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I actually wanted to better understand what about the engine running caused the transmission to engage but to release when the car is off. Obviously a different design than "other" cars. Is it vacuum?
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All automatic transmissions use hydraulic pressure to engage and disengage the friction elements at each gearset. When the engine is not running, the input shaft of the transmission is not turning and the hydraulic pump in the transmission is not either. Some transmissions, like these, have a sort of tow-to-start capability whereby pump pressure builds if the output shaft is turning, but this is well beyond my understanding...
That the car will roll away in Drive with the engine off is not unique to M-B. Any automatic will do so. The difference is you can't take the key out on most cars unless you shift to park.
Added complexity aside, I think it's a pretty good idea.
Ted