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Harnessing
the
Internal
Combustion Engine's Power:
The Transmission
Trans`mis´sion
n. (Mech.) The mechanism within a
vehicle which transmits rotational power from the engine to the axle
of the wheel propelling the vehicle; it includes the gears and
gear-changing mechanism as well as the propeller shaft.
There
is more to making a car go than an explosion in the cylinder
pushing a piston
and turning a crankshaft
as explored in the previous section
"The
Automobile's Engine"
.
A
vehicle needs a way to handle its rotary energy efficiently. It needs
a way to transmit the power the engine generates, and to transmit it effectively.
A car
needs a transmission.
What
is a transmission?
The
transmission is a device that is connected to the back of the engine
and as the name suggests, a car's transmission "transmits"
or sends, power from the engine
to the drive
wheels.
Essentially,
a car moves forward through a combination of speed and torque,
or twisting power. The transmission sets these two characteristics
so that the driver gets the smoothest, most efficient ride at the
speed they want.
A
Transmission Converts Torque
Torque
is the power of the twisting action that is sent to the wheels once
the reciprocal
(up
and down motion)
energy
of the pistons
is converted to rotary energy.
What the transmission does is increase and decreases the torque
according to the demand that the driver places on the automobile.
With a manual transmission, the driver selects the amount of torque.
With an automatic transmission, the transmission automatically shifts
according to demand.
If the
car is moving up a steep hill, it requires more torque. If the car is
moving on a flat interstate at highway speeds, it requires much less torque.
The
greater the torque, the greater the pulling power of the engine.
Torque,
along with engine RPM (revolutions
per minute),
is how horsepower is measured.
If your
vehicle had no transmission system, it would be difficult for the
vehicle to make the best use of all the possible combinations of
speed and torque.
This
would compromise the efficiency of the engine.
A
Transmission Is a Torque Multiplier
The
transmission delivers multiplied torque to the propeller shaft. The
shaft sends the power to the axle, and the axle to the tires. In low
gear and in second gear, an auto transmission is a torque multiplier.
Low
gear, the gear used when climbing a steep hill, or sometimes when
pulling a very heavy load, has a torque multiple of around 3 to 1.
Second
gear has a torque multiple of about 2 to 1. High gear, used at
highway speeds, is a direct drive gear, meaning that there is no
torque multiplier.
And
if you have a car equipped with "overdrive," the torque
ratio is in the neighborhood of 0.8 to 1. Every auto transmission
establishes its multiples with slight differences, depending on what
the designers determine to be the most efficient torque multipliers
for a given engine in a specific car.
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Highway speeds require less torque conversion from the transmission.
If your car is equipped with overdrive, use it
on the highway. Properly used, overdrive will reduce engine wear,
increase gas mileage, and promote longer engine and transmission life. |
Understanding
Torque Multiples
Imagine
riding a ten-speed bicycle. Your legs are the pistons
pumping up and down, the sprocket converts the energy to rotary
power, and the bicycle chain and the derailleur (the
gadget that moves the chain through the gears)
are the transmission. If the bike is in first gear-the chain is
connected to the smallest front sprocket and to the largest rear
sprocket- you may be able to climb even a steep hill and stay seated.
Your legs pump rapidly but without a lot of effort, and the bike
moves slowly up the hill: high torque, low speed. It is practically
impossible to start off in tenth gear and climb the hill. Tenth gear
generates low torque but much higher speed for the same amount of leg effort.
The
transmission of a car does the same work, but other gears replace the
chain. The gears themselves are made of much stronger and heavier
metals, and designed to withstand the strains of the automobile's
weight and speed.
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What
happens when pulling a heavy load in too high a gear?
Think
about peddling a bike up a hill in the wrong gear. If the gear is too
high (too little torque), a heavy load strains the engine,
causes it to heat up, and over time will reduce engine life. |
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So . . .
How
does the transmission actually work?
Well,
with each small
explosion in the four-stroke engine,
a connecting
rod turns
the crankshaft,
sending power through the transmission to a set of gears that adjust
the speed and torque to meet the conditions of the road.
Whether
you have a manual
transmission
or an automatic
transmission,
the
principle is the same:
higher
gears allow the engine to run more slowly on long or flat stretches
of road,
and
lower gears help the engine turn faster to produce more power and
conserve fuel.
In manual
transmission, the driver has to shift gears when they want to
increase or decrease their rate of speed.
With an automatic
transmission, invented
in 1932,
the gears are shifted automatically.
Other
popular transmission variants found on today's cars allow for both -
a manual and an automatic mode, as well as a fifth gear for better
fuel efficiency at higher speeds.
Types
of Transmissions
There
are two types of transmissions:
the manual
transmission
and
the automatic
transmission.
If
you're mashing a clutch
with your left foot and changing gears with your right hand,
Learn How To Drive A
Stick Shift
you
have a manual transmission.
If all
your car requires of your feet is that your right foot alternate
between the gas
pedal and the brake
pedal,
your
car is equipped with an automatic transmission.
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Every
car company develops its own transmissions. For that reason it is
impossible to offer one diagram that shows all the parts of every transmission.
But
every model of transmission uses the same basic principles to do its job. |
Automatic
Transmissions
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.
Hopefully
in this article we will help you understand the concepts behind what
goes on inside the transmission and what goes into repairing them
when they fail.
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
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usually
mounted to the back of the engine and is located under the hump in
the center |
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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
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 above 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.
Automatic
Transmission Components
The
main components that make up an automatic transmission include:
 Planetary
Gear Sets which
are the mechanical systems that provides 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.
Planetary
Gear Sets
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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.
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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. |
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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-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.
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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
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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 |
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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.
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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. |
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The three elements of the torque converter are the Pump, the
Turbine, and the Stator.
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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 |
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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 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.
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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 is
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.
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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. |
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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, stop light switch position, etc. to control exact shift points
as well as how soft or firm the shift should be. Some
computerized transmissions even learn your driving style and
constantly adapt to it so that every shift is timed precisely when
you would need it.
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.
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 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.
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