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 pedaling
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 it from turning.
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 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
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.
'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
The 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 (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, as required.
The valve body is the
brain 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 should occur.
important valve, and 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
computer uses sensors on the engine and transmission to detect such things
as throttle position, vehicle speed, engine speed, engine load, stop light
switch position, etc. to control exact shift points as well as how soft or
firm the shift should be. Some computerized transmissions even learn your
driving style and constantly adapt to it so that every shift is timed
precisely when you would need it.
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
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.
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
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 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.
Seals and Gaskets
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
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
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.
Spotting problems before
they get worse
for leaks or stains under the car
If there is a
persistent red oil leak that you are sure is coming from your car, you
should have your shop check to see if it is coming from your transmission or
possibly from your power steering system (most power steering systems also
use transmission fluid and leaks can appear on the ground in roughly the
same areas as transmission leaks.) If all you see is a few drops on the
ground, you may be able to postpone repairs as long as you check your fluid
level often (but check with your technician to be sure.) If transmission
fluid levels go down below minimum levels serious transmission damage can
occur (the same advice goes for power steering leaks as well.)
Check fluid for color and odor
require that you check transmission fluid levels when the vehicle is running
and on level ground. Pull the transmission dipstick out and check the fluid
for color and odor. Transmission fluid is a transparent red oil that looks
something like cherry syrup. If the fluid is cloudy or muddy, or it has a
burned odor, you should have it checked by your technician who will most
likely advise you to have a transmission drain and refill or transmission
tune-up. See the Maintenance section below for details on this service.
Be sensitive to
new noises, vibrations and
A modern transmission should shift
smoothly and quietly under light acceleration. Heavier acceleration should
produce firmer shifts at higher speeds. If shift points are erratic or you
hear noises when shifting, you should have it checked out immediately.
Whining noises coming from the floorboard are also a cause for concern. If
caught early, many problems can be resolved without costly transmission
overhauls. Even if you feel that you can't afford repairs at this time, you
should at least have it checked. The technician may be able to give you
some hints on what to do and not do to prolong the transmission life until
you can afford the repair.
Transmission fluid should be changed periodically. Your owner's manual
should give you the recommended intervals which could be anywhere from
15,000 miles to 100,000 miles. Most transmission experts recommend
changing the fluid and filter every 25,000 miles.
transmissions have drain plugs to drain the old fluid. In order to get
the fluid out, the technician removes the transmission oil pan. This is
quite a messy job and generally not recommended for the casual
do-it-yourselfer. Even if the transmission has a drain plug, the only way
to also change the transmission filter is to remove the pan. When the pan
is down, the technician can check for metal shavings and other debris
which are indicators of impending transmission problems.
cases during these transmission services, only about half the oil is able
to be removed from the unit. This is because much of the oil is in the
torque converter and cooler lines and cannot be drained without major
disassembly. The fluid change intervals are based on the fact that some
old fluid remains in the system.
When the transmission is serviced, make sure that the correct fluid is
used to re-fill it. Each transmission manufacturer has their own
recommendation for the proper fluid to use and the internal components are
designed for that specific formula. GM usually uses Dexron, Ford uses Type
F, Toyota sometimes uses Type T. A transmission will not work properly
or may even slip or shudder with the incorrect fluid, so make sure. Your
owner's manual will tell you which fluid is required.
There are several problems that can be resolved with an adjustment (A simple
adjustment is one that can be made without removing the transmission from
If a transmission is shifting too early or too late, it may require an
adjustment to the throttle cable. Since throttle cables rarely go out of
adjustment on their own or due to wear and tear, these mis-adjustments are
usually due to other repair work or damage from an accident. If the vehicle
has a vacuum modulator instead of a throttle cable, there is an adjustment
that can be made using an adjustment screw in some modulator designs. In
vehicles with modulators, however, it is very important that there are no
vacuum leaks and the engine is running at peak efficiency. Engine vacuum is
very sensitive to how well the engine is running. In fact, many technicians
use a vacuum gauge to diagnose performance problems and state-of-tune. Many
problems that seem to be transmission problems disappear after a tune-up or
engine performance related repair was completed.
In some transmissions, bands can be adjusted to resolve "slipping"
conditions. Slipping is when an engine races briefly when the transmission
shifts from one gear to the next. There are no adjustments for clutch packs
A transmission is resealed in order to repair external transmission fluid
leaks. If you see spots of red oil on the ground under the car, your
transmission may be a candidate for a reseal job. In order to check a
transmission for leaks, a technician will put the car on a lift and examine
the unit for signs of oil leaks. If a leak is spotted at any of the
external seals or gaskets and the transmission otherwise performs well, the
technician will most likely recommend that the transmission be resealed.
Most of the external seals can be replaced while the transmission is still
in the car but, if the front seal must be replaced, the transmission must
first be removed from the vehicle in order to gain access to it, making it a
much costlier job.
Replace accessible parts
There are a number of
parts that are accessible without requiring the removal of the complete
transmission. many of the control parts including most of the electrical
parts are serviceable by simply removing the oil pan. The parts that are
accessible, however, vary from transmission to transmission and most
transmission repair facilities would hesitate to provide meaningful
warrantees on external repairs for the simple reason that they cannot see if
there are any additional internal problems in the components that are only
accessible by transmission removal.
In a complete
overhaul (also known as rebuilding a transmission), the transmission is
removed from the vehicle and completely disassembled with the parts laid out
on a workbench. Each part is inspected for wear and damage and then either
cleaned in a special cleaning solution, or replaced with another part
depending on its condition. Parts that have friction surfaces, such as bands
and clutches are replaced as are all seals and gaskets. The torque converter
is also replaced, usually with a remanufactured one. Technical service
bulletins are checked to see if the auto manufacturer recommends any
modifications to correct design defects that were discovered after the
transmission was built. Automobile manufacturers often make upgrade kits
available to transmission shops to resolve these design defects.
Replacement unit vs. overhaul existing unit
When a transmission requires an overhaul, there are generally two options
that you may have. The first is to remove your existing transmission and
overhaul it, then put the same, newly rebuilt unit back in your car. The
second option is to replace your existing unit with another unit that has
already been rebuilt or remanufactured.
The second option will get you out of the shop and on your way much faster
but may cause you problems down the road. The reason for this is that, in
some but not all cases, a particular transmission model can have dozens of
variations depending on which model car, which engine, which axle ratio,
even which tire size. The problems you could experience could be as simple
as a speedometer that reads too high or too low (the speedometer is usually
connected by cable to a gear in the transmission output shaft.) You may also
experience incorrect shift points or even complete transmission failure
because your engine may be more powerful then the one the replacement unit
was originally designed for. This is not the case with all transmission
models so voice your concerns with your technician. Most shops will
rebuild your existing unit if you request it as long as they can afford to
have a lift tied up with your car while the transmission is being rebuilt.
Of course this is only important if you are sure that the transmission you
have is the original one and has never previously been replaced.
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