Transmission - Causes of failure
Automatic transmissions fail in as many ways as there are car models. In contrast, there are only a few reasons why anyone needs automatic transmission repair. I break it down into four major categories: neglect, abuse, design flaws, and normal wear and tear.

Transmission Neglect
The number one area of neglect is inadequate routine maintenance. As the temperature of the oil increases, the chemical components start to break down. Over time, the lubricating properties of the oil are lost. If not replenished, bearings and bushings begin to wear. If these bushings and bearings wear out, they can cause internal leaks which cause pressure losses, which lead to clutch, band and geartrain failure requiring the need for a rebuilt transmission. Fluid and filter changes (routine transmission service) should be performed at regular intervals from 15,000 to 25,000 miles, depending on the type of use. Vehicles used strictly for family transportation and for commuting to and from work should be serviced every 25,000 miles. Vehicles used for fleet service and medium duty hauling need servicing every 15,000 miles unless an auxiliary cooler is installed. More about coolers later.

A thorough transmission service procedure includes a road test to check for proper shift points, shift quality, inspection for external leaks, proper throttle pressure adjustment, proper linkage adjustment, a look inside the pan for abnormal signs of wear or damage, and finally replacement of the filter and addition of new fluid. In case you're wondering, there is no way to reasonably change all of the fluid. The pan usually holds four to six quarts of fluid while the torque converter can hold up to nine quarts. Most converters cannot effectively be drained without drilling and plugging. Most vehicles don't allow access for this. Some older transmissions have a drain plug in the converter to allow draining of this component. Not even then will all of the old fluid be removed.

Some shops have recently invested in machines that claim to be able to give your transmission a complete transfusion by connecting to the cooler lines and pumping new fluid into the system while at the same time removing the old. The problem with this procedure however, is that the converter acts as a reservoir. As new fluid enters, it does not force the old fluid out. Rather, the new fluid mixes with the old fluid inside the torque converter before it can be expelled from the transmission. In my opinion, to properly flush the system in this manner, one would need to circulate two to three times the capacity of the transmission through the system to get the job done. Even if this procedure were 100% effective, you would still need to remove the pan for filter replacement and inspection.
It's kind of like "the best cure for snake bite is not to get bit." The best answer is not to let it get ahead of you in terms of mileage. Changing the filter and moderate amounts of fluid at regular intervals saves the life of the transmission and keeps you away from the dreaded transmission rebuild.

In addition, all automatic transmissions use rubber or paper gaskets and seals. When subjected to the ordinary heat generated inside the transmission they shrink and become brittle. As this occurs, leaks develop. If these leaks are allowed to go unchecked, low fluid level will result. This will cause damage to internal components. So there is more than one reason to let a qualified transmission expert service your vehicle as opposed to just any auto repair shop.

Transmission Abuse
This is an area of great controversy. What is abuse? Any type of use that the vehicle was not intended for. How do we know what was intended? We really don't know. If the manufacturers made an absolute statement about intended use, they would expose themselves to litigation on the matter.

In my opinion, any amount of extra weight that causes the driver to notice a difference in vehicle performance is approaching abuse to the degree that extra heat is generated. It's excess heat caused by extra stress that dramatically shortens the life of your transmission. If the heat cannot be adequately dissipated, then internal damage will result. It's that simple. In addition to the extra heat being generated, there's the extra load on all of the internal components. Clutches and bands are designed to hold tightly under the load of the normal weight and horsepower of the vehicle. Increases in either of these can cause trouble. Now, I know that not using your 3/4 ton truck to pull the family boat to and from the lake is not the answer you want to hear. But if you want to preserve your transmission to the maximum extent, then that's the correct answer. Personally, I pull my boat. But I don't do so without taking precautions. Please consider the preventative measures listed below. Transmission overhaul is getting considerably more expensive these days. Preventative maintenance and proper driving habits are the best ways to avoid costly transmission repair.

Transmission Design Flaws
This is the area that most people find the hardest to accept. How is it that my new $28,000 truck could have design flaws that will cause premature failure of my transmission? If a vehicle manufacturer were to spend the time and money necessary to debug every facet of design, that $28,000.00 truck would cost $128,000.00 or more. Why do Rolls Royce and other comparable automobiles cost so much? They take more time in designing them and then work out the little problems in the design before they go to production. The government with all of its regulations, the demand by the public to produce vehicles quickly, and plain old human error simply make it impossible for any manufacturer to build anything perfect. That's why you see so many changes in design during the current model year and why TSB's (Technical Service Bulletins) exist.

Comfortable shifts, less strain on the engine by the transmission oil pump, reduced weight of transmission components, and other parts of the vehicle in order to conserve fuel, more heat and other factors, all contribute to premature wear inside the transmission. To be fair, most manufacturers have made great progress in improving transmission reliability over the last 10 years. They are more dependable today and last longer. But that doesn't mean they are flawless.

Transmission Normal Wear & Tear
Anything mechanical can, and will, at some point in time break down or wear out. Transmission failure is no exception. Bushings and bearings are usually the first components to go, with dynamically applied clutches and bands following. Dynamic meaning a band or clutch used to stop something in motion as opposed to preventing a stationary (static) object from starting to move. A reasonable service life can be from 40,000 - 50,000 miles up to 150,000 miles or more depending on the type of use. General family use on the high end. Fleet and commercial use on the low end.

Transmission Breakdown Prevention
First there's the heat problem. Installation of a good quality auxiliary oil cooler is the first step. Why do I need an auxiliary cooler? Doesn't the factory provide one for me? Most vehicles are designed to circulate transmission fluid up to the radiator and back again through two steel lines. The coolant inside the radiator is what cools the transmission fluid. Yes, that's right. 200+ degree antifreeze tries to cool 300+ degree transmission fluid. Doesn't make a lot of sense, does it? Auxiliary transmission oil coolers use the outside air temperature to cool the fluid which is far more efficient. Sometimes, if your vehicle has a towing package for instance, the factory will install an auxiliary cooler on the vehicle. Otherwise you must order it as an extra accessory.

The addition of one of these coolers can more than double the life of the fluid and the transmission. The best coolers, in my opinion, are built by the aftermarket. Ask your local transmission shop about the details.

Second, on many late model vehicles, reprogramming the transmission is also necessary in order to get enough fluid to the cooler, help the bands and clutches hold better, and provide an increased volume of lubrication oil to the geartrain. On certain 1997 and later domestic vehicles this reprogramming has its limitations. Due to the way the computer controls the transmission, changes made internally effect what the computer sees and how it controls the shifts, sometimes canceling out the benefits of reprogramming. To be sure, ask a qualified expert. By qualified, I mean a technician who specializes in modifications and reprogramming, such as shift kits, as separate transmission services.

I emphasize routine fluid changes. It's the single most important aspect of preserving your automatic transmission and preventing the need for a rebuilt or overhauled transmission.