There are many tasks on a vehicle that are relatively easy to undertake, and for some, getting out the tools and having a go yourself is preferable to paying a small fortune to your local mechanic. Sometimes this is a very misguided notion—it’s very easy to create problems on a vehicle when you’re not sure what you’re doing—but for others, it’s a great way to save money.

Typically the kind of jobs the home enthusiast take on are minor in scope. Most people would be fine with the notion of checking their engine oil, but very few people would have the inclination, knowhow, and tools to swap an engine.

Automatic transmissions are a bit tricky, however. Many professional mechanics will shy away from touching these complicated beasts, such is the ease with which they can develop problems if not properly handled.

Surely servicing one isn’t a big deal, right? Let’s look at what’s involved.

A 4L60 Transmission in the Vehicle
Credit: BA Transmissions LTD

Servicing your automatic transmission will mean getting under your vehicle, like with this 4L60 Camero gearbox.

What You'll Need

The first thing you’re going to need is a way to get under your vehicle. You’ll need something to put your old transmission fluid in—and a responsible way of disposing of it—as well as the new fluid to replace it with. The type and amount of fluid is critical. Automatic transmission fluid is not simply a lubricant, it is used as a hydraulic agent to power the transmission. This means that fluid with the wrong properties, or insufficient fluid, could very well damage your transmission.

Dropping the old fluid can be tricky; many American vehicles don’t have drain bungs, leaving you to unbolt the entire sump and try your best not to get covered in oil as it all gushes out. In the case of transmissions with a dipstick for checking the level, it is possible to pump the oil out through the tube, though the tools needed to do this aren’t exactly common household items.

If you plan to change the filter, do some research on your specific transmission beforehand. Every automatic transmission has a different filter. Some are very easy to change, such as the six speed DQ250 (DSG) transmission, which simply screws onto the top of the gearbox. Others, such as the Audi 01J transmission, have their filter deep in the guts of the gearbox, and can only be removed by dismantling the whole thing!

Finally, you’ll need a way of getting the fluid into your transmission. For gearboxes with a dipstick, a simple jug and funnel will suffice. Pour the oil down the dipstick tube (making sure you get the transmission tube and not the engine!). For transmissions without a dipstick tube, however, you’ll need to pump the oil in from underneath.

Can Changing your Transmission Fluid Cause Damage?

In this video, YouTube's ChrisFix looks into whether changing your transmission fluid can actually cause more problems.

What to do

Dropping the Oil

Firstly, as mentioned above, you’re going to need to get the old oil out of your transmission. The easiest way is undoing a drain bung and letting the oil drop into a container. If there is no drain bung, however, you're going to have to remove the entire sump and do your best not to make a mess. If you do have to remove the sump, make sure you have a large container to catch the oil, such as a wide tray. Pumping the oil out will require a suction pump with a long, thin tube. And, as stated above, is only an option on transmissions with a dipstick tube.

One final thing to note on this is that some transmissions have a “levelling tube” in the drain hole (more on that in a moment), which will need removing as well to properly drain the oil.

Changing the Filter

This will vary depending on your transmission, and there are so many different types that I couldn’t possibly list them all here. The most common type of easily changeable automatic transmission filter would require you to remove the sump (which you might have had to do anyway) and the filter will be just inside. Sometimes it’s a push fit, sometimes it will need a few fasteners undoing. More fluid will pour out of the filter hole when it’s removed so look out for that. Once that’s done, take your new filter and repeat the process in reverse.

If, for some reason, you end up with the wrong filter for your transmission DO NOT try and make it fit. This is a critical component in your transmission, and any imperfections in the seal between the filter and the transmission will cause lots of expensive damage to your gearbox.

Filling her up!

Here’s the crucial part. You need to make sure you get enough oil back into your transmission or you run a very real risk of damaging it. Over-filling isn’t as serious; it tends to cause oil to spit out of various orifices when driving., which will make a mess, but it shouldn’t damage the box. To find out how much oil you need you can look at technical data. Such information will often list two amounts; “Dry Fill” and “Drain and Fill”, or something along those lines. You can ignore the dry fill amount (which will be the larger of the two). This is the amount of oil you need if you have completely drained the system, something you can’t do without taking the transmission out.

Now, that being said, it’s much better to level your transmission properly, as capacities can vary depending on things like how much oil you managed to drain, and any custom modification on the vehicle, such as an upgraded transmission cooler. In order to properly level your fluid, you’ll need to find out how your transmission accommodates checking this. Dipsticks are pretty self explanatory, although some transmissions have a dipstick tube but no dipstick. In those cases, you’ll need to buy or borrow one. Mercedes transmissions are bad for this trick, but the dipstick can be easily found on sites like eBay.

For transmissions with no dipstick tube, things are a bit messier. They are levelled by putting the oil in until it reaches a certain level, where it will begin pouring back out. Sometimes this is a hole on the side of the box (you’ll need to undo that bung as well) or in a raised section of the sump. More awkwardly, it is sometimes in the drain bung itself. A small tube is wound into the drain hole, preventing the oil from pouring back out until it reaches the top of the tube.

Now, the bit that many people stumble on. You need to fill your transmission up until it’s level, and then start the engine and do it again. When running, the transmission fluid is pumped around the system and the effective level of fluid in the box drops by nearly half! If you don’t finish levelling the transmission with the engine running, you will fall well short of the necessary amount of oil.

So there you have it. As someone who repairs automatic transmissions for a living, I would still recommend you leave this kind of job to the professionals, but if you’re going to do it yourself, hopefully you’re well enough informed to do it right!