Looking closer at your photo above, your pressure line to the switchover valve is broken off. It should be connected where the tape is on that peg sticking forward. With that broken off, you'll never get boost pressure at the ALDA.

That switchover valve is the "overboost protection". In an overboost condition, it opens the ALDA to atmospheric pressure to cut fuel and limit power. Pretty well exactly what's happening here with no boost pipe hooked up!

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]