In a Turbo 350 transmission, the vacuum modulator is responsible for regulating the amount of vacuum that is applied to the diaphragm in the modulator. This in turn regulates the amount of pressure that is applied to the transmission fluid, which ultimately affects the shifting of the transmission.
The amount of vacuum that is applied to the diaphragm is controlled by the engine's vacuum. When the engine is under load, the vacuum is reduced and the modulator allows more fluid to be applied to the transmission, which results in a firmer shift. When the engine is under light load or no load, the vacuum is increased and the modulator allows less fluid to be applied to the transmission, which results in a softer shift.
The vacuum modulator is an important part of the transmission and its proper function is essential for proper shifting. If the modulator is not functioning properly, it can cause the transmission to shift hard or soft at the wrong time, which can lead to transmission damage.