Hydraulic Control System
The hydraulic control system is a set of hydraulic
parts and passages that performs the following functions:
Applies and releases the holding members to obtain
the needed gear ratios at any vehicle speed and throt-
tle position.
Controls system pressures for proper shift feel and
long holding member life as loading and acceleration
vary.
Keeps transmission fluid flowing to the torque con-
verter, transmission cooler, and lubricating system.
The major component of the hydraulic control sys-
tem is the valve body, which contains the control valves
and related springs. The valve body also contains the
manual valve, which is connected by linkage to the shift
lever in the passenger compartment. Moving the manual
valve directs pressure to other parts of the hydraulic sys-
tem. The valve body may also contain accumulators,
check balls, and spacer plates. A typical valve body is
shown in Figure 1-29. Other parts of the hydraulic system
include governor valves, throttle linkage, band servos,
clutch apply pistons, and the passages that connect them.
Some of these passages and parts may be located in the
transmission case.
Most modern transmissions and transaxles use an
electronic control system to operate the hydraulic com-
ponents. An on-board computer processes information
from input sensors. It then uses this information to operate
solenoids and other output devices installed in the trans-
mission or transaxle to control pressure flow through the
hydraulic system. See Figure 1-30. Figure 1-31 illustrates
the input and output devices used with a typical electron-
ically controlled transmission. The construction and oper-
ation of hydraulic and electronic control components will
be discussed in detail in later chapters.
Gaskets and Seals
Gaskets and seals are used to keep fluid from leaking
out of the transmission or transaxle and to keep pressure
from leaking internally. Gaskets are used where major
components are joined together. For example, a gasket is
used to seal the oil pans to the case. Gaskets are also used
in the valve body and where the front pump and extension
housing are attached to the case. See Figure 1-32.
Chapter 1 Introduction to Automatic Transmissions and Transaxles 25
Driven
gear
Crescent
Drive gear
Vacuum
Atmospheric
pressure
Inlet
Fluid
squeezed
out
Outlet
To system
Oil sump
screen
Sump
Spring
Valve
To oil pan on
pump intake
To rest of
hydraulic
system
To oil
pump
Boost pressure
from other
transmission valves
Figure 1-27. Simplified hydraulic pump. Fluid is drawn in as the
gears move apart. It is then carried around by the gear teeth
and pressurized as the teeth come together. (Ford)
Figure 1-28 . Simplified pressure regulator. If system pressure
becomes too great, the valve moves against spring pressure
and oil is routed back to the oil pan.
Manual valve
Valve body
Solenoid
Connector
Wiring harness
Figure 1-29. Valve bodies contain the hydraulic valves, check
balls, springs, restrictors, screens, and other components
needed to control the holding members. Note that this valve
body uses electrical solenoids to control transmission
operation.