How to Change Transmission Fluid

Adding New Transmission Fluid

Curtis Blackburn, a blind mechanic at Moonlight Automotive in Greenwood, Ind., switches sockets while working on a truck’s transmission on March 24, 2004. His lack of vision doesn’t hinder his ability to diagnose and fix car problems.
AP Photo/Daily Journal, Andy Costello

Before you add new transmission fluid, you should clean the pan. Use a rag to wipe away the residue from the inside of the pan. Don't be alarmed if you find metal debris -- even healthy transmissions will leave small amounts of metal in the pan. Excessive metal debris, however, could indicate a more serious issue. If your pan has a particle magnet, which keeps metal away from the moving parts of the transmission, you should clean that, as well.

Next, remove any traces of the gasket from the edges of the pan. You should be able to peel away most of the gasket by hand, but some sticky residue will remain. Use a knife or flat-head screwdriver to scrape off this residue, being careful not to scratch the surface of the pan. Brake cleaner will remove any stubborn remnants of gasket material. You can also apply brake cleaner to the pan bolts.


While the pan and pan bolts dry, install the new filter. Make sure it's mounted in the same position and that the O-ring is properly positioned. Reattach any bolts or clips that secure the filter to the transmission. Then you can reinstall the pan, using the gasket -- either rubber or cork -- recommended by the manufacturer. Many rubber gaskets are self-sealing, but some rubber gaskets and most cork gaskets are not. If your gasket is of the latter variety, apply fastening cement before you install the gasket. Start every pan bolt by hand for two turns before tightening. You don't want to overtighten the bolts, as this will flatten the gasket and cause leaking. If no recommended torque is given, make the pan bolts "screwdriver tight."

Now you're finally ready to add new transmission fluid. Most vehicles allow you to do this through the dipstick tube, using a longneck funnel. Pour a little less than the amount of automatic transmission fluid (ATF) you collected in the milk jugs. Next, start the engine and let it run idle for at least one minute. With the parking and service brakes applied, move the gear selector through each position, ending in park. Recheck the fluid level and add fluid to bring the level to an eighth-inch below the add mark. Run the engine until it reaches normal operating temperature, and then recheck the fluid level -- it should be in the hot region.

A word of caution about adding ATF: Overfilling can be just as bad as having too little fluid. So go slowly, check the dipstick often, and in no time you'll be back on the road with a happy and healthy transmission.

Keep reading for more car maintenance tasks you might want to try.

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