Ditto on the full manifold vacuum for the modulator! The modulator serves the same purpose as mechanical throttle linkage on other transmissions- - - -the further open the throttle goes, the higher the modulator pressure goes in the transmission. Low manifold vacuum = high TV pressure. This determines shift points and timing of the shifting, also how firm the shift is. An automatic trans does a balancing act between three different pressures- - - main line, throttle pressure, and governor pressure (driveshaft speed).
Jerry
