[Gregski]

Originally Posted by rich weyand

The vacuum modulator, together with the governor, controls the shift points on the TH350 transmission. All stock TH350s were manufactured the same; their differences in shift characteristics for specific engines and vehicles were set with the governor and modulator.

The governor lives behind a cap secured by a bail on the drivers side of the transmission in front of the tail cone. It is a little widget that spins with the output shaft. There are flyweights and springs on this spinning widget, and when the flyweights overcome the springs and hydraulic pressure, the flyweights push open a rod valve that lives in the center of the governor shaft. Adjusting the governor weights and springs can be done to adjust the wide-open throttle shift points of the transmission.

The vacuum modulator adjusts the hydraulic pressure to the governor dependent on the engine vacuum. At wide-open throttle (low vacuum), the full hydraulic pressure is provided to the governor and the wide-open throttle shift points are selected. When there is engine vacuum on the modulator, the hydraulic pressure to the governor is reduced, and the governor finds it easier to overcome the hydraulic pressure to open the shift valve, so earlier shifts are selected. Adjustment or replacement of the vacuum modulator can be used to adjust the part-throttle shift points.

The kick-down cable on the TH350 bypasses the modulator and provide full pressure to the governor, enabling the wide-open throttle shift points. The result is that, when the pedal is depressed past the kick-down point, the modulator is bypassed, and the full hydraulic pressure provided to the governor, which will cause the transmission to downshift if it is below the RPM for the wide-open throttle shift point.

Does that help?

I don't care if this thread is 2 years old, this reply is spot on MONEY! so i just had to tip my hat to it, thank you so much for a great and simple explanation