All Other Models
The boost control unit is nitrogen filled and contains a diaphragm and control valve. When intake manifold vacuum exceeds a pre-determined value, the diaphragm opens the control valve. This allows manifold vacuum to act upon the air by-pass control unit.
The air by-pass control unit consists of a diaphragm and air control valve. When intake manifold vacuum by-passes the boost control unit, it activates the diaphragm. The diaphragm opens the air control valve which supplies additional air into the intake manifold.
The amount of air entering the manifold is controlled by servo-action of the air control valve and vacuum control valve. Manifold vacuum can be kept very close to the pre-determined value during deceleration by this tightly controlled circuit.