the transmission. In this way, the vacuum modulator
can match transmission shift points to engine loads.
The governor valve (fig. 4-39) senses engine speed
(transmission output shaft speed) to help control gear
shifting. The vacuum modulator and governor work
together to determine shift points. The governor gear is
meshed with a gear on the transmission output shaft.
Whenever the vehicle and output shaft are moving, the
centrifugal weights rotate. When the output shaft and
weights are spinning slowly, the weights are held in by
the governor springs, causing low-pressure output and
the transmission remains in low gear. As the engine
speeds increases, the weights are thrown out further
and governor pressure increases, moving the shift
valve and causing the transmission to shift into higher
Figure 4-39.Operation of the governor.
Figure 4-40.Shift valve.