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How Does it Work /
Automatic
Transmission
How an Automotive Transmission Works
There are two types of transmissions most commonly used today, automatic and manual (stick
shift). The power generated from the engine is transferred into the transmission
through an input shaft. Most automatic transmissions have a
service filter
in the transmission pan that should be changed at manufacturer's specified intervals.
When servicing an automatic transmission a complete fluid flush is recommended
to clean the many passages inside an automatic transmission.
The driver selects a mode by maneuvering the gear shift lever, usually placed
on the steering column or next to the driver's seat in the center console. In most cars it
is necessary to depress the brake peddle before the gear selector can be moved from
the Park position. Gear selection options usually include (P)park, (R)reverse,
(N)neutral, (O)overdrive, (D)drive, (2)second and (L or 1)first gear in that order. The park or P mode
locks the transmission mechanically. This restricts movement of car in any direction.
This is achieved using a metal rod that engages the output shaft and prohibits movement.
An automatic transmission functions by automatically changing the gear ratios
while determining
the speed and load of the engine. Automatic transmissions have been available
since the 1950s. The automatic transmission is controlled by selecting a
desired gear from the hydraulic gear engagement system.
Automatic Transmission Gear Range Selector
Inside the Automatic Transmission
The automatic transmission consists of: a transmission case, planetary gear-sets,
valve body, fluid cooler (in radiator) and a torque converter or fluid coupler.
A torque converter utilizes transmission fluid as a coupling agent allowing the
engine to run while the car is stopped without stalling the engine. And then re-engaging
to make the car move again once the RPM of the engine has increased. A series of
clutch and band controlled planetary gear sets provide mutable forward gear ratios
with a set reverse gear. The valve body is the hydraulic control center which receives
pressurized fluid via the main transmission pump. This system is controlled by the
PCM (power control module).
Automatic Transmission cut-away Image
Transmission Planetary Gear Set
To prevent damage to the transmission the car should be at a complete
stop before engaging the Park mode. Park mode is also one of the
modes in which the car can be started, the other is the N or neutral position.
The neutral or N mode is used to disengage the transmission from
the drive wheels to let the car move freely. The drive or D mode allows the car
to vary its speed through a range of forward gears, usually 4 forward gears exist,
but companies like Audi, BMW, Lexus and Mercedes Benz have developed a direct shift
gearbox that has up to 8 forward gears.
The D4 mode is best suited for highway speeds and D3 can be used
for around town driving. The first, 1 or L mode is meant to lock the transmission
in first gear. In this mode a vehicle will move slowly but have more power that
can be used when towing or on steep grades. The second, 2 or S mode is used to lock
the transmission in the first two gears. This is used in extreme weather conditions
like ice and snow and to govern vehicle speed.
Automatic Transmission Computerized operating system with ABS Brake System
Automatic transmissions make specific noises when a malfunction
occurs. An automatic transmission is a hydraulic pressure driven system and it
can make different noises than a manual transmission. If the transmission filter
becomes plugged due to debris, it can make a whining noise. If
the fluid level is low, you might hear a gurgling sound, caused by the pump scavenging
fluid inside the transmission pan. Most internal failures are due to bearing, clutch
or hard part failure. When such a failure occurs the transmission can make
grinding, whurrring sounds or no noise at all. When a transmission has a major failure you
might hear a loud pop which could mean a drive component inside the transmission has
failed and the car will stop moving.
Some manufacturers have developed a continuously variable transmission
or CVT. This type of trnsmission has become popular during the past few years.
Instead of having set gear ratios, the system can change speed over a wide range
of speeds. The varying diameter of two cones or wheels are used to change the gear
ratio. Hydrostatic drives use a variable displacement pump and hydraulic motor to
vary the ratio continuously.
