercedes-Benz M100 engine, the biggest Mercedes-Benz engine in post-war history,[10] exclusively used in the W116 450 SEL 6.9.

The W4B 025 (type 722.1) is the K4C 025, where the fluid-coupling is replaced by a torque converter.[2] Used in L4, L5 and L6 engines due to its lower torque output. For V8-engines see W3A 040 and W3B 050 (type 722.0) above.

In normal situations, it rests statioary in 2nd gear, but it will use 1st gear when the vehicle starts moving and throttle is applied[11] or if L position is selected in gear selector.

The W4A 018 (type 720.1) was derived from the W4B 025 (type 722.1) for vans up to 5.600 kg (12,345 punds) and off road vehicles, the W4B 035 from the W4B 025 (type 722.1) and K4A 040 (type 722.2) for light trucks up to 13.000 kg (28,660 punds).[8] The main difference is the use of straight toothed planetary gearsets instead of helical toothed ones for better fuel efficiency at the price of lower noise comfort.