inward by specially calibrated springs that allow the
gear position. This is how a forced downshift (passing
weights to move from a fully retracted to a fully extended
gear) is initiated. A hypothetical shift valve is illustrated
position within a desired output shaft speed range. The
spool valve is in a position where all line pressure is
e. Operation. The following subparagraphs
blocked when the governor weights are fully retracted.
As the weights move outward, the spool valve gradually
describe the operation of a hypothetical hydraulic system
opens the line pressure port to the governor pressure
for a two-speed automatic transmission in all of its
port. The result is an approximate linear speed and
modes of operation. It must be noted that a reverse
governor pressure increase. A hypothetical governor is
band rather than a reverse clutch is used in the
illustrated in figure 23-15. A graph showing the typical
hypothetical transmission. The reason for this is to allow
relationship between governor pressure and speed is
a wider variety of holding elements to be illustrated.
shown in figure 23-16.
(1) When the shift lever is placed in neutral
d. Shift Valve. The shift valve is a simple
(N) (fig. 23-18), line pressure is blocked at the manual
balance-type spool valve that selects between low and
valve and none of the frictional elements are applied.
high gear when the manual valve is in the drive (D)
position. Governor pressure acts on the spool valve in
(2) When the shift lever is placed in drive-
one direction, trying to push the spool towards the high-
low (L) (fig. 23-19), the manual valve delivers line
gear position. At the same time, spring pressure
pressure directly to the low band. The compound
modulated by the position of the accelerator pedal tries
planetary gear train (para 23-9) then is in low gear.
to push the spool toward the low-gear position. The
decision of the shift valve is dependent on which
(3) When the gearshift lever is placed in
pressure is greater: governor or throttle. The shift valve
reverse (R) (fig. 23-20), the manual valve delivers line
also contains a device known as a throttle plug. At full
pressure directly to the reverse band. The compound
throttle the throttle plug makes physical contact with the
planetary gear train (para 23-9) then is in low near.
shift valve spool, forcing it into the low-
Figure 23-15. Operation of the Governor.