My car has a 327 with a 2 speed PowerGlide. The modulator on the PG goes to a port that has full vacuum on my intake. What is the purpose of having constant vacuum on the modulator?
Why wouldn't it go to a controlled vacuum port such as the one on the carb?

Clearly I don't understand the point of the modulator.

Could someone with patience please explain how this is supposed to work.

Thanks!!

Vic

 

Full manifold vacuum is the right connection.
There are a few things that controls how the trans shifts. First is the governor, and it adjusts fluid pressure by using centrifugal weights that spin based on the cars speed. More speed makes the weights fly out further, and it adjusts the shift valves on the valve body.
Second is the modulator. It's a diaphragm that also has a spring inside. Higher vacuum at lower speeds pulls the diaphragm in one direction and moves the throttle valve in the valve body to give softer sooner shifts. As your right foot goes down (putting a load on the engine), engine vacuum drops, and the modulator moves the valve in the opposite direction to give firmer later shifts.
Third is the kickdown cable. It too tells the trans how and when to shift (up and down) based on the position of the throttle. All three things work together as one.

 

Thanks Chuck!

So even though the connection gets "full manifold vacuum" there is still variation based throttle plate position.

I guess this works opposite to the vacuum that goes the timing advance which gets vacuum when the throttle plate starts to open...?

Vic

 

Right, the modulator has a calibrated spring. No vacuum, or low vacuum, and the spring does the moving in one direction. Higher vacuum at low rpm moves it in the opposite direction, against the spring.