Vacuum Modulator

The engine vacuum signal is provided by the vacuum modulator, which consists of an evacuated bellows, a diaphragm and a spring. These are so arranged that the bellows and spring apply force that acts on the modulator valve so that it increases modulator pressure. Engine vacuum and a spring oppose the bellows and spring to control modulator pressure.

To reduce the effect of altitude on shift points, the effective area of the diaphragm is different than that of the bellows. Atmospheric pressure acts on the resulting differential area to reduce modulator pressure.