With automatic transmissions, on the other hand, gear
selection decisions are made by an automatic control sys-
tem. Instead of a manual clutch to connect and disconnect
the engine from the transmission, automatic transmissions
use fluid couplings or torque converters to transfer power
from the engine to the transmission.
Automatic transmissions use planetary gearsets,
which do not slide in and out of engagement. In operation,
one of the gears in the gearset is locked in place. The
remaining unlocked gears are driven by engine power and
comprise the input and output. Different gear ratios are
achieved by different combinations of locked and
unlocked gears. The gears are operated by holding mem-
bers called clutches and bands. The clutches and bands
are controlled by a hydraulic control system. Late-model
automatic transmissions have hydraulic systems controlled by
on-board computers. Vehicles with automatic transmissions
are easier to drive than those with manual transmissions.
They are also more durable for heavy-duty operation, such
as trailer towing. The major differences between manual
and automatic transmissions/transaxles are shown in
Figure 1-3.
The ideal transmission will transmit engine power
with no slipping. Slipping can be defined as failure to
transmit all engine power to the other drive train compo-
nents. In other words, a slipping transmission will lose
both speed and torque between its input and output shafts.
Early automatic transmissions were so inefficient and
slipped so much that they were called “slush boxes.”
Modern automatics are efficient and smooth. Except for
first and reverse gears, modern automatic transmissions
permit no slippage. They transmit as much engine power
as manual transmissions. It is now possible for an auto-
matic transmission to be more efficient than a manual.
With an automatic transmission, there is no need to release
the accelerator pedal during shifts, and then reaccelerate
to maintain vehicle speed.
Transmissions and Transaxles
Until about 20 years ago, nearly all vehicles had a
rear-wheel drive arrangement that used a transmission to
transfer power to the rest of the driveline. The rear-wheel
drive transmission transmits power in a straight line, from
the front of the vehicle to the back. The differential and the
final drive assembly are contained in a separate housing at
the rear axle.
Today, most automobiles use front-wheel drive sys-
tems equipped with transaxles. Transmissions and
transaxles perform the same function. The major differ-
ences include the arrangement of the parts and the fact that
the differential and the final drive assembly (sometimes
called the ring-and-pinion assembly) are an integral part of
the transaxle. Transaxles have two output shafts, one for
each wheel. These shafts are attached to the CV axles.
Engine power is transmitted sideways through a chain or
gears at some point in the transaxle.
The advantages of transaxles include reduced weight
and increased fuel economy. On trucks, however, weight
and fuel economy are less of a factor than durability. The
rear-wheel drive train is usually used on these vehicles.
Therefore, it is important that both transmissions and
transaxles be understood completely.
Four-wheel drive vehicles have a transfer case
attached to the rear of the transmission. The transfer case
sends power to the front wheels. Figure 1-4 illustrates the
layouts of modern rear-wheel drive vehicles with auto-
matic transmissions and front-wheel drive vehicles with
automatic transaxles.
Automatic Transmission and
Transaxle Development
The modern automatic transmission was not the result
of a single invention. Some components used in automatic
transmissions were developed long before the automobile
itself. Planetary gear principles were known during the
time of the Roman Empire and eventually appeared on the
Ford Model T. Fluid couplings were used to drive machin-
ery in 19th century mills. The first automotive fluid coup-
lings were used on English cars in the 1920s and on
Chrysler vehicles in the mid 1930s.
The 1938 Oldsmobile is widely considered the first
car to have an automatic transmission. These early
Oldsmobile Hydra-Matics had planetary gears operated by
Chapter 1 Introduction to Automatic Transmissions and Transaxles 13
Transmission Function Manual Transmission Automatic Transmission
Driver operated clutch.
Sliding gears.
Sliding gears operated by driver.
Fluid coupling or torque converter.
Planetary gears.
Planetary gears operated by a
hydraulic control system.
Engaging and disengaging engine drive
wheels.
Reversing directions.
Changing gear ratios to match vehicle
speed.
Figure 1-3. Although their basic function is the same, there are differences between automatic transmissions and manual
transmissions.