Mechanics

Automatic Transmission

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Article explains how an automotive automatic transmission and torque converter works.

Many 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 valve body, torque converter and fluid cooler. 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. It is necessary to depress the brake pedal before the gear selector can be moved from the "park" position.

Gear selection options include (P) park, (R) reverse, (N) neutral, (O) overdrive, (D) drive. 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 early 1950s.

Automatic Transmission

The automatic transmission consists of: a transmission case, planetary gear-sets, valve body, fluid cooler (in radiator) and a torque converter. A torque converter utilizes transmission fluid as a coupling agent allowing the engine to run while the car is stopped without stalling. Then when the engine speed is increased, re-engaging the transmission to make the car move. A series of clutches and bands control the planetary gear sets to provide multiple forward gear ratios. The valve body is the hydraulic control center that receives pressurized fluid via the main transmission pump. This transmission gear system is controlled by the computer PCM (power control module.)


Automatic Transmission cut-away Image

Park and neutral are the only gear selectors modes the engine will start in. 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 to 6 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 which 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 to slow vehicle speed.

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.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-19)