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The flap in the intake doesn't change "vacuum" because there is no vacuum in the turbo charge (intake) manifold.
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I agree that when the flap is opened, there is no vacuum in the intake manifold. But when it closes, I think we should have negative pressure, even if the EGR is opened : the EGR port is half an inch in diameter and the unflapped intake is over 2 inches (the flap is downstream of the turbo : turbo is pushing on a closed door)
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The overboost protection is totally separate
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In 1992 model year, the overboost protection relay of the alda has been discontinued : the alda is connected directly only to the intake and the EDS pressure sensor. It's weird : if you have an EDS error code that cut signal to the flap, you are totally unprotected from a stucked closed wastegate (that situation happens if the vacumm transducer for the control flap/wastegate is defective).
I don't have access to a detailed description of the EDS system and I'm not the man who engineered it : most is interpolation and extrapolation from the sparse data found in the shop CD, direct observation of a specimen and crushed fingers field experience. Just try to share.