The older ones shut off the ALDA if boost hit about 1.1 to 1.2 BAR, that might give you a clue. I don't know if the overboost protection kicks in at the same place on a 90-93 or not.

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'79 300SD W116
'86 190E 2.3-16V W201
'92 300D 2.5 W124
'99 E320 4MATIC S210