I do not know the 240D but I have a 1981 300SD. The vac modulator on the tranny is round and has a ridge in the center so you can turn it CW or CCW to adjust the shift. A vac line runs from the vac modulator (there is a nipple) to the VCV (vacuum control valve) up front at the injection pump. In the pic below the VCV is the white plastic gizmo with vac lines attached. It is supposed to supply about 10-15 ihg vac to the vac modulator until you hit the accelerator at which time the vac goes quickly to zero so you get a smooth upshift. For downshift it applies vac through the modulator to give a smooth downshift.

For the 240D I am guessing you have the same setup. So run the vac line from the VCV back to the vac modulator. Without you will get really bad upshifts, jarring. But the downshifts should be OK without it.