Vacuum Diaphragm: Description and Operation
DESCRIPTIONAltitude Compensating Vacuum Diaphragm:
This unit, the Altitude Compensating Vacuum Diaphragm, is used in some transmissions. The unit incorporates an evacuated bellows that is sensitive to barometric pressure.
OPERATION
Barometric pressure decreases about 1 in Hg for each 1,000 ft altitude increase. The bellows decreases throttle pressure at higher altitude to make the shift feel comparable to lower altitude. The engine power loss at high altitude is offset by a lower throttle pressure and the shifts remain smooth.