Friday, January 22, 2010

How an Automatic Transmission Works

There are two types of transmissions most commonly used today, automatic and  manual (stick shift). Most automatic transmissions have a service filter in the  transmission pan that should be changed at manufacturer specified intervals. When servicing an automatic  transmission a complete fluid flush is recommended to clean the many passages  inside an automatic transmission and to get the fluid that is trapped in the torque  converter.
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 pedal 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, then re-engaging  to make the car move again once the RPM of the engine has increased. A series of  clutches and band controlled planetary gear sets provide multiple forward gear ratios  with a set reverse gear. The valve body is the hydraulic control center that receives  pressurized fluid via the main transmission pump. This system is controlled by the  PCM (power control module) in newer cars and mechanically in older ones (Pre-computer  controls).

Automatic Transmission cut-away Image

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 will  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, whirring 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 transmission has become popular during the past few years.  Instead of having set gear ratios, the system can change the amount of forward acceleration  over a wide range of speeds. Two cones or wheels of varying diameter are used to  change the gear ratio. Hydrostatic drives use a variable displacement pump and hydraulic  motor to vary the ratio continuously according to the amount of throttle being given  and the amount of load on the vehicle.

Popular Problems Checks
1. Car will not go into gear:
  • Gear selector cable has failed
  • Brake lock solenoid/brake light swtich 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 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 either a vacuum modulator or a throttle   control valve cable (TV Cable). If the vacuum is impaired to the modulator or   the throttle valve 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

Transmission Planetary Gear Set
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 a 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 onetime   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.

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