Transmission fluid is a vital part of your car’s gearing system. Without it, the whole thing would seize up and break within a few minutes.
Underfilling your transmission is obviously bad, but what about overfilling it? Is adding too much transmission fluid terrible for your car?
In a word, yes.
I’ll explain why and how to remedy the situation in this guide.
- Transmission fluid should be between the relevant notches on the dipstick.
- The main symptoms of too much transmission fluid are high dipstick levels and foamy liquid. The warning light might also illuminate.
- An overfilled transmission means the unit will get too hot and eventually break.
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What Is Transmission Fluid?
Transmission fluid is, you won’t be surprised to know, the fluid that goes in your transmission. It lubricates and protects the internal components.
It also powers the torque converter, allowing you to slow down and stop without a traditional clutch.
We’re talking about automatics here, hence the common term ATF – automatic transmission fluid.
Manuals (‘standards’ or ‘stick-shifts’) need a gearbox oil (or ‘gear oil’).
Transmission fluid (automatics) and gearbox oil (manuals) do essentially the same thing. However, each is appropriate for its relevant component.
For example, ATF has a low SAE viscosity rating to ensure the transmission efficiently transfers power to the wheels. It has plenty of additives to increase its life expectancy and reduce foaming (etc.).
In contrast, manual gearbox oil is much thicker (‘more viscous’). This cushions the gears when they come together and preserves the components under high temperatures and pressures.
On this page, I’ll focus on automatic transmissions. That said, similar principles apply to manuals – you’re just extremely unlikely to ever top up their oil yourself.
How Much Transmission Fluid Do You Need?
The answer to this question can be found in your owner’s manual and by using the ATF dipstick.
There is no one-size-fits-all.
Transmissions are also designed, manufactured, assembled, and engineered into the vehicle by each manufacturer. While the general principles are the same, the specifications vary from car to car.
Unless you completely drain the transmission, you shouldn’t ever need to top it up from empty.
You will, however, need to check your ATF level and keep it topped up. Use the correct type for your car! Again, you’ll find this in the owner’s manual.
- Read your owner’s manual for specific information about your vehicle. In general, you should drive your car for a few minutes to warm the engine and lubricate the gears. When checking the transmission fluid level, you often need to leave the engine idling in Park.
- Locate the relevant dipstick (ensuring you aren’t looking at the motor oil one).
- Pull it out from its seating and use a rag or paper towel to clean off the end.
- Push the dipstick back in and draw it out again.
- Read the level.
It should be between the two notches (‘MIN’ and ‘MAX’).
If the level rests below the ‘MIN’ mark, top it up.
If the wet ATF mark sits above the ‘MAX’ line, there’s too much transmission fluid in the system. This can only happen if too much was added at the last top-up.
Don’t forget to replace the dipstick.
What Happens If You Put Too Much Transmission Fluid In?
Too much transmission fluid? It might not seem like a big deal, but your car is suffering.
Here’s a basic overview of what’s happening.
An overfilled transmission results in excess pressure within the housing.
You may notice bubbling or foaming within the ATF if too much has been added. (These could also be signs of old fluid.)
This chemical breakdown means the transmission fluid isn’t cooling the components as effectively as it should. In other words, your transmission is overheating, subject to more strain, and more likely to break.
A failing automatic transmission is something that should make everyone wince. The financial implications are rarely minor and often significant.
Dangers: Can You Drive With Overfilled Transmission Fluid?
Every time the transmission operates, extra strain is put on it.
I’d say it’s best to cut out all risk and avoid driving until your ATF is at an acceptable level.
Unless you’ve massively overfilled the transmission, the car shouldn’t immediately seize up. But it will begin wearing down the internal components straight away.
If you decide to drive to a garage – and, again, that’s not my recommendation – go slowly, keeping the engine revs down as much as possible. This ensures there’s minimal strain on the gears and clutches.
If you need to go somewhere in a hurry (to the hardware store for tools?), ask to borrow someone else’s car. Public transport may also be an option where you live.
How Much Transmission Fluid Is Too Much?
When you miss the train, it doesn’t matter how late you are. Whether it’s a second or an hour, you’ve missed it, and that’s it.
The same principle applies here. If the ATF is higher than the mark on the dipstick, there’s too much, and you need to drain some.
It’s as simple as that.
In the end, it’s up to you. You could take the risk – but why would you? What if you’re the unlucky one, and your transmission wears down? It’ll cost you much more in the future!
Overfilling the transmission means you need to remove some. I’ll cover that in a little more detail below.
How Do You Remove Excess Transmission Fluid?
Most cars have a transmission drain plug, just like the one for the motor oil.
If you know what you’re doing, use a drip pan and the relevant drive or socket to drop some fluid. Of course, you’ll also need to safely jack the vehicle up and wear the appropriate PPE.
An alternative is to use a syringe-style extraction hose. You’ll see these listed online and stocked in certain hardware stores.
These can be useful because they often have measuring gauges etched onto the syringe. As a result, you can see how much fluid you’re removing at a time.
Only use these techniques if you’re comfortable. Otherwise, engage the services of a mechanically-minded friend or family member to help.
These are by far the cheapest way to remove excess transmission fluid.
The other, more expensive (but more thorough and careful) option is to go to a mechanic. The flat-bed truck towing fee will be costly enough. You’d then pay an extra cost (determined by the shop) to drain the fluid.
You could also opt for an entire system flush if you prefer. $250 is a reasonable average estimate for this service.
Whatever you do, don’t drive with too much (or too little!) transmission fluid! Always keep yourself and others safe on the road.