Transmission Flush
The fluid responsible for allowing your engine to turn your automatic transmission so that you can move your vehicle when you slide the gearshift lever into the D position is called the transmission fluid! A lot of people don't realize how much stress is put on this fluid and how temperature extremes wear down this stuff significantly. The going rate for this service is approximately $139 and is worth every penny!
The automatic transmission is a modern marvel of how hydraulics work. Fluid is pushed through many valves, a filter, a torque converter, and variable rates of pressure to control what they want when they want it. The engine actually spins a part in the torque converter, which in turn pushes fluid against a different part in the torque converter, to actually transfer the power from the engine to the transmission. That's why we don't have to push a clutch!
It's other responsibilities include lubricating the gears and clutches and cooling the transmission. So transmissions are what they call transaxles, because they contain a differential assembly as well. That means the fluid also has to lubricate those gears.
That is why the average interval for most vehicles is 30K miles for each fluid change. Some manufactures recommend longer intervals, such as 50K, 60K, or 100K miles changes. There are so many different types of transmission fluids out there, and acceptable use generally has a lot to do with what the manufacturer recommends.
For example, there is Dextron for GM's, Mercon for Fords, and ATF+ for Chrysler products. What makes this even more confusing is that most of those have numbers after them (example is Mercon IV) that signify what standard they are meeting. Most are backwards compatible. There are universal fluids out there that meet multiple standards of fluids.
Sometimes the manufacturer also recommends that the filter be changed as well. Most shops will tackle this light duty transmission service. A few vehicles have external transmission filters, some can't be serviced at all, but most are accessed by removing the transmission pan.
If your car has one, the transmission cooler needs to be flushed out as well. Some vehicles have additional external coolers, but most coolers are located inside of the radiator assembly to be cooled by engine coolant. A busted radiator can actually leak transmission fluid into your cooling system.
A hundred dollars every year or two can save you a 4K dollar transmission replacement! Don't forget to service your other drivetrain components, like differentials and transfer cases.