Saw this - I have wondered a long time exactly what happened inside the tranny and why we need to replace or adjust the vac modulator:

How did automatic transmissions “know” when to change gears before computers? Today it seems pretty straightforward, but how was this done in the 70s?
It was even simpler. There were usually two mechanical moving parts in the transmission. One of them was called a governor. It was driven by a gear on the transmission output shaft, which means it spun at a speed that was proportional to the vehicle speed. It looked something like this:.

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Those “wing" looking appendages on both sides are held in place by springs. The faster it spins, they start to "fly out", beginning to overcome the spring tension. As they do, they move a valve in the center shaft that controls the path of the fluid through the transmission and control which gear is active.

But, you say, the transmission shifts at different speeds depending on how much I'm pressing on the gas.

Yes it does. That's controlled by the second part, called a vacuum modulator.

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That's exactly what it does: it “modulates" the action of the valve inside the governor, and therefore the shift points, based on how much manifold vacuum is currently present.

At full throttle, manifold vacuum is very nearly zero, such means the vacuum modulator is essentially inoperative, so all the shifting “decisions” are controlled only by the governor — how fast it's spinning. At very light “sedate" throttle there is a lot of manifold vacuum, which means the modulator will be very active, causing the shift points to occur much earlier and at lower vehicle speed than they do at full throttle.

Side Note here: If you have a car with an automatic transmission that uses a vacuum modulator, and you encounter a situation where the transmission always shifts at a very high engine speed regardless of how much throttle you’re applying, then suspect that either the modulator itself has gone bad or perhaps the vacuum line to it is leaking, disconnected, or completely plugged. This situation will prevent the modulator from ever sensing any vacuum and therefore the transmission will “think” that you’re at full throttle all the time and it will act accordingly. Conversely, if it always shifts EARLY no matter how much you’re into the throttle, then suspect a bad/”stuck” vacuum modulator. Such is a rare occurrence, but it can happen. Vacuum modulators are usually located on the side of the transmission, relatively easily accessible, relatively easy to replace yourself, and usually not very expensive.

Thanks to Paul Fleming for pointing out to me that the Chrysler 727 didn’t use a vacuum modulator. Instead it had a mechanical connection to the car’s throttle linkage that controlled a valve in the transmission that moderated the shift points based on the position of the throttle. This was another means of modifying shift points based on engine loading.

Like I said, simple.

Even many modern computer controlled transmissions still use vacuum modulators for the same purpose.