This article is broken down into five sections:

• What is a transmission breaks down in the simplest terms what the purpose of a transmission is.

• Transmission Components describes the general principals behind each system in simple terms to help you understand how an automatic transmission works.

• Spotting problems before they get worse shows what to look for to prevent a minor problem from becoming major. 

• Maintenance talks about preventative maintenance that everyone should know about.  

• Transmission repairs describes the types of repairs that are typically performed on transmissions from minor adjustments to complete overhauls. 

What is a transmission?

The transmission is a device that is connected to the back of the engine and sends the power from the engine to the drive wheels.  An automobile engine runs at its best at a certain RPM (Revolutions Per Minute) range and it is the transmission's job to make sure that the power is delivered to the wheels while keeping the engine within that range.  It does this through various gear combinations. In first gear, the engine turns much faster in relation to the drive wheels, while in high gear the engine is loafing even though the car may be going in excess of 70 MPH.  In addition to the various forward gears, a transmission also has a neutral position which disconnects the engine from the drive wheels, and reverse, which causes the drive wheels to turn in the opposite direction allowing you to back up.  Finally, there is the Park position.  In this position, a latch mechanism (not unlike a deadbolt lock on a door) is inserted into a slot in the output shaft to lock the drive wheels and keep them from turning, thereby preventing the vehicle from rolling.

There are two basic types of automatic transmissions based on whether the vehicle is rear wheel drive or front wheel drive.  

 On a rear wheel drive car, the transmission is usually mounted to the back of the engine and is located under the hump in the center of the floorboard alongside the gas pedal position.  A drive shaft connects the rear of the transmission to the final drive which is located in the rear axle and is used to send power to the rear wheels.  Power flow on this system is simple and straight forward going from the engine, through the torque converter, then through the transmission and drive shaft until it reaches the final drive where it is split and sent to the two rear wheels.

 On a front wheel drive car, the transmission is usually combined with the final drive to form what is called a transaxle. The engine on a front wheel drive car is usually mounted sideways in the car with the transaxle tucked under it on the side of the engine facing the rear of the car.  Front axles are connected directly to the transaxle and provide power to the front wheels. In this example, power flows from the engine, through the torque converter to a large chain that sends the power through a 180 degree turn to the transmission that is along side the engine.  From there, the power is routed through the transmission to the final drive where it is split and sent to the two front wheels through the drive axles.

 There are a number of other arrangements including front drive vehicles where the engine is mounted front to back instead of sideways and there are other systems that drive all four wheels but the two systems described here are by far the most popular. A much less popular rear drive arrangement has the transmission mounted directly to the final drive at the rear and is connected by a drive shaft to the torque converter which is still mounted on the engine. This system is found on the new Corvette and is used in order to balance the weight evenly between the front and rear wheels for improved performance and handling.  Another rear drive system mounts everything, the engine, transmission and final drive in the rear.  This rear engine arrangement is popular on the Porsche.

Transmission Components

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:

• Planetary 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.
• Seals 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 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.

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 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

 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.

• Watch 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
  Most manufacturers 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 cough 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 shift behavior
  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.  

Maintenance

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.

Few 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.

In most 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, Fords prior to 1983 use Type F while later models use Mercon.  Late model Chrysler products use ATF +3 +4 (Not using the correct fluid for Chrysler transmissions is the most common reason for their transmission problems.)  Toyota sometimes uses Type T which is only available through Toyota and Lexus Parts departments.  Honda also specs out their own formula which is available from Honda or Acura parts departments.   A transmission will not work properly or may even slip or shudder with the incorrect fluid, so make sure that you double check.  Your owner's manual will tell you which fluid is required.  Naturally, the owner's manual will try to convince you to only use the manufacturer's branded fluid, but they will also provide you with the specs for the oil.  If the aftermarket product indicates on its container that they meet or exceed the specs for a particular type of transmission fluid, it is generally ok to use that product.

Transmission repairs

• Adjustments and In-Car Repairs
  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 the vehicle.) or minor repair.
  
  If a late model transmission (computer-controlled transmissions started becoming popular in the early '90s) is not shifting properly, it is often the result of a computer sending incorrect signals due to a faulty sensor, or the transmission is not reacting to the computer command because of a bad connection or defective solenoid pack.  These problems can be corrected while the transmission is in the car for considerably less money then a complete overhaul.
  
  If a non computer-controlled 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 older 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 however.
  
• Reseal job
  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. 
• Complete Overhaul
  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.