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:
1) 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.
2) 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
Seals and Gaskets are used to keep the
oil where it is supposed to be and prevent it from leaking
out.
The
The
On newer vehicles, shift points are controlled by
Planetary Gear
Sets
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.
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 i
n four and now, five, six and even seven-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 newer vehicles, 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 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.
3)
Spotting problems before they get worse
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 belowonclick="javascript:find()" 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.
4) 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.
5) 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.
