ATF fluid service
Difficulty: 2/5
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Introduction 

Unlike the manual transmission, you can't just drain the fluid and add the suggested amount back.  The service manual requires you to first warm up the car so that there is fluid circulating through the ATF pumps and coolers.  Suggested change intervals are every 40,000-80,000 miles or as needed, depending on use and personal preference.  Unlike manual transmission, OEM fluids are highly recommended for automatic transmissions.  

If you go to the dealership, they may tell you that VW/Audis have lifetime fluid and is not serviceable.  What they mean is that when the fluid breaks down and damages the transmission, it ends the economically useful lifetime of your car, so come back and buy a new car.  Lifetime might also mean the lifetime of your car's warranty.  There is no such thing as fluid that has an infinite life.  In theory, lifetime fluids could last anywhere between 50,000 mi or 500,000 mi without issue.  The problem is that it gets contaminated with water, new car break-in debris, age related debris, faulty auto transmission filters, or leaks/seeps out.  Below is a picture from an Audi transmission pan with lifetime fill.  Something was wrong with the transmission and changing the fluid prevented further damage.  Those are metal shavings stuck to a magnet.  Changing the fluid saved this transmission, do you know what is going on in yours?

Also note that in manual transmission, the differential and transmission share gear oil.  In an automatic transmission, the differential and transmission are sealed off from each other and use different fluids!

• FOR TRANSMISSION ONLY: 5.3 liters/5.6 quarts total capacity (I would suggest getting a little more because some will spill) VW# g052 162 a2 (or a1) full synthetic ATF transmission fluid or 1 liter pentosion equivalent VW TL52162 spec ATF fluid
• FOR DIFFERENTIAL ONLY: 0.75 liters 75w90 VW# g052 045 a1 (or a2) synthetic gear oil
• Replacement fill plug  (01M 321 435 B and 01M 321 432 A)
• screwdriver  g052 162 a2
• optional but recommended depending on mileage: ATF filter VW# 01M-325-429

You may also have heard that if a high mileage car has never had the ATF changed then you should not change it because it will leak.  The reason why leaks could occur is that residue has built up on the gaskets and seals and is preventing the fluid from seeping out.  Adding new fluid that contains fresh detergents could clean the residue out and cause the gaskets to seep fluid.  On the other hand, not changing the fluid could also cause problems in the future.  Either way, you are looking at possible problems, so it's up to you.  The best thing is to regularly change the fluid.

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Procedure

First, start the car and warm it up until the transmission pan is medium warm.  The factory service manual has a specific procedure to determine the temperature, a VAG-COM is needed to do this.  If you don't have a VAG-COM, warm the car until the transmission reservoir pan is warm to the touch.  The specified test fluid temperature is 95^oF-113^oF.

Engage the parking brake, jack up the car using the factory jack points, rest car securely on jack stands, and make sure the car is safe and secure before doing anything else.  Note that the car MUST be level, otherwise the fluid level will not be accurate.  If you feel more comfortable with the car even more secure, use wheel blocks as an extra level of safety in addition to jacks and jack stands as suggested in 1000q: wood blocks.  Disclaimer - these are not a substitute for jack stands on the factory jack points.

Draining the fluid

Check the temperature of the transmission pan.  If it's not warm enough, run the engine in neutral or park until it is warm enough.  Once it is warm enough, shut off the car. The specified test fluid temperature is 95^oF-113^oF.

Remove the transmission plug and overflow pipe.  Make sure you are not removing the engine oil drain plug!  The engine oil drain plug will be attached to the bottom of the engine and the oil pan.  See 1000q: engine oil change to see what the oil drain plug looks like!

Let it drain out and remove the red drain hole insert.  This should have removed most of the fluid - some will still be in the torque converter, etc.  Replace the red drain hole insert and begin to replace the ATF.  Open the fill hole with a screwdriver.  It will probably be damaged when you remove it, so make sure you bought a replacement, part number is listed above.  Here is a cutaway drawing.  3 and 4 are the fill hole cap.  1 is the drain hole and 2 is the red drain hole insert.  

