Vehicles' vital fluids normally perform two important functions: lubricating and cleaning. As the fluid circulates through the parts, it gathers the dirt and metal shavings that can accumulate over time. If you're lucky, this debris will settle on the bottom of the pan or housing and not circulate through the system.
Nothing prolongs vehicle life more than regular fluid changes. In automatic transmissions/transaxles, the recommended interval is about every 30,000 miles or 30 months. (Check your owner's manual or service manual for your car's specifics.) The automatic transmission fluid (ATF) should be changed sooner if its dipstick reveals dark or burnt-smelling fluid.
Even those of us who change our own oil often cringe at the prospect of draining ATF. Because many transmission pans don't have drain plugs, changing the fluid can be a messy proposition—the entire pan must be removed. But even on vehicles that do have drain plugs, the pan still must be removed to change the filter.
Changing ATF is one of those messy jobs that someone has to do. Doing the deed yourself will save money and possibly time. Just as Keith Richards allegedly gets his blood changed in Switzerland at regular intervals, fresh ATF can make your gearbox perform young beyond its years.
Fluid drains better at operating temperature. Raise and secure the vehicle, then lay down a tarp, cardboard or a newspaper under at least a 2-gallon catch pan. Next, remove the bolts from one side of the transmission pan, being cautious of hot exhaust parts and fluid.
Gradually loosen the other bolts, which should allow the pan to tilt and begin to drain. Once all bolts are removed, lower the pan and dump the remaining fluid into the drain pan. Gently break the gasket seal with a screwdriver if necessary.
Clean the gasket surfaces on both the pan and the transmission housing. Inspect the pan for metal shavings or other signs of internal damage, and then clean it with solvent.
Remove the old filter and O-ring. The filter contains fluid, so keep the drain pan underneath.
Install the new filter, making sure that its O-ring seats in the appropriate orifice.
Attach the new gasket to the pan with oil-soluble grease -- not gasket sealer or adhesive.
Refer to the service manual about using thread sealer on any or all of the trans-pan bolts then screw in all fasteners finger-tight.
Torque the pan bolts to spec in a spiral pattern starting at the center. Maximum torque is often about 12 lb.-ft.
Lower the vehicle and fill the transmission with the recommended amount of fluid.
Start the vehicle, warm it up, then shut it off and check for leaks. If leak-free, run the vehicle up to operating temperature on level ground, move the shifter through all gears, return to Park, and check the dipstick while the engine idles.
I hope this answers the question, How do I change my own transmission fluid?