Transmission fluid is the lifeblood of your automatic transmission. Without it, everything goes wrong.
In brief, you must always drive with a safe level of transmission fluid. Too much or too little – both are dangerous.
This guide examines what you should expect to see if you overfill it – even by a little bit.
- Check your transmission fluid regularly using the dipstick.
- Watch for foamy transmission fluid, warning lights, and gear-related noises or jerks.
- Drain some transmission fluid if you’ve added too much.
- Don’t drive if you’ve overfilled your transmission fluid.
- Key takeaway #5
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What Is Transmission Fluid?
Transmission fluid lubricates and cools your automatic transmission. It’s an entirely separate thing from the gearbox oil used in standard manuals.
Automatic transmission fluid – which I’ll occasionally refer to as ATF – is also used in the torque converter. It’s a complicated system composed of an impeller, a propeller, and a stator. This allows you to stop without stalling the engine.
Traditional transmission fluid was, unsurprisingly, refined from crude oil. These days, they’re much more likely to be synthetic.
As might be becoming clear, transmission fluid is vital. Overfilling it could lead to severe problems.
How Do I Know If I’ve Overfilled My Transmission?
The most clear-cut sign of overfilling your transmission with ATF is the dipstick. You should use this while filling up to ensure you never exceed the limit.
The dipstick might show your transmission fluid levels are higher than the MAX mark. That means – at the risk of sounding pretentious – there’s too much in it!
Driving on an overfilled transmission could be dangerous. The internal components and torque converter feel excess strain and are increasingly likely to fail.
You’ll likely notice several other symptoms along with the high ATF level on the dipstick.
Symptoms Of Overfilled Transmissions Fluid
Here are some of the most apparent signs of overfilled transmission fluid.
High Transmission Fluid Level
I’ve mentioned this one before. It’s incredibly obvious but worth listing here anyway.
You’ll see it on the dipstick if you have a high transmission fluid level.
Of course, you need to check this manually, following the instructions in your owner’s manual.
Transmission Warning Light/Notice
Most modern cars will warn you when there’s a problem with the transmission. Sensors often pick up on issues long before we realize they’re present.
You might notice a transmission warning light or a notification on your screen. If it’s about the transmission and you’ve added too much ATF, chances are they’re related symptoms.
Heavy Gear Changes
When you’re in Drive and the car changes gear, you might feel more of a significant *thunk* than usual. If this happens on repeat, you can be sure there’s a developing transmission problem.
In reality, this could stem from any internal component (or a mount), but the transmission fluid is a common culprit.
In short, the ATF isn’t lubricating the gears. This lack of cushioning means they ‘clunk’ into position instead of smoothly gliding.
Rough Revving (Especially Acceleration And Idle)
The above video shows how a torque converter works. It’s particularly relevant for this brief section.
A rough idle and poor power while accelerating could be symptoms of almost anything. This includes high levels of transmission fluid.
Too much transmission fluid might damage the torque converter or affect its flow.
It’ll also otherwise stop the gears from meshing efficiently. This results in that feeling of the car struggling.
Your transmission could start to overheat, even though you have too much fluid as opposed to not enough.
Because there’s too much ATF in your transmission, it isn’t circulating as it should. Higher pressures due to more volumetric space occupation also mean higher temperatures.
The fluid breaks down and starts to lose its cooling properties.
These combine to result in an overheating transmission. This is often displayed via a dedicated warning light or message.
Sounds From The Transmission
Transmissions aren’t quiet by any stretch of the imagination. But they shouldn’t be clunking, excessively vibrating, screeching, or juddering.
Any unusual sounds coming from your engine bay could be caused by the transmission. In turn, the root issue could be overfilled transmission fluid.
Car Struggles When Slowing Down (In Gear)
The torque converter is also responsible for slowing down. Without it, the transmission would be fighting you as you brake – a dangerous situation!
Torque converters utilize the transmission fluid. Problems with it can be caused by high ATF levels.
Foamy Or Bubbly Transmission Fluid
The transmission fluid should still be vibrant and clear. Bubbles or foam means the liquid is being churned up by the internal components too much.
This happens when there’s too much of it.
You’ll also notice it when the transmission fluid is old. Either way, you need a system flush!
This is the worst-case scenario and should only happen if:
- The transmission has been massively overfilled
- You drive your car for an extended period with overfilled transmission fluid.
The torque converter is the most likely part to go. It’s handling the brunt of the pressure applied by excess fluid.
That said, any mechanical component could break. They’re all lubricated by the fluid and thus aren’t being protected or appropriately cooled.
Is It Bad To Overfill Transmission Fluid?
You mustn’t overfill transmission fluid, the same way you mustn’t overfill motor oil.
Anything above the MAX level on the dipstick is too much.
If you put too much transmission fluid in, you should remove some. Only drive once you’re at a level safe for your car.
What Should You Do If You’ve Overfilled Transmission Fluid?
I wouldn’t advise driving anywhere because it’ll damage your transmission. Yes, you might get away with a quick trip to the mechanic, but I don’t condone risking it.
Overfilling your transmission can be easily remedied at home. You might need help from a mechanically-minded friend, though.
Here are two methods you could try if you know what you’re doing.
1. Transmission Drain Plug
Almost all modern cars have a transmission drain plug.
This works in exactly the same way as an engine oil drain plug. It’s loosened (or removed entirely) to allow the transmission fluid out.
Only do this if it’s safe to do so and you know what you’re doing.
Using axle stands, a drain pan, and an appropriate tool, the drain plug can be loosened. In some cases, it’s easy to only let a small amount out. Sometimes, this is more difficult, and it might make more sense to flush the whole lot and refill it.
Remember to reinstall the plug and never start your car until you’re out from underneath it and it’s lowered off the axle stands.
You don’t need to remove the bottom half of the transmission (like some blogs say) unless it’s a sealed unit. But that’s not a DIY job; that’s one to leave to the professionals.
2. Extraction Tool
The alternative is getting yourself an extraction hose.
Poke the end down the transmission dipstick housing. Next, use the syringe on the opposite end to extract a certain amount of transmission fluid.
Now, recheck the levels. Repeat this process (and add more if necessary) until the ATF is at an acceptable level.
3. Call-Out Mechanic
If it comes to it, use a call-out mechanic. This is undoubtedly the most expensive option, but it’s the best way forward if you’re really out of your depth.
You shouldn’t drive if you have too much transmission fluid, so a call-out mechanic is the next best thing.
They’ll most likely perform one of the two techniques mentioned above. You’ll be on your way again in no time.
For peace of mind, this is the option for you, and thus the one I officially recommend.
So, to sum up – your transmission fluid must be at a safe level. If it’s too high (or too low), you’ll find yourself in a costly or even perilous situation.
It’ll save you a very expensive transmission rebuild or replacement.
Keep an eye on your transmission fluid, and check it as part of monthly maintenance. That way, you can be sure you’re doing your bit to look after your vehicle.
Stay safe out there!