The automatic transmission in your car is essentially a gigantic hydraulic pump. It uses transmission fluid to transfer engine torque to the drive wheels through a “torque converter” to a series of valves and pumps to control fluid flow and pressure.  As with the other fluid systems on your car, much is expected of this fluid and it deteriorates rapidly with heat, contaminants, and oxidation--it will tend to thicken over time and not flow as needed and reduce the efficiency of this unit, as well as shorten its useful life.

Most transmissions should have a fluid exchange or a filter service at least every 30K miles to renew the fluid and protect all the components in the unit.  With city driving, the demands of your transmission are higher than highway driving as they receive more wear and tear in this stop-and-go environment.  Many shops will recommend more frequent service based on your particular vehicle and your driving habits.

There are two types of transmission service available to the public—a Transmission Flush or a Filter Service or both depending on the shop and their level of expertise.  A true transmission flush uses a transmission flush kit (a cleaner and a conditioner pack) with a piece of equipment to completely exchange (90-95%) new fluid for old and has become a very popular service.  Be aware of a fluid exchange without a flush kit as this procedure will not remove gum and varnish or condition the moving parts and is merely pumping out old fluid without the full benefits available of this procedure.  A filter service will replace the old filter, gasket, and about half of the old fluid and should be done every other time your vehicle is in for service.

When Automatic Transmission Fluid oxidizes it breaks down, thickens, and loses vital additives that will shorten transmission life without regular fluid replacement. Scheduled AUTOMATIC TRANSMISSION SERVICE will keep the transmission clean, assure positive shifting, and keep transmission wear to a minimum!

For more information on auto trans operation, check out : HowStuffWorks - "Automatic Transmission"