The Advantages of an Automatic Transmission:

Automatic Transmission Advantages

(1) Automatic transmissions are easier for the driver to operate: no clutch pedal or gears to shift. This frees the drivers attention, and lessens the training needed to operate the vehicle. Fatigue in stop and go traffic is lessened.
It's harder to over-rev the engine with an automatic transmission, so it can be easier on the motor.
An inexperienced driver can operate an automatic transmission vehicle easier than a manual transmission: learning to use a clutch is sometimes hard for beginning drivers. (All Army vehicles are being ordered with automatic transmissions because a lot of recruits can't drive manual transmissions!)

(2) Longevity and reliability: A manual transmission has a clutch which must be serviced eventually. Each time a manual transmission starts you must allow the clutch to slip, and that slipping wears out the clutch disc eventually. This normally means removing the transmission to do a clutch job. If an automatic transmission is never run low on fluid and has periodic filter and fluid changes it will last practically forever. (200,000+ miles!)

Automatic Transmission Disadvantages

(1) Slight horsepower loss: Although with a lockup-type transmission the "slip" from the torque converter has been eliminated, and many have variable displacement oil pumps, the oil pump which operates the transmission uses some power. Ditto for all those released wet clutch packs as they slide together. An automatic transmission is often a good deal heavier than a manual transmission.

(2) Slight loss of control of vehicle: an automatic transmission shifts when the transmission wants to, not when YOU want to. Lots of folks prefer a manual transmission where THEY shift the thing when THEY want to!

HOW THEY WORK:

A planetary gear set consists of a sun gear in the center, 3 or more planet gears meshed with and encircling the sun gear, and a ring gear with teeth on the inside, encircling the planet gear assembly.
The speeds are obtained by holding one of the three gears, applying power to another, and taking power off the third. For example: power the sun gear, hold the planet gear assembly, and get reverse off the ring gear. Hold the sun gear, turn the planet gear, get a gear ratio off the ring gear. A two speed transmission needs only a single planetary gear set. Normally each additional speed takes another planetary gear set (a 3 speed transmission needs 2 planetarys, a 4 speed transmission takes 3), but with compound planetary gear systems a multiple speed transmission can be designed using fewer planetary gear sets..

Here's a link to a PLANETARY GEAR ANIMATION on youtube: GOOD STUFF!

An automatic transmission has one or more sets of planetary gears, they are stopped by clutches and bands to get all the speeds and reverse.

A band circles a drum and looks almost like a western style leather belt , but made of sheet metal covered with composite material. A hydraulic piston tightens this around the drum to stop a planetary gear element.

Clutches have two types of elements: steel plates and composition on steel discs. The steel plates have splines ("teeth") on the outside. The composition clutch material circular plates have splines on the inside. Usually several (often 6 or so of each) steels and composition discs are stacked together, increasing the "area" of the clutch pack. A hydraulic piston presses these together to hold a planetary element.

An early automatic transmission had one set of planetaries and had 2 forward speeds. A newer 3 speed transmission has 2 sets of planetaries, and the 4 speed transmission has 3 sets. I recently rebuilt a Chrysler front wheel drive transaxle, and they were getting 4 forward speeds using only 2 sets of planetaries! A clever design, but it had a VERY complex set of servos to accomplish this!

WHAT CONTROLS THEM?

For the newer cars, the answer is simple! (Imagine that!)

THE COMPUTER CONTROLS IT, taking in consideration your speed, throttle position, engine load, and other factors! Little solenoids supply fluid to those pistons and shift the transmission!

For older (and many new) cars the answer is more complex. They have a "hydraulic computer" which compares hydraulic pressures versus spring pressures and applies and releases pistons (also called servos) to control the planetarys.

The simplest ones shift using two inputs: throttle position, and governor pressure. A link goes from the gas pedal to the transmission. It moves a valve in the tranny. The more you press the pedal, the more it moves the valve, and the higher the THROTTLE VALVE PRESSURE is.
The governor has two weights which are opposed by two springs. As the transmission spins, centrifugal force moves these weights outward against the spring pressure.

The shift valves actually route fluid to the various hydraulic pistons (called servos) which operate the bands and clutches. Throttle valve pressure opposes governor pressure, delaying the shift when you're "floorboarded". When governor pressure gets high enough, the shift valve overcomes its spring pressure and shifts to the next higher gear.

HOW THEY FAIL

The actual clutches and bands usually last as long as the rubber seals in the servos. As soon as the pistons (servos) start to leak, the clutch elements fail rapidly. A "slipping" will be felt, or a "flare" where it slips when it first goes into gear then links up.

A "flare" is when the engine revs up after a shift, gradually slowing down as the transmission stops slipping. Other shift problems are: hard shift, late shift, or a gear that "just isn't there".

Usually, but not always, a slipping transmission which is not low on fluid will require an overhaul. Overhauls come in different "levels": the most basic involving new clutch discs and rubber seals: nothing else. a major overhaul involves replacement of all the bushings and steel plates also. If you' going to spend the money to overhaul a transmission you might as well do a major overhaul. It's important to keep fluid in your transmission at all times. This is the main thing, especially newer vehicles. Transmission cooler lines are a big source of leaks. Check them and check your fluid regularly!

FLUID AND FILTER CHANGES

It used to be GM recommended no fluid change until 100,000 miles!!!

They don't say that anymore, although many people DO go that long and longer without a change. As with many things, READ YOUR OWNER'S MANUAL!!!

When in doubt, change the fluid at 50,000 miles.

ALSO: Look at your fluid! when you check it, it should be a pink color. If it's brown, change it. If it's nice and pink, you probably don't need to change it. They don't make the transmissions as tough as they used to, and an inexpensive fluid change can avoid or postpone a very expensive rebuild!!!