for an automatic shift from first to second gear, but
tubing arranged in rows. The rows of tubing pass
blocks an automatic shift from second to third gear.
through a series of fins. As the oil passes through the
tubing, its latent heat is transferred to the air via
(2) Vehicle Speed Versus Engine Loading.
conduction through the fins. This type of cooler usually
The shifting of the automatic transmission is controlled
is located in front of the engine radiator where it is
by two pressure signals that are indicators of speed and
subjected to airflow. This condition is important for its
engine loading. These signals, as will be described later
in this para- graph, work against each other to produce a
f. Lubrication. After the oil is passed through
the oil cooler, it is piped through internal passages in the
transmission case to the rear section of the transmission
The center of all shift control is the valve body. This unit,
where it provides lubrication for components such as the
which can be thought of as a hydraulic computer,
planetary gearsets and the output shaft bearings. The oil
receives information from the indicators described
then drains back to the sump. (At this time, it should be
above. Based on this information, the valve body
noted that the oil sump also dissipates heat from the
switches line pressure to the proper planetary holding
transmission oil and is considered to be a source of
elements to produce the required gear range. As the
information changes, the valve body changes its line
pressure outputs accordingly. For the sake of simplicity,
23-14. Range Control System (Fig. 23-13).
a two- speed transmission will be used to explain the
operation of all of the typical components. A
a. General. The range control system provides
hypothetical, automatic transmission hydraulic system
automatic or operator-controlled shifting of the
will be constructed in this paragraph . It must be
transmission. The shifting of the transmission is
stressed that this hypothetical system by no means
controlled by the following two indicators:
contains all of the devices necessary in a real
transmission. It is useful for the sake of learning,
(1) Manual Selection. The position of the
because it clearly illustrates how the decision and the
gearshift lever selected by the operator chooses the
action of shifting is initiated.
desired shifting program. The selections available to the
b. Manual Valve. The manual valve is the
operator of a typical two-speed unit are:
device that selects the desired shift program through the
(a) Neutral (N). In neutral (N), the
position of the gearshift lever. A manual valve is
engine freewheels, providing no driving force to the
basically a multiport spool valve that switches line
pressure to selected passages as it is moved through its
operating positions. A hypothetical manual valve for a
(b) Drive (D). in drive (D), the
two-speed transmission is illustrated in figure 23-14 in all
transmission provides automatic shifting through the low-
of its operating positions.
to high-gear ranges.
c. Governor. The governor modulates line
(c) Drive-Low (L). in drive-low (L), the
pressure to produce a signal that is an indication of
transmission is locked in the low range and no automatic
vehicle speed. This signal is used by the valve body to
formulate gear-range selections for the transmission.
The governor uses a spool valve that is operated by
(d) Reverse (R). Reverse (R)
centrifugally operated weights. The weights are rotated
reverses the direction of engine torque to drive wheels,
by the output shaft of the transmission because it is
and the vehicle is driven backwards.
solidly linked to the drive wheels and, therefore,
consistently provides a true indication of vehicle speed.
(e) Park (P). Park (P) is the same as
As the weights rotate they are acted on by centrifugal
neutral except that the drive wheels are locked by a
force which tends to pull them outward from the axis of
rotation. The weights are pulled
positive latching device within the transmission.
The three-speed transmission has all of the above
selections plus drive second. This position allows