If you have ever looked through your car's owners manual, it probably says that the car is maintenance free in a lot of areas. In fact, newer cars may even advertise you never have to change things like the automatic transmission fluid. I would not suggest you follow that advice. Engines create a lot internal heat because of the friction of the churning of the engine inside the torque converters, clutch plates, gears and bearings. This friction, or grinding, is what accumulates tiny bits of debris that eventually leads you back to the car lot to buy another automobile.

Most mechanics recommend you the change the fluid and filter every 30,000 miles for older vehicles.  Newer cars that run on Dexron III ATF are supposed to be able to go 100,000 miles ATF Dipstick Manual ChangeCredit: Opensourcebefore you need to flush and replace the fluid.

If you are interested in flushing all of the old automatic transmission fluid out of your engine, the following process explains how it can be done in your garage, even by a novice mechanic, and save you money.

I should note that the following procedure was performed on a 2000 Nissan Maxima, but the procedure applies to all vehicles. I suggest you look online for a forum dedicated to your type of vehicle if you have any questions.

How to Flush an Automatic Transmission
Credit: mjpyro

Before You Begin

The first thing you need to do is go and get all of the materials you will need for the transmission fluid change because once you get your car jacked up and start emptying it out, there is no stopping and going back to get what you forgot. Well, unless you have another car.

First make a list of what you need. I recommend starting out with 10 quarts of AT fluid. My preference is Mobil 1 Synthetic, but you will also want to buy some regular type of fluid, meaning less expensive, because you will be using it to flush out the old fluid, then replace, then flush again, then replace.

So rather than using the expensive synthetic fluid to flush the old out, use the less expensive fluid first, and end the process with the Mobil 1 Synthetic Automatic Transmission Fluid which is the fluid that will remain in the engine.

Now You Are Ready to Begin

Changing ATF fluidCredit: OpensourceThe whole procedure can be done in 30 minutes or less but if it is your first time doing something like this, take your time.

Jack up either the front or drivers side of the car and put jack stands underneath the car.

Remove the drivers side engine undercover that protects the bottom of the engine. Be careful not to break the grommets that hold the cover in place.

Remove the drain plug from transmission and drain fluid into a proper container. This will drain approximately 4 quarts.

Insert the drain plug back into the pan and tighten.
Pour the same amount (approximately 4 quarts) of new transmission fluid into the transmission. The transmission should now be back to stock level.

Disconnect the fluid supply line at the bottom of radiator. This is the bottom line connected to the transmission. Be very careful removing this line, it’s very easy to break the hose nipple on the radiator. 

Note: If the transmission fluid comes out of the bottom of the radiator instead of the hose you are holding you have the wrong line. Hook that one back to the radiator and remove the other one.

Put the end of the hose into a clean, clear or white plastic 1 gal container. While you are holding the hose get someone to start the car until the transmission pumps out approximately 2 qts  and the gallon container is half full. This should take 15-20 seconds.

Flush 2 quarts at a time to prevent draining the transmission. Remember, you are only taking out 2 quarts at a time from a system that holds 10 quarts.Refill ATF fluidCredit: Opensource

Refill with 2 fresh quarts of fluid after taking out 2 quarts.  By only removing 2 quarts at a time, it allows the transmission to remain filled with enough fluid to prevent damage and also have enough new fluid in it to "push" the old fluid thru the converter and out the return line as you go.

Repeat the previous steps until fluid changes color. Since you already put 4 qts in initially and the transmission holds 9 qts we are talking about approx 5 qts.  If you buy 10 quarts and repeat the process 2 to 4 time, it will ‘push’ out all of the old fluid.  If your fluid is in fairly good condition the color change will be very small and may not be noticeable, so stop at 10 qts.
Remember to use the Mobil 1 Synthetic last when you are finished flushing it through the hose.
Reinstall the hose to the radiator.

Replace the engine cover, and lower the car.

Check the transmission fluid level by removing the dipstick to make sure you have the correct amount in the system.

Summarizing the Process

What you have basically done is removed all the fluid from the pan and the internal pump and system and replaced all fluid internal to the transmission without shocking your transmission  like the force flush machines do at the automotive shops.

This is much simpler than it sounds. I personally use clean, empty water  jugs and compare the fresh fluid level in one jug with the level of old fluid in the other jug so that I could replace the old fluid with the exact amount I took out. It is ok to eyeball the 2 quart level. Just refill the same amount, then repeat the drain process, then add another 2 quarts. The entire process should take about 30 minutes, less if you have a helper.

Flush an Automatic Transmission