The modern automatic
transmission consists of many components and systems that are designed
to work together in a symphony of clever mechanical, hydraulic and
electrical technology that has evolved over the years into what
many mechanically inclined individuals consider to be an art form.
We try to use simple, generic explanations where possible to describe
these systems but, due to the complexity of some of these components,
you may have to use some mental gymnastics to visualize their operation.
The main components that
make up an automatic transmission include:
Gear Sets which are the mechanical systems that provide
the various forward gear ratios as well as reverse.
- The Hydraulic
System which uses a special transmission fluid
sent under pressure by an Oil Pump
through the Valve Body to
control the Clutches and the
Bands in order to control the planetary gear sets.
and Gaskets are used to keep the oil where it is supposed
to be and prevent it from leaking out.
- The Torque
Converter which acts like a clutch to allow the vehicle
to come to a stop in gear while the engine is still running.
- The Governor
and the Modulator or Throttle
Cable that monitor speed and throttle position in order
to determine when to shift.
- On newer vehicles,
shift points are controlled by
Computer which directs electrical solenoids to
shift oil flow to the appropriate component at the right instant.
Automatic transmissions contain many gears in various combinations.
In a manual transmission, gears slide along shafts as you move the
shift lever from one position to another, engaging various sized
gears as required in order to provide the correct gear ratio. In
an automatic transmission, however, the gears are never physically
moved and are always engaged to the same gears. This is accomplished
through the use of planetary gear sets.
The basic planetary gear
set consists of a sun gear, a ring gear and two or more planet gears,
all remaining in constant mesh. The planet gears are connected
to each other through a common carrier which allows the gears to
spin on shafts called "pinions" which are attached to the carrier
One example of a way
that this system can be used is by connecting the ring gear to the
input shaft coming from the engine, connecting the planet carrier
to the output shaft, and locking the sun gear so that it can't move.
In this scenario, when we turn the ring gear, the planets will "walk"
along the sun gear (which is held stationary) causing the planet
carrier to turn the output shaft in the same direction as the input
shaft but at a slower speed causing gear reduction (similar to a
car in first gear).
If we unlock the sun
gear and lock any two elements together, this will cause all three
elements to turn at the same speed so that the output shaft will
turn at the same rate of speed as the input shaft. This is like
a car that is in third or high gear. Another way that we can use
a Planetary gear set is by locking the planet carrier from moving,
then applying power to the ring gear which will cause the sun gear
to turn in the opposite direction giving us reverse gear.
The illustration on the
right shows how the simple system described above would look in
an actual transmission. The input shaft is connected to the ring
The Output shaft is connected to the planet carrier (Green)
which is also connected to a "Multi-disk" clutch pack. The sun gear
is connected to a drum (yellow)
which is also connected to the other half of the clutch pack.
Surrounding the outside of the drum is a band (red)
that can be tightened around the drum when required to prevent the
drum with the attached sun gear from turning.
The clutch pack is used,
in this instance, to lock the planet carrier with the sun gear forcing
both to turn at the same speed. If both the clutch pack and the
band were released, the system would be in neutral. Turning
the input shaft would turn the planet gears against the sun gear,
but since nothing is holding the sun gear, it will just spin free
and have no effect on the output shaft. To place the unit in first
gear, the band is applied to hold the sun gear from moving.
To shift from first to high gear, the band is released and the clutch
is applied causing the output shaft to turn at the same speed as
the input shaft.
more combinations are possible using two or more planetary sets
connected in various ways to provide the different forward speeds
and reverse that are found in modern automatic transmissions.
Some of the clever gear
arrangements found in four and now, five, six and even seven and
eight-speed automatics are complex enough to make a technically
astute lay person's head spin trying to understand the flow of power
through the transmission as it shifts from first gear through top
gear while the vehicle accelerates to highway speed. On modern
vehicles (mid '80s to the present), the vehicle's computer monitors
and controls these shifts so that they are almost imperceptible.
clutch pack consists of alternating disks that fit inside a clutch
drum. Half of the disks are steel and have splines that fit into
groves on the inside of the drum. The other half have a friction
material bonded to their surface and have splines on the inside
edge that fit groves on the outer surface of the adjoining hub.
There is a piston inside the drum that is activated by oil pressure
at the appropriate time to squeeze the clutch pack together so that
the two components become locked and turn as one.
d is connected directly to the input shaft of the transmission
providing power to move the vehicle. The stator is mounted
one-way clutch so that it can spin freely in one direction but not
in the other. Each of the three elements have fins mounted in them
to precisely direct the flow of oil through the converter
With the engine running,
transmission fluid is pulled into the pump section and is pushed
outward by centrifugal force until it reaches the turbine section
which starts it turning. The fluid continues in a circular
motion back towards the center of the turbine where it enters the
stator. If the turbine is moving considerably slower than the pump,
the fluid will make contact with the front of the stator fins which
push the stator into the one way clutch and prevent it from turning.