Popular Problems Checks
1. Car will not go into gear:
- Gear selector cable has failed
- Brake lock solenoid has failed not allowing the gear selector to move out
of "Park"
- Excessively low transmission fluid (note: if car is operated for an extended
amount of time with low fluid level the transmission will fail prematurely)
- Shorted electrical component not allowing the PCM to control the transmission.
Example: shorted fuse
- Flex plate (flywheel) is broken completely not transferring engine power
to the transmission.
2. Car goes into gear but then fades out of gear or is slipping while driving:
- Transmission fluid is low
- Transmission clutch discs or bands are worn out or burned
- Faulty transmission shift solenoid.
3. Car goes into gear but does not shift out of first:
- Blown fuse to the PCM controller
- Faulty vehicle speed sensor (VSS)
- Shorted second gear control solenoid
- Faulty transmission controller (PCM)
Tips About Common Problems and Fixes
- When the vehicle is cold or going around a corners the transmission fades
in and out of gear: In most cases this means the transmission fluid is low.
The transmission
will lose hydraulic pressure causing the transmission to drift in and out of
gear. Check your transmission fluid when the car is on flat ground with the
engine idling in park (some Chrysler products must be checked in neutral), add
fluid as needed and recheck level.
- Transmission is shifting too late or not at all. On most cars the transmission
is controlled by the PCM (powertrain control module) if the vehicle speed sensor
fails the PCM has no input so the computer will not shift the transmission properly.
The best way to check this sensor is to make sure the speedometer is operating
correctly. If it isn't, replace the VSS and recheck. On older cars the transmission
shifting is controlled by a vacuum modulator or a throttle control cable. If
the vacuum is impaired to the modulator or the throttle position cable has failed
the transmission will not shift correctly.
- The transmission skips second gear, shifts from first gear to third gear
and the "service engine soon" MIL is illuminated.
Scan the PCM to locate the
transmission control solenoid that has malfunctioned, replace the solenoid then
clear trouble codes and recheck operation.
Fluid Level and Leaks
One of the most common complaints with an automatic transmission
is leaking fluid. Leaks can occur from the driveshaft seals, the input shaft seal,
pan gasket, or the ATF (automatic transmission fluid) cooler or line connections.
When adding transmission fluid, do not overfill. Doing so could cause the fluid to become aerated, which will affect
transmission operation. If there are no visible leaks, check the radiator for ATF
in the coolant. The ATF cooler inside the radiator may be leaking and cross-contaminating
the radiator coolant. You should also check the condition of the fluid, some discoloration
and darkening is normal as the fluid ages, but if the ATF is brown or has a burnt
smell, it is badly oxidized and
transmission service needs to be performed.
Most transmission problems can be prevented by changing the ATF and filter
according to manufacturer
specifications. In extreme conditions installing an aftermarket auxiliary ATF
cooler parallel to the OEM ATF cooler is recommended. This prevents fluid overheating
on vehicles used for towing or performance applications.
Fault Codes:
Fault codes can be set when the transmission controller or PCM detects a malfunction.
Codes can be set by the computer when a command is sent, such as a 2-3 shift, but
the transmission does not respond.
Diagnostic Procedure
-
Clear all DTC's with a
engine scan tool (diagnostic
trouble codes)
-
Start the engine and observe the MIL, if it does not illuminate
continue to next step (malfunction
inductor lamp or
check
engine light)
-
Drive the vehicle while trying to maintain a constant throttle
position as it accelerates up through all four gears. If the transmission is
shifting properly, it should be in 4th gear by the time you reach 45 to 50 mph
on level ground. Repeat this procedure from a standing start 3 to 5 times. Rescan
the PCM for trouble codes, if none appear the problem could have been a one
time occurrence. If
trouble
codes have returned repair as needed and recheck system.
Tips: Never allow little noises go unattended, a small noise can cause
a large noise and transmission failure. Avoid overloading a vehicle or towing beyond
capacity this can cause premature transmission failure.
How Does it Work
/ Automatic Transmission
Check Manufacturer Specific Automatic
Transmission Questions and Answers
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