Deceleration Valve: Description and Operation

Fig. 32 Boost controlled deceleration device.:

Fig. 83 Boost controlled deceleration device.:

The Boost Controlled Deceleration Device (BCDD), Figs. 32 & 83, is used to reduce HC emissions emitted during coasting. The BCDD, installed under the throttle chamber as a part of it, supplies additional air to the intake manifold during coasting to maintain the manifold vacuum at the proper operating pressure 470 mm Hg (18.5 inches Hg).
There are two diaphragms in the device unit. Diaphragm I detects the manifold vacuum and makes the Vacuum Control Valve open when the vacuum exceeds the operating pressure. Diaphragm II operates the Air Control Valve according to the vacuum transmitted through the Vacuum Control Valve. The Air Control Valve regulates the amount of additional air so that the manifold vacuum can be kept at the proper operating pressure.
Diaphragm I monitors the manifold vacuum; when the vacuum exceeds a pre-determined value, it acts so as to open the vacuum control valve. This causes the manifold vacuum to be introduced into vacuum chamber II and actuates diaphragm II.
When diaphragm II operates, the air control valve opens the passage and introduces the additional air into the manifold.
The amount of air is controlled by the servo-action of the air control valve and vacuum control valve so that the manifold vacuum may be kept at the pre-determined value.