With the stator stopped, the fluid is directed by the stator fins
to re-enter the pump at a "helping" angle providing a torque increase.
As the speed of the turbine catches up with the pump, the fluid
starts hitting the stator blades on the back-side causing the stator
to turn in the same direction as the pump and turbine. As
the speed increases, all three elements begin to turn at approximately
the same speed.
Since the '80s, in order
to improve fuel economy, torque converters have been equipped with
a lockup clutch (not shown) which locks the turbine to the pump
as the vehicle speed reaches approximately 45 - 50 MPH. This
lockup is controlled by computer and usually won't engage unless
the transmission is in 3rd or 4th gear.
Hydraulic system is a complex maze of passages and tubes that sends
transmission fluid under pressure to all parts of the transmission
and torque converter. The diagram at left is a simple one
from a 3-speed automatic from the '60s. The newer systems
are much more complex and are combined with computerized electrical
components. Transmission fluid serves a number of purposes
including: shift control, general lubrication and transmission cooling.
Unlike the engine, which uses oil primarily for lubrication, every
aspect of a transmission's functions are dependent on a constant
supply of fluid under pressure. This is not unlike the human
circulatory system (the fluid is even red) where even a few minutes
of operation when there is a lack of pressure can be harmful or
even fatal to the life of the transmission. In
order to keep the transmission at normal operating temperature,
a portion of the fluid is sent through one of two steel tubes to
a special chamber that is submerged in anti-freeze in the
radiator. Fluid passing through this chamber is cooled and then
returned to the transmission through the other steel tube.
A typical transmission has an average of ten quarts of fluid between
the transmission, torque converter, and cooler tank. In fact,
most of the components of a transmission are constantly submerged
in fluid including the clutch packs and bands. The friction
surfaces on these parts are designed to operate properly only when
they are submerged in oil.
The transmission oil
pump (not to be confused with the pump element inside the torque
converter) is responsible for producing all the oil pressure that
is required in the transmission. The oil pump is mounted to
the front of the transmission case and is directly connected to
a flange on the torque converter housing. Since the torque
converter housing is directly connected to the engine crankshaft,
the pump will produce pressure whenever the engine is running as
long as there is a sufficient amount of transmission fluid available.
The oil enters the pump through a filter that is located at the
bottom of the transmission oil pan and travels up a pickup tube
directly to the oil pump. The oil is then sent, under pressure to
the pressure regulator, the valve body and the rest of the components,
valve body is the control center of the automatic transmission.
It contains a maze of channels and passages that direct hydraulic
fluid to the numerous valves which then activate the appropriate
clutch pack or band servo to smoothly shift to the appropriate gear
for each driving situation. Each of the many valves in the
valve body has a specific purpose and is named for that function.
For example the 2-3 shift valve activates the 2nd gear to 3rd gear
up-shift or the 3-2 shift timing valve which determines when a downshift
The most important valve,
and the one that you have direct control over is the manual valve.
The manual valve is directly connected to the gear
shift handle and covers and uncovers various passages
depending on what position the gear shift is placed in. When
you place the gear shift in Drive, for instance, the manual valve
directs fluid to the clutch pack(s) that activates 1st gear. it
also sets up to monitor vehicle speed and throttle position so that
it can determine the optimal time and the force for the 1 - 2 shift.
On computer controlled transmissions, you will also have electrical
solenoids that are mounted in the valve body to direct fluid
to the appropriate clutch packs or bands under computer control
to more precisely control shift points.
computer uses sensors on the engine and transmission to detect
such things as throttle position, vehicle speed, engine speed, engine
load, brake pedal position, etc. to control exact shift points as
well as how soft or firm the shift should be. Once the computer
receives this information, it then sends signals to a solenoid pack
inside the transmission. The solenoid pack contains several
electrically controlled solenoids that redirect the fluid to the
appropriate clutch pack or servo in order to control shifting. Computerized
transmissions even learn your driving
style and constantly adapt to it so that every
shift is timed precisely when you would need it.
Because of computer controls,
sports models are coming out with the ability to take manual control
of the transmission as though it were a stick shift, allowing the
driver to select gears manually. This is accomplished on some
cars by passing the shift lever through a special gate, then tapping
it in one direction or the other in order to up-shift or down-shift
at will. The computer monitors this activity to make sure
that the driver does not select a gear that could over speed the
engine and damage it.
Another advantage to
these "smart" transmissions is that they have a self diagnostic
mode which can detect a problem early on and warn you with an indicator
light on the dash. A technician can then plug test equipment
in and retrieve a list of trouble codes that will help pinpoint
where the problem is.
