Figure 2-9.Planetary gear system.
Power can be transmitted through the planetary
gearset in various ways. A shaft from the engine may
be connected to drive the sun gear. It may be connected
to drive the planet carrier or the shaft may be connected
to drive the ring gear. The propeller shaft may also be
connected to anyone of these members; however, power
can be transmitted in the planetary gear system only
when (1) the engine is delivering power to one of the
three members, (2) the propeller shaft is connected to
one of the other members, and (3) the remaining
member is held against rotation. All three conditions
must be satisfied for power to be transmitted in the
system. Automatic transmissions provide for holding a
member through hydraulic servos and spring pressure.
Automatic Transmission Operation
Most automatic transmissions are basically the
same. They combine a fluid torque converter with a
planetary gearset and control the shifting of the
planetary gear with an automatic hydraulic control
system. The fluid torque converter is attached to the
engine crankshaft and serves as the engine flywheel.
This design means that when the engine runs, engine
power flows into the converter and drives the converter
output (turbine) shaft. There is no neutral in the torque
converter. Neutral is provided in the planetary gearset
by the release of bands and clutches.
The transmission automatically multiplies and
transmits engine torque to the drive shaft as driving
conditions demand. The speeds at which the coupling
point and the gearshifts occur are controlled partially by
the operator. The operator has only a partial control in
the D-drive position, because the transmission in the
D-drive position shifts the planetary gearset into the
higher gears to prevent engine overspeeding regardless
of throttle position.
The operation of automatic shift vehicles is quite
simple; however, it is imperative that the professional
operator learn to operate them smoothly and properly.
In vehicles equipped with automatic transmissions,
initial gear selection is controlled with a selector lever.
When in drive (D or DR), shifting from drive to low (L)
and returning to drive is controlled automatically by the
Most vehicles have four or five of the following
P-PARK POSITION. On light vehicles, such as
sedans and pickups, this position is used for locking the
transmission so the vehicle cannot roll while parked. In
some heavier vehicles, the park position does not lock
the transmission. In vehicles with a park position, the
engine should be started from the park position.
N-NEUTRAL POSITION. Engines of vehicles
not equipped with a P-park position are started from the
In this position, the engine is
disengaged from the drive shaft of the vehicle.
D-DRIVE POSITION. With the shift lever at D
or DR, the vehicle moves forward as you depress the
accelerator. After starting the engine in neutral or park
position, step on the brake and change the selector to D
or DR for forward movement. To avoid premature
forward movement, keep pressure on the brake while in
the drive position until you are ready to place the vehicle
in motion. Without further operator action, the
transmission automatically shifts to higher gears as
L-LOW or POWER POSITION. T he
transmission will not shift automatically to higher gear
ratios when the lever is in the low position. The low
position is used when negotiating steep grades and
rough terrain or when the braking power of the engine
is required. When low range is no longer needed,
release the accelerator temporarily and move the shift
lever to the drive position for normal gear progression.
In the drive position, the low range is engaged
automatically when engine speed is reduced. If the
accelerator is suddenly fully depressed, the low range
becomes engaged. (This procedure may be used to
provide a sudden burst of speed for passing.) When a
predetermined engine speed has been attained, the
transmission automatically returns to driving range.