Here is how it works on a 300SD; I would expect the 240D might be the same. There is a stop valve in the engine compartment with a lever actuated by vac. If you put a Mityvac on the nipple and apply vaccum the interior lever pulls back toward the driver's compartment and the engine goes off. This stop valve is inside the back of the ALDA (in the 300SD it is just in front of the oil cannister). There is also a manual stop exterior lever with a clearly marked big red push sticker in case the engine continues to run.

Inside the cabin the ignition switch also has a vacuum nipple and vac hose. When you turn the engine off the vac kicks in and transfers to the stop valve up front. This turns the engine off. If the hose is off that ignition switch nipple or if the ignition mechanism is bad you get no vac and therefore the engine will not shut off without opening the hood and pushing on the manual stop valve.

Guys - this is from a layman's view. If I am not correct or if the 240D does not work this way, please make the correction.