Shift Modulator: Description and Operation

Fig. 7 Altitude Compensating Vacuum Diaphragm:

ALTITUDE-COMPENSATED VACUUM DIAPHRAGM
The unit incorporates an evacuated bellows that is sensitive to barometric pressure. Barometric pressure decreases about one inch Hg for each 1000 feet 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.