Introduction
Automatic transmissions and transaxles have been
used for more than 60 years. They have been consistently
modified and improved, evolving from early inefficient
designs to the smooth-shifting, efficient units of today.
Most modern transmissions and transaxles are controlled
by an onboard computer and provide almost the same fuel
economy as manual models.
To service late-model automatic transmissions or
transaxles, the technician must possess considerable
knowledge and skill. This chapter will introduce you to the
fundamentals of automatic transmissions and transaxles.
The basic principles covered here will be expanded upon
in later chapters.
The Purpose of Transmissions
All transmissions, whether manual or automatic, have
the same basic purposes:
To transmit power from the engine to the drive wheels
when necessary.
To disconnect the running engine from the drive
wheels during gear changes and when the vehicle is
not moving.
To reverse the direction of power flow when the
vehicle must be backed up.
To multiply engine torque as needed.
In simplest terms, a transmission modifies engine
torque and speed to match the vehicle’s needs. For exam-
ple, moving a vehicle from a stop requires a great deal of
engine torque, or turning force. At low speeds, however,
an engine produces relatively little torque. The transmis-
sion must multiply engine torque to get the vehicle mov-
ing. It does this by reducing speed to increase torque.
The relationship of the speed of the transmission’s
input shaft to the speed of its output shaft is called the gear
ratio. The transmission uses a set of at least two gears that
cause the output shaft speed to be much lower than the
input shaft speed. This set of gears is called a reduction
gear. Figure 1-1 shows a simple reduction gear.
At higher speeds, the vehicle does not require as
much engine torque to keep it moving. The engine would
be turning very fast if the transmission output speed
remained slower than the input speed. High engine speed
will cause poor fuel economy and rapid engine wear.
Therefore, the transmission must be shifted into succes-
sively higher gears as vehicle speed increases. Shifting into
higher gears changes gear ratios, so the speed of the out-
put shaft approaches and eventually equals or exceeds the
speed of the input shaft.
Most modern transmissions have at least four forward
gears, with the highest gear being either direct drive or
overdrive. A direct drive gear causes the input and output
shafts to turn at the same speed. The overdrive gear causes
the output shaft to turn faster than the input shaft, Figure 1-2.
Overdrive allows the engine to turn at a relatively slow
speed, increasing fuel economy and reducing engine wear.
Both manual and automatic transmissions can
accomplish any of the previously mentioned jobs. There
are many similarities between manual and automatic
transmissions. All transmissions have a way of keeping the
engine from stalling when the vehicle is stopped, all use
gears and shafts to obtain different ratios, and all have a
way to reverse the direction of vehicle travel. However,
with a manual transmission, the gear selection decision
must be made by the vehicle’s operator. The driver slides
the transmission gears in and out of engagement using a
gearshift lever. The gears are meshed in different combina-
tions to achieve the desired gear ratios. The driver must
also operate a manual clutch to connect and disconnect
the engine from the transmission when stopping or chang-
ing gears.
12 Automatic Transmissions and Transaxles
3 Revolutions
Drive gear
90-tooth gear
Driven
gear
Power out
1 Revolution
30-tooth
gear
Power in
Figure 1-1. Gear reduction produces the torque needed to
move the vehicle from a stop. In this illustration, a 30-tooth gear
is turning a 90-tooth gear. The 30-tooth gear must make three
revolutions to turn the 90-tooth gear once.This multiplies torque
three times but cuts speed to one-third.
30-tooth
driven gear
3 ft-lb of
torque
Power in
90-tooth
drive gear
1 ft-lb of torque
Power out
Figure 1-2. In this example, overdrive is accomplished by using
the 90-tooth gear to turn the 30-tooth gear. This increases
speed three times but cuts torque to one-third.