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What Is an Automatic Transmission?

A transmission without the housing.
Cutaway view of a transmission.
Car clutch mechanism.
Repairing or rebuilding an automatic transmission requires training and expertise.
Automatic transmissions are equipped with three forward gears, reverse, park and neutral.
A sports car with an automatic transmission.
Article Details
  • Written By: Lori Kilchermann
  • Edited By: Lauren Fritsky
  • Last Modified Date: 19 October 2014
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An automatic transmission uses a hydraulic fluid pushed through a series of clutch plates and servos to drive a vehicle. The fluid is spun through a torque converter to multiply the amount of torque that is produced by the transmission. This torque determines shift points and acceleration, as well as vehicle power output. The torque converter links the automatic transmission to the engine at the flywheel or flex-plate. As engine speeds increase, the torque converter multiplies the amount of power the automatic transmission applies to the drive axle.

The shifting and gear changes in an automatic transmission are accomplished through the flow of transmission fluid through the transmission. Much like the blood flow in the human body, transmission fluid is pumped through a complex maze of passages and channels throughout the automatic transmission. As the fluid flows through the automatic transmission, it applies pressure to the different clutches and drives a specific gear. The gear will change accordingly as the transmission speed is changed either up or down.

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There are many varieties of automatic transmissions, and the main difference between them is the number of gears the transmission has. The traditional automatic transmission was equipped with three forward gears: reverse, park and neutral. Modern transmissions are equipped with at least four forward gears and can have as many as eight. There are also variable torque transmissions available that use a single speed that changes from low to high speed as the engine is brought up to speed. This type of transmission works much like a fluid coupling or hydrostatic drive type of transmission.

Most vehicles that can be equipped with a manual transmission offer an automatic gear box as an option. Typically, base model vehicles are equipped with a manual transmission while the upper-end vehicles are offered with an automatic transmission at an extra charge. Americans typically prefer an automatic-equipped vehicle, while Europeans will often choose a manual-type transmission. Many sports cars and crossover vehicles offer high-performance automatic transmissions complete with paddle shifters. While standard shifters are typically column- or floor-mounted, the paddle shifter places the gear selectors on the steering wheel.

By simply tapping the paddle up or down, the transmission can be shifted through the gears without the driver ever lifting his hands off the steering wheel. This type of performance-shifting selector has made its way from the Formula 1 ranks to street-driven vehicles. Other performance-type automatic transmission shifters offer both manual and automatic shifting through the flip of a switch.

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SarahSon
Post 2

@andee - Speaking of problems with your transmission, when I was in college and had no money I had an old car that I had to really baby along. One day it just wouldn't start and I needed to replace the transmission. Because it was so much cheaper to rebuild the automatic transmission than buy a new one - that is what I did. I figured I just needed to keep that car long enough to get through college.

It lasted as long as I kept the car. I am not saying there weren't plenty of other problems, but at least the transmission was good for awhile!

andee
Post 1

I won't consider anything other than an automatic transmission! Yes - I do know how to drive a manual, but don't like them at all. I have had my share of automatic transmission problems thought.

I know every car has their issues, but we had a van that we had to replace the transmission at least twice that I can remember. We have not had a van since then. We kept it around a long time for a spare, but it finally just gave out.

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