Vacuum Modulator, Throttle Cable
These three components
are important in the non-computerized transmissions. They provide
the inputs that tell the transmission when to shift. The Governor
is connected to the output shaft and regulates hydraulic pressure
based on vehicle speed. It accomplishes this using centrifugal force
to spin a pair of hinged weights against pull-back springs.
As the weights pull further out against the springs, more oil pressure
is allowed past the governor to act on the shift valves that are
in the valve body which then signal the appropriate shifts.
Of course, vehicle speed
is not the only thing that controls when a transmission should shift,
the load that the engine is under is also important. The more
load you place on the engine, the longer the transmission will hold
a gear before shifting to the next one.
There are two types of
devices that serve the purpose of monitoring the engine load: the
Throttle Cable and
the Vacuum Modulator.
A transmission will use one or the other but generally not both
of these devices. Each works in a different way to monitor
The Throttle Cable simply monitors the position of the gas pedal
through a cable that runs from the gas pedal to the throttle valve
in the valve body.
The Vacuum Modulator monitors engine vacuum by a rubber vacuum hose
which is connected to the engine. Engine vacuum reacts very
accurately to engine load with high vacuum produced when the engine
is under light load and diminishing down to zero vacuum when the
engine is under a heavy load. The modulator is attached to
the outside of the transmission case and has a shaft which passes
through the case and attaches to the throttle valve in the valve
body. When an engine is under a light load or no load, high
vacuum acts on the modulator which moves the throttle valve in one
direction to allow the transmission to shift early and soft.
As the engine load increases, vacuum is diminished which moves the
valve in the other direction causing the transmission to shift later
and more firmly.
An automatic transmission
has many seals and gaskets to control the flow of hydraulic fluid
and to keep it from leaking out. There are two main external
seals: the front seal and the rear seal. The front seal seals the
point where the torque converter mounts to the transmission case.
This seal allows fluid to freely move from the converter to the
transmission but keeps the fluid from leaking out. The rear
seal keeps fluid from leaking past the output shaft.
A seal is usually made
of rubber (similar to the rubber in a windshield wiper blade) and
is used to keep oil from leaking past a moving part such as a spinning
shaft. In some cases, the rubber is assisted by a spring that holds
the rubber in close contact with the spinning shaft.
A gasket is a type of
seal used to seal two stationary parts that are fastened together.
Some common gasket materials are: paper, cork, rubber, silicone
and soft metal.
Aside from the main seals,
there are also a number of other seals and gaskets that vary from
transmission to transmission. A common example is the rubber O-ring
that seals the shaft for the shift control lever. This is
the shaft that you move when you manipulate the gear shifter.
Another example that is common to most transmissions is the oil
pan gasket. In fact, seals are required anywhere that a device
needs to pass through the transmission case with each one being
a potential source for leaks.
A one-way clutch (also
known as a "sprag" clutch) is a device that will allow a component
such as ring gear to turn freely in one direction but not in the
other. This effect is just like that of a bicycle, where the pedals
will turn the wheel when pedaling forward, but will spin free when
A common place where
a one-way clutch is used is in first gear when the shifter is in
the drive position. When you begin to accelerate from a stop, the
transmission starts out in first gear. But have you ever noticed
what happens if you release the gas while it is still in first gear?
The vehicle continues to coast as if you were in neutral. Now, shift
into Low gear instead of Drive. When you let go of the gas
in this case, you will feel the engine slow you down just like a
standard shift car. The reason for this is that in Drive, a one-way
clutch is used whereas in Low, a clutch pack or a band is used.
A band is a steel strap
with friction material bonded to the inside surface. One end
of the band is anchored against the transmission case while the
other end is connected to a servo. At the appropriate time
hydraulic oil is sent to the servo under pressure to tighten the
band around the drum to stop the drum from turning.
On automatic transmissions,
the torque converter takes the place of the clutch found on
standard shift vehicles. It is there to allow the engine to
continue running when the vehicle comes to a stop. The
principle behind a torque converter is like taking a fan that is
plugged into the wall and blowing air into another fan which is
unplugged. If you grab the blade on the unplugged fan, you
are able to hold it from turning but as soon as you let go, it will
begin to speed up until it comes close to the speed of the powered
fan. The difference with a torque converter is that instead
of using air, it uses oil or transmission fluid, to be more precise.
A torque converter is
a large doughnut shaped device (10" to 15" in diameter) that is
mounted between the engine and the transmission. It consists
of three internal elements that work together to transmit power
to the transmission. The three elements of the torque converter
are the Pump, the Turbine, and the Stator. The pump
is mounted directly to the converter housing which in turn is bolted
directly to the engine's crankshaft and turns at engine speed.
The turbine is inside the housing.