Filled For Life? The Truth About Automatic Transmissions

March 3rd, 2023

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“Oh, you don’t have to change the automatic transmission fluid,” he said.  “It’s filled for its lifetime.” He was a university professor (in the field of biomechanics) so I assumed he knew what he was talking about.  However, things didn’t go so smoothly when his automatic transmission tried to change gears later.  Turns out that biomechanics experts aren’t actual mechanics!

The idea of having a fluid in your car that doesn’t ever need to be replaced or topped up sounds great. We should be topping up everything else on a regular basis, from the radiator to the window washing fluid to the oil. Not having to do this for the transmission fluids sounds almost too good to be true.

And you know what they say about things that sound too good to be true…

The trap that a lot of us can easily fall into is the whole idea of the “lifetime” automatic transmission fluid.  What does a lifetime actually mean?  Does it mean forever until the end of the universe (short answer: no)? Does it mean for the rest of your lifetime?  The car’s lifetime? Or something else?

It turns out that the lifetime in question is the planned lifetime of the car. This is not the same as the actual lifetime of the car.  Car manufacturers, who are always coming out with nice new models want you to buy those nice new models. No harm in that and if you want a new car, why shouldn’t you get one? However, some manufacturers have a sneaky way to push you into buying a new car possibly sooner than you want to, known as “built-in obsolescence”. This means that the car manufacturers expect that a vehicle will wear out – and need to be replaced – at some point.  You can get an idea of what the expected lifetime of a car is by looking at the warranty, which is either going to be the age of the car or the mileage. After the car has clocked up that many kms or that many years, you and the manufacturer can expect things to start showing a few signs of wear.

Don’t blame the car manufacturers too much for this. There is nothing that can be done about the law of entropy, and it’s in the nature of things to break down and wear out over time.  You can see evidence of this fact when you look in the mirror (and the biochemist could tell you more about that in great detail). I know my face doesn’t look as smart and new as it did 15 or so years ago.

However, some of us like our cars and we’re rather fond of them. We don’t want to move on something as comfortable and familiar as an old friend. We would like to keep them for longer, thank you. There are those of us who are into classic cars, and there are those who, despite some of the great offers out there, are more likely to get a second-hand car that has passed the threshold of the magical number in the warranty. What happens then?

The fact is that you’re going to have to do something about the fluids in your automatic transmission. That fluid gets old and deteriorates over time, and when it does, then it won’t work as well as it used to… and neither will your automatic transmission. To keep the gears changing the way they should, then it’s time to do something about the fluid.

I don’t want to hear that line again about not changing the automatic transmission fluid because it’s got a lifetime guarantee. Remember those cheap watches that had a “lifetime guarantee”? That “guarantee” that meant that the watch wouldn’t break until it came to the end of its life, and you knew when it came to the end of its life because it would break, i.e., it was guaranteed not to break until it broke.  The same thing applies here. The lifetime is the lifetime of the fluid, so it won’t need to be replaced until it comes to the end of its lifetime – which is shorter than the lifetime of the car, to say nothing of yours.

So what comes next? To flush or not to flush the automatic transmission, that is the question.

There’s a bit of debate about whether one should flush an automatic transmission, or whether it’s better to simply change the fluids. If you’re new to the world of motoring – which we all are at some point – then let’s start by describing the difference between them.

• Changing the fluids means that the old fluids are simply drained out without the help of any special equipment, new fluids are put in, then the filter is replaced and there you go. You can do this yourself. Some of the old fluid will still be left inside the system and will mix with the new fluid.
• Flushing the automatic transmission involves the use of a pump that will ensure that every single bit of the old fluid, along with any junk and debris that’s got caught in it, gets removed from the system and completely new fluids will be added.

You can get a rough idea of this by picturing a bottle of hand sanitiser or similar goop. Changing the fluids is like squeezing or pouring out the contents and putting fresh stuff in (try it; you’ll see bits of the old stuff stuck on the sides). A full flush is like giving it a full scrub out under the tap before putting new stuff in.

Some say that you should only change the fluid, as the forces involved in flushing can move that inevitable debris from the corners and get it into the working parts, which can damage the transmission. This argument has some weight to it, and the risk is real. However, a mechanic who knows what she/he is doing will have the right equipment and will be able to do it properly. Because this can be a bit pricey and it’s something of a deep clean for your car’s transmission, it shouldn’t be done that often. Have a look at the fluid in question. If its still a nice translucent scarlet, a flush isn’t needed. If it’s thick and black, then it’s flushing time.

Merely changing the fluids can be done more frequently – about every 2 years or thereabouts for your average driver. It will need to be done more often (as will a full flush) if you put your automatic gearbox through a workout on a regular basis – lots of towing or lots of very short trips being the main ways to stress the auto gearbox.

How often should you change the fluids in your automatic transmission or get it flushed?  The answer will vary. However, one thing’s for certain: the gearbox is not filled for life and when it comes to automotive maintenance, listen to the person in the blue overalls with black smears, not the person in a white lab coat.