Your car's automatic transmission is a handy piece of equipment. It shifts gears for you automatically, takes a lot of abuse, and is generally ignored by many car owners.
Transmission fluid level should be checked regularly, according to owner manual instructions. And fluid should be added if needed.
Also important -- every 30,000 miles or so the oil transmission filter and transmission fluid should be removed and replaced.
To do it yourself, remove the transmission dipstick. With a clean white cloth, wipe the oil from the dipstick. Look at the cloth. Ther shouldn't be any metal particles, the oil should be clear reddish color, and it shouldn't have a strong burned smell. Look at the dipstick. There shouldn't be any varnish buildup on it.
If the dipstick and fluid don't measure up so far, take the car to a garage and let them check it. If they're OK, you can continue.
Get a pan to drain the old fluid into. Most automatic transmissions don't have a drain, so draining the fluid out without taking a bath in ti can be a little difficult. But you can do it if you do it right.
Elevate the car safely so you can reach the drain pan. Use ramps or jack stands to support your car. Remove all but two of the bolts that hold the transmission pan on. These two bolts that you don't remove should be on opposite sides of the pan. The drain pan should now be undrneath the transmission pan (the drain pan should be wider than the transmission pan, because fluid will shortly be coming out from all around the edge of the transmission pan).
Now loosen those two remaining bolts enough so that you can pry the pan down slightly from the bottom of the transmission. If you can't pull the pan down with your fingers, use a screwdriver. It won't be hard to pry loose, because gasket sealer isn't used on a transmission pan gasket.
Now, push the pan back up against the transmission and remove one bolt. Let the pan drop down gently so it pivots on the remaining bolt. Let the fluid drain. When it stops draining excessively, push the pan up again. Remove the other bolt and lower the transmission pan down in the drain pan.
While the transmission finishes draining, remove the transmission filter. Note how the filter is held in place. Some are held in by screws. If so, usually there's a gasket between the filter and valve body of the transmission. Remove the gasket and filter. When most draining has stopped, it's time to put things back together again.
Scrape the bottom of the transmission, where the new gasket will touch, to remove any old gasket or foreign material that might be present and clean the transmission pan with a cleaning solvent (available in auto-parts stores).
Be sure the pan is not damaged. Lay the pan on a flat surface to see that the flange, the part of the pan that the gasket fits against, is flat. Lay the new pan gasket (new gasket and filter can be purchased at auto-parts stores) on thee pan, being sure to line up the holes in the gasket with the bolt holes in the pan. To install the new filter, simply reverse the removal procedure.
Push the pan up against the transmission and put the bolts in place. If you have trouble with the gasket shifting position, use a light coating of grease on the flange to hold the gasket in place. Never use any form of gasket sealer on a transmission gasket.
Tighten the bolts that hold the transmission pain in place.
The next step is to drain the transmission torque converter if it has a drain plug. Many cars do not. If yours doesn't, then don't worry about it -- you're ready to refill the transmission. Here's how. Put three quarts of transmission fluid in. (Use the type of transmission fluid recommended in your owner manual. Use a narrow-neck funnel to pour the fluid down the dipstick tube.)
Start the car. Take a measurement with the dipstick. Is fluid showing on the bottom of the dipstick? It should. Add fluid gradually until the fluid level reaches the "add" line. Go into the car, put your foot on the brakes, and shift through all the gears. Bring the gear shift lever back to the position the manual calls for when checking the fluid -- either neutral or park. Continue adding fluid gradually -- don't overfill -- until you're within 1/4" of the full mark. The transmission is cold now. If you fill to within 1/4" of the full mark, the fluid level will be at the full mark when the transmission heats up. If you did drain the torque converter, to -- refill the transmission and converter, put four quarts of fluid in initially. Start the engine and continue to add fluid until it shows on the bottom of the dipstick. Then continue to add fluid until the fluid level reaches the "add" area.
Shift through the gears. Come back to the position the maker calls for when checking the fluid level. Add fluid until the fluid level of the dipstick is within 1/4" of the full mark. The engine should be running all the time you're adding fluid.
That's it. Check for leaks, and you're done.