Legal Notice
All information
provided is believed to be accurate and reliable. We will make changes, updates
and deletions as required and make every effort to ensure the accuracy and
quality of the information provided. However, Bear Transmission or its franchisees
assumes no responsibility for any errors and are not liable for any damages
of any kind resulting from the use of, or reliance on, the information contained
herein. All information can be withdrawn or changed without notice.
Communications made through this electronic mail and message system shall
in no way be deemed to constitute legal notice.
Franchise opportunities are being offered in Florida. In other states new
franchises will be added soon.
Toll Free
866·435·BEAR
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.
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.
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.
Solenoid PackThe 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.
Automatic transmission 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 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.
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.
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.
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.