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 pedaling backward.
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.
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.
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
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 Impeller, the Turbine,
and the Stator. The Impeller 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 and is connected directly to
the input shaft of the transmission providing
power to move the vehicle. The stator is
mounted to a
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
the engine running, transmission fluid is pulled
into the Impeller 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 Impeller, 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 Impeller at
a "helping" angle providing a torque
increase. As the speed of the turbine
catches up with the Impeller, the fluid starts
hitting the stator blades on the back-side causing
the stator to turn in the same direction as the
Impeller and turbine. As the speed increases,
all three elements begin to turn at approximately
the same speed.
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
dependant 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 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, as
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
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.
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.
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
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.
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.
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 engine load.
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
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.
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.
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.
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.
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.