Just so we're all clear on how the transmission works, it was designed for a gas engine, then with some complicated plumbing adapted to work on the Diesels.

On a gas engine, when you press the gas pedal, manifold vacuum drops. The further the throttle is open, the lower the vacuum. Low vacuum means high load, and requires a firmer shift. High vacuum means low load and requires a softer shift to prevent the abrupt jerking associated with a firm shift at low RPM.

The diesels don't have a manifold vacuum (especially if they're turbocharged) and have a relatively fixed vacuum source via the vacuum pump. The older models have the VCV, the newer ones have the blue "UFO" vacuum amplifier that modulates vacuum to the transmission to mimic the manifold vacuum changes of a gas engine.

On any engine (gas or diesel) with the Mercedes transmission, without a vacuum source you will get firm, jerky shifts and possible weird/early shift points. If you don't have the VCV or equivalent in place and functioning properly so that you just have full vacuum applied all the time, you'll have mushy/slushy shifts and flares between gears.