If the plastic tubing that runs from the manifold banjo bolt across the firewall to the overboost switch then the ALDA looks old and feels really stiff, examine it carefully for splits.

I chased a similar problem on my w116 300sd and finally found that the tubing was split on one spot on its underside, absolutely not visible unless you grabbed it and turned it. So the ALDA was getting only a fraction of the pressure it was supposed to.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22