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How Automatic Transmissions Work

Introduction to How an Automatic Transmission Works
Why Your Car Needs a Torque Converter
How it Works
Benefits and Shortcomings of the Torque Converter

If you have ever driven a car with an automatic transmission, then you know that there are two big differences between an automatic transmission and manual transmission:

• There is no clutch pedal in an automatic transmission car
• There is no gear shift in an automatic transmission car. Once you put the transmission into Drive, everything else is automatic!

In this FAQ we'll work our way through an automatic transmission. We'll start with the key to the whole system: planetary gearsets. Then we'll see how the transmission is put together, learn how the controls work and discuss some of the intricacies involved in controlling a transmission.

Figure 1. Location of the automatic transmission, On DSM's, since they are FWD based, just turn the engine and tranny sideways.

Some Basics
Just like that of a manual transmission, the automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds.

Without a transmission, cars would be limited to one gear ratio, and that ratio would have to be selected to allow the car to travel at the desired top speed. If you wanted a top speed of 80 mph, then the gear ratio would be similar to third gear in most manual transmission cars.

You've probably never tried driving a manual transmission car using only third gear. If you did, you'd quickly find out that you had almost no acceleration when starting out, and, at high speeds, the engine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly undriveable.

So the transmission uses gears to make more effective use of the engine's torque, and to keep the engine operating at an appropriate speed.

The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission...

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Table of Contents:

Introduction to How an Automatic Transmission Works
Planetary Gearsets & Gears
The Clutches and Bands
The Hydraulic System and How It Shifts

• Automatic Transmissions, how do they work?
• Torque Converters... What? What are those?
• Launch/Staging Tips - 3,2,1 GO!
• NOS - Nitrous Oxide
• Turbo selections, what works, what doesn't
• A/T Digest - Talk to our group of enthusiasts and vendors daily!
• Fastest A/T DSM's - Please remove shoes and bow before entering :)
• Members pages (new!)
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