If you are talking about the boost pressure line, the solenoid is part of the over-boost protection system.It never gets vacuum, diesels don't create manifold vacuum (no throttle plate). The solenoid is normally open. When the pressure switch on the intake manifold is grounded(by too much boost pressure)the solenoid is energized and prevents the pressure from reaching the aneroid compensator. If the switch is by-passed, you have no over-boost protection.
Regards,
Randy D.

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W.R.Durrance
ASE Master Tech
Mercedes Tech 19 Years