[ss3011]

The basic differences between the modulators is a spring force , and the size of the diaphragm . It works by vacuum acting on the diaphragm area to back off the load on the spring . If no vacuum (WOT) then just the full load of the spring acts against the modulator valve which regulates a pressure that makes line pressure and also works on the shift valvetrains . This is max modulator pressure . When you have high vacuum (closed throttle) ,then the diaphragm works against the spring , reducing it to zero . This means zero modulator pressure which gets you minimum line pressure , usually 60 psi , and minimum shift points . So all these different modulators are just combinations of spring loads , and Diaphragm areas . The problem in this thread is sounding like the diaphragm area is too big so under medium engine load it backs off the modulator pressure and gives an early shift . It would be nice to have a chart of different modulator part numbers with spring load , diaphragm area , and ID stripes . Also note that the adjustment screw is usually adjusted at the factory to get a certain load at a certain vacuum , back when these were more common there was not a screw , but the adjustment was done by dimpling the top of the housing .