I believe the Vacuum Modulator controls shift quality & the B2 Piston controls 1st gear.
In my case the transmission doesn't create drive in any gear. It doesn't even try to engage. When in park you can't turn the rear wheels.

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80 280TE, 5 speed
83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts