7.4. Automatic Transmission (AT)

Principle of work of AKP
Hydrotransformer
Check of AKP
Possible malfunctions of AKP, their reason and ways of elimination



Even with full synchronization of transfers and their easy switching the address with a mechanical box of gear shifting all the same remains a difficult task for many drivers. Therefore on the Mercedes cars the unit removing a task of switching of speeds from the driver — the automatic transmission (AT) is also used. With its help the choice of necessary transfer depending on traffic conditions comes automatically, as a rule, from four transfers of the forward course. On the fourth generation of AKP which are released by Mercedes-Benz since the beginning of 1995 norm are five main transfers. Models CDI of the E-class are serially equipped with five-step AKP of the latest development with electronic control.

Fig. 6.9. Compact five-step AKP in a section: 1 – hydrotransformer; 2 – multidisk coupling; 3 – planetary mechanism; 4 – the locking parking mode gear wheel; 5 – driveshaft flange; 6 – the intermediate lever operated by the lever of the choice of transfers on the central console


For five transfers of the forward course to AKP of new generation only three planetary mechanisms 3 (fig. 6.9) and six elements of multidisk coupling 2 for torque transmission without rupture of a stream of power are used. Still the same number of transfers required four planetary mechanisms and seven elements of coupling.
Instead of the multidisk friction clutches (couplings) and tape brakes used for these purposes engineers of Mercedes applied multidisk friction clutches and multidisk brakes in AKP of new generation. Advantage consists that during all service life of the unit quality of their work remains optimum. Reduction of number of planetary mechanisms allowed to make AKP of more compact: no five-step AKP of this class of other producers, four-stage can even approach AKP Mercedes in this parameter. Its length makes only 600 mm that is 150 mm shorter than other comparable AKP. Approximately also four-stage AKP Mercedes, but in it one transfer less has the same size and there is no hydrotransformer with the blocking coupling. New AKP and on weight differs. Its weight equal to 80 kg honor one third less analogs. Use by production of the AKP case of Mercedes of sheet metal instead of heavy cast and shod designs allowed to achieve such result.

Fig. 6.10. Five-step AKP in disassembled state clearly demonstrates noticeable reduction of number of details in comparison with the previous samples


Thanks to a modern design new AKP consists of only 630 details (rice 6.10). It is 45% less than comparable five-step models.