control units to make the right selection (gear ratio) for
difference being that the disc of the wet disc must be
the engine load and vehicle speed and to make the
bathed in oil to function. The disc face of either type
selection at the right time.
(wet or dry) is covered with friction material, generally
a bonded sintered bronze facing. The discs are splined
A typical system of hydraulic control units
to a hub, and the hub is splined to a shaft. The steel
c o n s i s t s o f m a ny c o m p o n e n t s . S o m e o f t h e s e
clutch plates are mounted between the friction discs
components sense vehicle speed, others sense engine
and are splined to a clutch housing or retainer. The
power, some supply hydraulic pressure, and still others
clutch plates and disc are commonly referred to as a
regulate and control the hydraulic oil and pressure. A
clutch-pack. The clutch retainer contains the actuating
typical system of hydraulic control units is shown in
piston and return or release spring. When hydraulic
figure 2-65. Most of the transmission's valves and oil
pressure is applied to the clutch piston, the discs and
passages are housed in the control units. The control
plates are squeezed together and rotate as a unit. The
units are usually made up of several valve bodies
release spring is also compressed at this time. When
bolted together, or the valve bodies are mounted on a
hydraulic pressure is removed from the piston, it is
metal plate that has the oil passages machined or cast
returned to the release position by the release spring.
into the plate and valves.
This causes the discs and plates to separate.
The front pump, driven by the torque converter at
The clutch hub is attached to one member of a
or near engine crankshaft speed, is usually mounted on
planetary gear unit, while the clutch retainer is
the transmission input shaft at the front of the
attached to another member of the same planetary unit.
transmission. The pump delivers oil from the sump
Therefore, when the clutch discs and plates are
(transmission oil pan), through a filter, to the proper
squeezed together causing these two units to rotate as
hydraulic control units for routing to the operating
one unit, the planetary gear unit is in direct drive (1 to 1
units as directed by the various valves.
ratio). The clutch is used for holding two planetary
The regulator valve maintains a controlled oil
members, while the servo unit holds only one.
pressure to the control units. The manual valve permits
the operator to manually select the desired operating
Hydraulic Control Units
range. The shift valve initiates the upshifting or
In the manual selective type of transmission, the
downshifting as determined by the governor and
operator must select the gear ratio he or she thinks is
throttle pressures. The throttle valve is positioned by
best for engine load and vehicle speed. However, in the
the accelerator linkage; therefore, it is sensitive to
automatic transmission, it is the job of the hydraulic
HYDRAULIC CONTROL UNIT
Figure 2-65.--Typical hydraulic control unit and it's components.