How to Service an Automatic Transmission Yourself

Servicing an automatic transmission is a part of vehicle maintenance that often gets forgotten—or worse, intentionally avoided. As with many things related to automatic gearboxes, servicing is not the relatively simple task that servicing an engine is, and it can scare a lot of people (including mechanics), especially in European countries where automatics are less prevalent.

Knowing how often to service the fluid is part of the battle, as information on what to do is often hard to find in the standard vehicle documentation. This matter is further confused by certain manufacturers such as BMW placing optimistic “Lifetime Oil” stickers on their transmissions and insisting you should leave it alone. The truth of the “lifetime oil” probably lies in the fact that the transmission itself will fail before the oil degrades to the point where it causes a failure, but regular servicing can help prevent automatic transmission faults from occurring, and will likely extend the life of your transmission.

As a rule of thumb, however, you should service your transmission every 30,000 miles.

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Doing It Yourself

Before we get into the details of how to service your automatic transmission, be aware that I am referring to a fully automatic transmission (see this article on transmission types for the differences). Also, please understand that there are many, many models of transmission, and many variants of those models. I can’t provide exact instructions for every transmission in this lone hub, but hopefully you’ll have enough to go on from the information below.

How to Service an Automatic Transmission Yourself

Things You Might Need

  • Spanners/Allen Keys
  • Funnel
  • Oil Pump
  • A Tray for Catching Oil
  • Transmission Fluid

Determine Your Transmission's Levelling System

The first thing you need to do is establish what kind of levelling system your transmission uses. There are three main ways in which automatic transmissions can be levelled with the correct amount of oil.

  • Dipstick

  • Levelling Bung

  • Levelling Tube

The easiest of these (and unfortunately the least common) is the dipstick. If your transmission has a dipstick then you level the transmission in the same way you level your engine oil; by dipping the transmission and adding more fluid until the level is correct.

If your transmission uses a level bung, you will need a way of pumping the fluid into the transmission from underneath, and you will also need something to catch the oil.

The level bung will be somewhere on the transmission at a higher point that the drain bung, and the levelling process involves pumping transmission fluid into the box until it starts coming out of the levelling bung.

A level bung will be higher than the drain bung, often facing out rather than down.
A level bung will be higher than the drain bung, often facing out rather than down. | Source

DON'T just guess which bit to undo when removing a sump or level bung. Some transmission have components fixed in place from the outside, and undoing these by accident will likely result in the transmission needing to be removed and dismantled.

A levelling tube often sits in the hole through which the transmission should be drained, and must be removed in order to drain the oil.
A levelling tube often sits in the hole through which the transmission should be drained, and must be removed in order to drain the oil. | Source

Finally, we have the levelling tube. The levelling tube works in the same manner as the levelling bung in that you pump fluid into the transmission until it starts to come back out. The problem is that the levelling tube is often inside the drain bung, meaning you have to check whether there is enough fluid using the same hole you are filling it through. Needless to say, this is the most difficult (and messiest) way of levelling an automatic transmission.

Draining Your Automatic Transmission

Understand that, in a typical transmission oil service, you will only be changing about two thirds of the oil in your transmission. Gravity can only get so much of the fluid inside, the rest remains in places like the transmission cooler, the torque converter, and various places inside the transmission itself. You can have your transmission “power flushed” to be sure you’re getting a complete oil change, but that requires specialist equipment and is not something you’re likely to be tackling yourself (unless you own a garage!).

With that in mind, you will need to establish which is your drain bung, find something to catch your old transmission fluid (and have a responsible way of disposing of it), and the means to get under your vehicle.

If your transmission employs a dipstick or a drain bung, simply place your waste oil receptacle so that it will catch the oil and undo the bung. If your transmission uses a levelling tube, you will need to undo the levelling bung first, then wind out the tube as well.

Some transmissions (mostly in American vehicles) have no drain bung. Unfortunately, with these transmissions, you'll just have remove the sump and do your best to catch all of the oil.

Leave your transmission draining until the oil coming out slows to a trickle. You can leave it longer if you wish, but be aware that it will continue to trickle out for hours if you let it. If you really want to be thorough, you could leave it overnight to get as much oil out as you can, but given that the best you can hope for is still around two thirds of the total transmission fluid, it’s probably not worth it leaving for too long.

There are many specifications of transmission fluid. Make sure you have the right one for your gearbox.
There are many specifications of transmission fluid. Make sure you have the right one for your gearbox. | Source

Levelling Your Transmission Fluid

Once you have drained the oil to your satisfaction, refit the sump bung (replace it if you want to be really thorough), unless your transmission uses a levelling tube, in which case just replace the tube.

Begin refilling your transmission with fresh fluid, ensuring the oil you’re using is the correct specification for your particular transmission. If you’re filling from underneath, pump the oil in while watching for the oil coming back out. If, in the course of filling the transmission, you are obstructing the levelling hole/tube, pause every so often to check that the oil is not trying to come back out. If your transmission has a dipstick, simply pour the oil through the dipstick using a funnel.

Once the oil is levelled… you’re nearly done.

You see, when an automatic transmission is running, it pumps the transmission fluid around the transmission system, including the cooler and torque converter. This is relevant to you because when the transmission is running, the oil level drops significantly as the oil is taken up into the system. It’s the oil level while running that you need to work with.

So. Once the oil is levelled, make sure your vehicle is in park, fire up the engine, and then do it again. This time, once the oil is levelled, you’re done. You can put everything back together and clean up the mess you inevitably made.

If possible, you should drive the vehicle, being sure to get all the gears, and then check the level again to be sure. However, as long as the level was not sitting on the minimum when checked, it should be fine.

The specification of fluid varies from transmission to transmission. Do some research and make sure you're using the right fluid for your transmission.

Other Things You Should Know

Stay on the Level

It seems obvious, but it’s worth pointing out that in order to get an accurate reading of the oil level, your vehicle needs to be as close to horizontal as possible. This means no sloped driveways, and no jacking the front end of the car up.


Changing an automatic transmission filter will, more often than not, involve removing the sump entirely, so if you’re not comfortable exposing the innards of your transmission, you may want to leave the filter alone (or take it to a transmission specialist). Of course, this only applies to transmissions where the filter is accessible without taking the whole gearbox apart!


There is an optimum temperature for the transmission fluid you are putting into your transmission. It varies from model to model but is usually around 40°C. Generally you’ll be fine filling your transmission at lower temperatures, just be sure not to overfill.

Transmission dipsticks will often have "hot" and "cold" markers. You'll have to use your judgement as to which end of the spectrum your oil is at.

No Dipstick?

Some transmissions with dipstick tubes do not actually have dipsticks inside them (Mercedes-Benz are guilty of this). In these cases, you'll need to buy (or borrow) a compatible dipstick to check your level.

How Much Oil?

It is possible to look up how much transmission fluid your gearbox should take using services such as Haynes, or Autodata. Do not do this. I can't stress enough how much variation you can get in automatic transmissions. Perhaps you didn't drain as much as you thought. Maybe the previous owner fit a larger cooler, increasing the oil capacity. Always level your transmission properly, don't trust technical data capacity figures.

Filling Through the Dipstick Tube

While filling through a dipstick tube is undoubtedly the easiest way to get oil into your transmission, it can be a little awkward to check the level. The oil tends to cling to the insides of the tube, causing the first few dips to be wildly inaccurate. I find it best to let the oil settle for a few minutes, and then dip it a few times until the level is consistent. If one side of the dipstick shows a different level to the other, always go with the lower side.

© 2016 John Bullock

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