Chapter 1 Introduction to Automatic Transmissions and Transaxles 33
Summary
Manual and automatic transmissions have the same
basic purposes. They must connect and disconnect the
engine and drive wheels, multiply engine power as dictated
by vehicle speed and load, and provide a way to reverse
the direction of power flow.
Manual and automatic transmissions have many sim-
ilarities. Major differences are that automatic transmissions
use of a torque converter instead of a clutch, planetary
gears instead of sliding gears, and a hydraulic control sys-
tem that makes shifting decisions for the driver. Modern
automatic transmissions and transaxles are almost as effi-
cient as manual models. Transmissions and transaxles dif-
fer only in the layout of parts and the fact that the transaxle
contains the final drive assembly and the differential.
The modern automatic transmission has been gradu-
ally refined over the years. The first automatic transmission
was introduced more than 60 years ago. Over the years,
automatic transmissions with torque converters, simplified
gear trains, and aluminum cases were developed and
placed in service. Lockup torque converters, transaxles,
and electronic control systems have been introduced and
gradually perfected during the last 20 years.
Fluid couplings and torque converters are fluid-filled
units that contain a set of impeller blades driven by the
engine and a set of turbine blades connected to the input
shaft. All modern transmissions use a torque converter
instead of a fluid coupling. Modern torque converters use
a lockup clutch to eliminate slippage.
All transmissions and transaxles have separate input
and output shafts and planetary gears. A basic planetary
gear consists of a central sun gear surrounded by planet
gears that are housed in a planet carrier. A ring gear with
internal teeth surrounds the sun and planet gears. Simple
planet gear sets can be arranged into complex gear units to
obtain different gear ratios.
The parts of the planetary gear assembly are held
or driven by holding members. Clutches and bands are
common holding members that are operated by hydraulic
pressure. One-way clutches are mechanically operated
holding members.
The transmission or transaxle case is the support for
the other parts of the transmission. It contains oil pressure
passages. The oil pan is a reservoir for extra fluid. The
hydraulic pump provides all the pressure used in the auto-
matic transmission or transaxle. The pressure regulator is
installed in the outlet line to the pump and controls over-
all transmission pressures.
The hydraulic control system provides pressure to the
holding members and determines when shifts should
occur. It also keeps transmission fluid flowing to the torque
converter, transmission cooler, and lubricating system. On
most modern transmissions and transaxles, an electronic
control system operates the hydraulic components.
Manual linkage connects the shift lever to the man-
ual valve inside the transmission or transaxle. Throttle
linkage connects the engine’s throttle plate to the trans-
mission’s throttle valve and controls shift speeds.
Transmission fluid is made of oils and various addi-
tives. The fluid is pressurized, and it operates the holding
members. It also lubricates moving parts, carries away
heat, and helps seal internal parts. The proper fluid is
important to transmission and transaxle operation.
To cool the transmission fluid, it is pumped from the
torque converter to a cooler in the radiator. A few vehicles
have direct-air coolers.
To remove debris from the transmission fluid, a filter
is installed on the pump intake. Filters can be screens or
some other type of filtering material in a plastic housing.
Filters should be changed periodically.
Automatic transmission operation varies from one
gear to another. In Neutral and Park, no holding members
are applied and the power flow stops at the planetary
gears. In Drive or Reverse, holding members apply and
cause the planetary gears to connect the input and output
shafts. Engine power goes through the impeller, through
the turbine and input shaft, and into the planetary gears.
The hydraulic control system varies gear ratios by applying
and releasing different holding members.
Review Questions—Chapter 1
Please do not write in this text. Write your answers on
a separate sheet of paper.
1. One of the primary jobs of any transmission is to dis-
connect the engine from the _____ _____ when the
vehicle is not moving.
2. Which of the following gears should be selected to
obtain the best acceleration from stop?
(A) Reduction gear.
(B) Direct drive.
(C) Overdrive.
(D) Passing gear.
3. If the transmission output speed is more than the input
speed, the transmission is in a(n) _____ gear.
4. Which of the following gears should be selected to
obtain the best fuel economy?
(A) Reduction gear.
(B) Direct drive.
(C) Overdrive.
(D) Passing gear.
5. Briefly explain why placing an automatic transmission in
drive with the brakes applied does not kill the engine.
6. Planetary gears are so named because they resemble
the _____ system.
7. Clutches and bands are examples of _____ _____.