If there is any automotive maintenance item that is overlooked more than changing automatic transmission fluid, we would be surprised. Most car owners know that transmission fluid needs to be changed periodically but they often forget about it -that is until something goes wrong.

What we are dealing with

An automatic transmission has a lot of parts inside of it; it's not an easy thing to understand. The main components of an automatic transmission include the torque converter, planetary gear set, hydraulic pump, hydraulic clutches, clutch bands, various sensors and the valve body. Frankly, they are a remarkable piece of machinery. Here are what the main parts do:

Torque converter – The torque converter is a hydraulic fluid coupling that connects the engine to the transmission. Consisting of a stator, impellor, and turbine, the torque converter does exactly what it says – it converts the engine torque into transmission torque.

Pump – Located between the torque converter and the planetary gear set, the pump pressurizes the oil for use throughout the transmission. If it fails (which is rare), the transmission will not work at all.

Planetary gear sets – The planetary gear sets are what do the shifting. They consist of a sun gear, planet gears, and a ring gear. The gear ratio is determined by how the planet gears are locked or unlocked, and the planet gears revolve around the sun gear.

Clutches & Bands – Clutches and bands are used to help the transmission shift gears by allowing gears to rotate, engage, or disengage.

Valve body – The valve body is a hydraulic control center that uses transmission fluid to run a network of spring loaded valves, check balls, and servo pistons. The valves determine what gear ratio is selected by sending fluid to the clutches and bands.

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Automatics "self-contaminate"

Since automatic transmissions are completely sealed, how does the fluid get dirty? Our service consultants at Reedman-Toll Chevrolet of Langhorne, a local Chevrolet dealer in Langhorne, PA said its simple: wear and tear.  Automatic transmissions have clutch bands that wear down over time. The point of a clutch is to "slip" a little during use but slipping causes some of the material on those clutches to rub away. This debris has nowhere to go but into the fluid itself. This is how the fluid self-contaminates itself with debris.


Debris-filled fluid being forced through small channels, orifices, and filters is a recipe for disaster. As the debris builds up, it will start to mess up certain parts of the gearbox. This can manifest in sticking valves, causing a harsh gear changes. It can also cause completely stuck valves, resulting in the complete loss of one or more gears.

Clean fluid

The solution to all this is simple. Replace the transmission fluid periodically with clean, fresh fluid. This will keep debris and gunk out of the transmission and allow it to function properly.

In general

At a minimum, change your transmission fluid according to the manufacturer's suggested intervals. The low end for an automatic transmission fluid change interval is around 30,000 miles. On the high-end is a change interval of 100,000 miles. To find the interval that your car's manufacturer suggests, consult your owner's manual. If you don't have one, go on-line and search or simply call your local dealer.