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The Basics: Automatic Transmission Servicing--complete article

When and Why Service an Automatic Transmission

Transmission fluid, like any other fluid, such as motor oil, antifreeze, and other fluids, tends to "break down" with heat and time. As the operating temperature rises, the fluid breaks down at a quicker rate, requiring replacement at much shorter intervals. This is not a direct proportional rate. If the transmission fluid lasts a certain amount of time at 200 degrees farenheit, the fluid may only last half as long at 230 degrees and half, again, as long at 250 degrees. These numbers are off the top of my head and are used, only as an example. The actual breakdown may (and probably is) be much different than these numbers. I only mention this to illustrate the importance of keeping the temperature under control within an automatic transmission. Most vehicles utilize a heat exchanger within the radiator, to keep the transmission cool. This is the best way to cool a transmission. Water is a much better heat-transfer medium than is air. This is why 70 degree air does not feel, to you, as cool as 70 degree water does. Most after-market coolers are hung in front of the radiator and use air as a heat-transfer medium, rather than water; for this reason, after-market coolers are not capable of getting rid of as much heat as the original radiator cooler, and should be used, only, in addition to the original radiator cooler; Not by themselves. The transmission runs much hotter than an engine, so there is no danger, as some people believe, of overheating the transmission with the engine coolant. Actually, having a heat exchanger within the radiator, helps the transmission in another way, by initially heating up the transmission and helping to act as a thermostat to keep the transmission at a more consistent operating temperature.
I have seen many recommendations, in terms of automatic transmission service intervals. Most, generally agree that a good "rule of thumb" is to service the transmission every 25,000 miles. Of course this will depend upon how the vehicle is used and what type of vehicle. Most newer cars, and trucks have lockup torque converters (What's a lockup torque converter?) which tend to create much less heat than older, traditional automatics of the '70's and early '80's, so for those that do, you may want to extend the service period, if you use the vehicle primarily on the highway, and do not tow, fourwheel, or use the vehicle in any other "severe use condition".

The term "fluid break down" refers to the destruction of additives within automatic transmission fluid, due to the combination of heat and time. The higher the heat, the less amount of time the fluid will last. When the fluid "breaks down" it turns dark in color, and begins having an unpleasant odor. The fluid becomes more acidic and, as a result, hardens the rubber seals within the transmission. When these seals harden up, they can no longer perform their job efficiently, and they begin to leak. When the seals begin to leak, they tend to allow clutches to slip. The clutches within an automatic transmission are not able to withstand much heat, so they eventually burn, which makes them slip more, which makes them burn more......until the transmission developes severe shifting problems or may, in some cases, quit moving at all. The only alternative, when the problem becomes this advanced is a complete overhaul, which, among other things, includes replacement all of the clutches and rubber seals.