The modern automatic transmission is by far, the most complicated mechanical component in today’s automobile.  Automatic transmissions contain mechanical systems, hydraulic systems, electrical systems and computer controls, all working together in perfect harmony which goes virtually unnoticed until there is a problem. This article will help you understand the concepts behind what goes on inside these technological marvels and what goes into repairing them when they fail.

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.

Now, (with caution – safety first!), shift the transmission lever into neutral. Apply the parking brake. With the engine idling, press the clutch pedal all the way, hold it down, and listen for noises. Then release the pedal and listen for noises again. There should be no loud noises at either positions.
The next step is the test drive.
Try to drive the vehicle at different speeds in all gears, one by one. Every gear should shift smoothly and easily without any noises or jerks. While driving at the second or third gear, try to press down sharply on the accelerator pedal for an instance. The clutch should not slip.
If you feel any slipping (e.g. the engine rpm increases but the speed remains the same), the clutch most likely has to be replaced.
Try to drive with acceleration and deceleration – there should be no grinding, whining or humming noise under any condition. All the gears should shift easily and noiselessly.

Check the CV joints

Broken CV joint boot

A Constant Velocity joint or CV joint is an important component of the drive train. The CV joint is packed with grease and protected by the rubber or plastic boot. Most common problem with the CV joints is when the protective boot gets damaged. Once this happens, the grease comes out and the moisture and dirt come in, eventually causing the CV joint to fail due to lack of lubrication and corrosion. In worst case, the CV joint may disjoin causing the vehicle to stop running. You can check the condition of the CV joint boots visually. You can see them with the front wheel turned outside, looking down from the front of a car. None of them should be damaged. Look at the photos. The CV-joint boot in upper image is damaged, in the lower image it looks OK. If a boot is damaged, it has to be replaced as soon as possible. If continue driving with broken boot, the CV joint or a whole drive shaft will need to be replaced. While drive test, If you hear clicking or popping noise when turning, the CV joint is probably already defective.

One of the indications of a transmission problem is delayed engagement, when there is a long delay between the moment you shift the shifter into “D” (Drive) or “R” (Reverse) and the moment the transmission kicks in.
It’s easier to note delayed engagement after a car was sitting for a while: With the transmission in “P” (Park) start the engine, and wait until the engine rpm has reduced to normal level (650 – 850 rpm).
With your foot holding down the brake pedal, shift to the “D” (Drive) position. Almost immediately the transmission should engage – it feels like the car wants to creep forward. This should happen very smoothly, without a strong jerk or clunk.
Shift to “N” (Neutral), and the transmission should disengage. Now, still holding the brakes, shift to the “R” (Reverse) position. Again, the transmission kicks in almost immediately – you will feel the car wants to creep backward. This also should be very smooth, without a jerk or clunk.
Now, still holding the brake pedal down, try to shift from D to R and back. There should be no strong jerk or clunk.
If there is a notable long delay (more than 1 seconds) between the moment you shift and the moment the transmission kicks in, such a transmission might be either too worn or has some problem, avoid this car.
If you feel a strong jerk or clunk while shifting, the car may have a transmission problem, avoid such a car.

Now it’s time to test drive the car.
With the shifter in “D” (Drive) position drive gently, with smooth and gradual acceleration. Until the vehicle reaches a speed of 30-37 mph (50-60 km/h) or you should feel the gears shifting at least twice (from first to second, and from second to third gear).
All shifts should be done very smoothly, without jerks or slipping.
You should be able to feel when the transmission shifts by the slight change in the engine tone or change in engine rpm. If the transmission is extremely worn it may shift with quite a strong jerk, shudder or a delay (especially from first to second gear).
Driving at a speed of 25-30 mph (40-50 km/h) if you press down the accelerator pedal for a few seconds, you should feel downshifting to the lower gear, if the automatic transmission works properly.
The next step: check overdrive.
While driving at 60-70 km/h or 35-45 mph on a level road, without using the accelerator, switch overdrive ON. You should feel an upshifting to the next speed. Switch it to “OFF,” and you should feel a downshifting.
Another thing that may indicate the transmission problem is the slipping. When the transmission is excessively worn it may slip – which means you press the accelerator, the engine rpm increases but the speed remains the same.

If during the drive test you feel any problem such as transmission seems to slipping or shifts with a jerk or shudder or if the transmission got stuck in some gear, or has trouble shifting into a particular gear (for example, from second to third), avoid buying such a car.
Test-drive the car as long as possible. Often the transmission may work well when it’s cold but when it’s warmed up it starts giving troubles or visa versa. So, it’s better to spend more time checking the transmission than later fixing it endlessly. Normally there should be no shudder, no noises or any kind of strong jerks at any speed and at any engine temperatures while any shifting. If the salesperson tells you that the jerks or shudder or any other abnormal transmission behavior is “normal” for this car or it’s just because the car is cold or anything alike, don’t trust them. If the “check engine” and/or a flashing overdrive light comes on while driving, have the problem assessed with your mechanic before buying a car.

• 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.Experienced engineers with ID lanyards can explain you better about the full process once you are at the sales office.

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.

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.

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.

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.

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. Developing solid metal hitch covers is a good choice.

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.

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.Use your business checks and upgrade your office automobiles for secure driving.

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.

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.

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.