When you are replacing the fluid, first add at least 3 liters of fluid, then start the car.  Move the shifter through all the gears.  Leave the car running, then add fluid until it flows out of the level fill hole.  If you shut the car off, the fluid can come out of the coolers and pumps, and come pouring out the fill hole.  Make sure you clean off the fill plug, then seal the transmission before you shut off the engine.  

Refilling the automatic transmission fluid

There are a few ways to get the fluid back in.  You can use a gravity pump (siphon) or funnel, where the bottle is higher than the fill hole and connected with a hose.  This can be used in the VW TDI because there is adequate clearance but can't be used in many other cars.  This method with a funnel and hose is the best because it wastes the least fluid.  You can also use a hand pump designed to force fluid out of a container.  I sometimes use compressed air to force the fluid into the fill hole when there is no clearance for the funnel/hose method.

To use the compressed air method of adding fluid, take a compressed air tank and regulate the pressure down to a reasonable amount, just enough to get the fluid flowing.  If you don't, the bottle of fluid could pop, so start low and then increase the pressure if the fluid isn't moving fast enough.

Use a short length of hose, just longer than needed to go from the bottom corner of the bottle to the transmission fill hole.  Make sure the hose has at least 1/4 diameter, otherwise the fluid will have a very hard time flowing through it. 

Poke a hole slightly smaller than the hose in the cap (smaller is better for a tight air seal), and a small hole (again, smaller is better) in the top of the bottle, marked by the green arrow in the below picture.  Now stick the hose into the cap and put the other end into the fill hole.  Since a picture is worth a thousand words, here you go.

Now press the compressed air nozzle into the hole in the top of the bottle marked with the green arrow in the picture above.  Warning!  Do this step last because you may accidentally press the trigger on the compressed air nozzle before the hose is in the fill hole.  If this happens, your expensive fluid is now all over the ground.  Apply gradually increasing amounts of pressure until the fluid is gone.  If the bottle stretches a little that is okay.  If you think it's going to pop, stop applying pressure.  I put my gloved hands around the bottle cap and nozzle to prevent any fluid from hitting me in case the bottle cap blows off.  As always, wear protective safety goggles.  

Like this tip?  There are many more tips for the mechanic at 1000q: mechanics tips.

Regarding the fill hose, note that the fill hole end of the hose has a coat hanger bent into an s-shape (so it stays in place) with a hook at the end.  I did this so that I could use the same hose for both manual and automatic transmissions and to minimize dripping at the end of the hose.  I put a green arrow to show an appropriate location for the air nozzle hose and used a green line to show the hose inside the bottle.  If the air intake hole is below the fluid level, obviously the fluid will leak out.  

Here is an example of an ATF pan from an Audi.  See how the tip needs to be bent over the edge of the fill hole?  There is also a cap (not pictured) on top of the hole so you must use a 90^o bend tip on Audis.  The hook at the end of the hose lets you use this hose on Audis and many other models of cars as well.  This does not apply to the VW TDI since it uses a different style of fill port.  The hook also makes sure that fluid doesn't drip down the outside of the hose too.

Make sure that the hose goes all the way to the bottom of the bottle otherwise you will get just foam instead of fluid.  This is important!  Too much foam will fool you into thinking there is more fluid than there actually is.  If this happens, let it settle down and overfill it very slightly.  Also make sure the hose has a large enough inside diameter.  If you can't breathe through the hose, it's too thin.  I suggest about 1/4 inner diameter hose.

When you go to the next bottle, just move the cap to the next bottle and poke another air intake hole in the next bottle.  Don't let the hose touch the ground because it will get dirty.  If it gets dirty just thoroughly wipe it off.

After you're done, hang the hose with a paper towel or two on the end and let it drip dry.