Controlled Deceleration Device: Operation

The BCDD system is used on all 1974-76 models, except B210 and F10. On 1976-78 models, the BCDD system comes into operation when engine is suddenly decelerated between 1800-2000 RPM. A diaphragm monitors manifold vacuum and will open vacuum control valve when vacuum manifold exceeds a certain level. As valve opens, air is allowed into a vacuum chamber and diaphragm assembly. As diaphragm moves, it allows additional air into the manifold. The amount of air is controlled by the air control valve and vacuum control valve so that manifold vacuum can be maintained at correct level.

The 1979 BCDD system contains two diaphragms. When vacuum in the intake manifold exceeds a pre-determined value, diaphragm I opens the vacuum control valve. This allows manifold vacuum into a vacuum chamber in the BCDD and actuates diaphragm II. See Figure.

When actuated, diaphragm II opens the air control valve on 280ZX and 810 models, or the mixture control valve on 200SX, 510 and Pickup models, allowing additional air or air/fuel mixture into the manifold to maintain the required air/fuel ration.

The amount of air or air/fuel mixture entering the manifold is controlled by the servo-action of the air or mixture control valve and vacuum control valve so that the manifold vacuum may be kept at the pre-determined operating pressure.

On all models except 280ZX and 810, the amount of mixture entering the manifold depends mainly upon the coasting air bleed, while the mixture ratio is determined by the coasting jet and coasting air bleed I. See Figure.

On California models, the intake manifold operating pressure (vacuum) changes depending on the altitude at which the vehicle is being operated. To compensate for this, an altitude compensator is incorporated into the BCDD on these models. The altitude compensator automatically adjusts the operating pressure in response to altitude.