My guess is you can see nearby where the two hoses which used to be connected to that switch have been re-connected together. Otherwise I think your ALDA won't supply fuel at all.

I have had mine bypassed on all my diesels to no harmful effect. I am not sure what happens at "overboost" except that excess fuel is dumped into the cylinders, causing excess smoke. A few others have also bypassed that switch. Two lines of thinking on this, one is that Mercedes engineers are clever fellows, the other is that any way to get a little extra zip out of 20+ year old cars is worth doing.

I reconnected mine recently, drove it for a few weeks, then reconnected it. Lots of highway miles, lots of hard acceleration. Hard to notice much difference, more smoke under hard acceleration, more boost sooner (under 2K rpms).

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Peter
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K