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
  gear (Blue), 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.

Many 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 Packs

A 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.



  One-Way Clutch

  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 backward. 

  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.


  Bands

  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. 

  Torque Converter

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 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.

  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 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

  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.

  Oil Pump

  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.

  Valve Body

  The 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 should occur.

  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 Controls

  The 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.

  Governor,  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 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 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.


  Information provided by Family Car Magazine.
  SmartTrac Computer Systems, Inc.