90-up models use a vacuum-operated wastegate, controlled by the ECU. If it senses anything wonky, it keeps the wastegate fully open so there is zero boost.

The engine is similar to the 3.0 but not the same. To test, try whatever modifications you want and see what happens.

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Perhaps its the 124 or 140 603's that did that. Ill look for vacuum line to wg when i get home.
So vacuum controlled wg on a 603s always had ecu control?

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Vacuum wastegate always has ECU control.

124 and 140 definitely had the vacuum wastegate as of USA model year 1990.

I am not sure about the V126 / 350SDL. Never owned one.