What happens if you overfill an automatic transmission

Transmission fluid is not something that all drivers pay too much attention to, but it is just as important as engine oil when it comes to keeping your car running smoothly.

Just like engine oil, problems will occur if your car has too much or not enough transmission fluid. If your transmission is overfilled by 1 quart, keep reading this article to find out what to do. 

Overheating, a build-up of pressure or leaking transmission fluid are the most common symptoms of overfilled transmission. To sort the issue, you’ll have to drain the excess transmission fluid through the transmission dipstick hole or by unscrewing the nut at the bottom of the transmission.

What Is Transmission Fluid 

Cars need several different fluids to function correctly. For example, without fuel, your car would have nothing powering it.

If there was no coolant in the system, the engine would overheat, no brake fluid would result in brake failure and without oil, the engine would not be lubricated and may melt down completely. 

A vehicle fluid that is rarely discussed is transmission fluid. Cars with automatic or manual transmission both rely on transmission fluid to work correctly.

In a manual transmission, the fluid works by lubricating the metal components and bearings inside the gearbox. It is thanks to transmission fluid that all the different metal parts don’t grind together and cause severe wear. 

However, the transmission fluid needs to do a little more in an automatic transmission. As well as lubricating the metal moving components, transmission fluid creates friction and hydraulic pressure to make the parts move and the gears shift.

Transmission fluid has another crucial role in both automatic and manual vehicles – it prevents the transmission from becoming too hot by working as a cooling agent. 

Different Types of Transmission Fluid: Automatic vs Manual

When your car shifts gears, it is the transmission fluid that allows this action to happen as smoothly as possible.

Automatic cars cannot use the same transmission fluid as manual cars. However, many modern manual cars are starting to use automatic transmission fluid. 

However, it is always important to check which transmission fluid you should use by referencing your owner’s manual. Automatic transmissions do not get as hot as manual transmissions as they work in a slightly different way and the transmission fluid will vary.

Manual transmission fluid will need to be changed approximately every 30,000 – 60,000 miles.

If you have an automatic vehicle, the transmission fluid will last a little longer, approximately 60,000 to 100,000 miles. If you change cars regularly, you may never have to worry about changing the transmission fluid. 

Automatic transmission fluid (ATF) is used in automatic cars, as well as some modern manual transmission vehicles too.

ATF works to lubricate the gears and aids torque converter and clutch friction operation, brake band friction, transmission cooling, and valve body operation. ATFs are often a clear color with a reddish hue, they also have a thin and light viscosity.

On the other hand, manual transmission fluid is heavier and darker than the fluid used in automatic transmissions, usually around 75W to 140W.

In fact, in older manual vehicles, the fluid in the transmission is often referred to as an oil or lubricant. If your vehicle does not have enough transmission fluid, you may notice slow gear engagement and slipping gears. 

Other signs of low transmission fluid include:

• Poor acceleration. 
• Check engine light is on. 
• Strange gumming or clunking sound. 
• Burning smell. 
• Leaking transmission fluid. 

If you notice any of the above, it is time to get your transmission fluid topped back up.

Before you just go and pour a whole bottle of ATF into your car, you need to find out how much transmission fluid your car actually needs.

Overfilling the transmission fluid can be just as problematic as not having enough ATF, keep reading to find out how much transmission fluid your vehicle needs and what to do if you add too much. 

How Many Quarts Should I Put in My Transmission?

It is very important that you add the correct amount of fluid to your vehicle’s transmission. The amount of transmission fluid that is needed will vary from vehicle to vehicle. However, on average, drivers can expect to use between 4 and 17 US quarts to fill up their transmission.

Vehicles with a manual transmission and greener vehicles will use less transmission fluid than luxury high-performance cars and pickup trucks, for example. To know exactly how much transmission fluid your vehicle needs, you should either ask a professional or refer to your owner’s manual. 

The amount of transmission fluid you add will be different for a flush, full replacement, and a simple top-up. The important thing is to avoid exceeding the maximum amount of fluid. You can check how much transmission fluid is in your car by performing a simple dipstick test, similar to checking the oil. 

Firstly, remove the transmission dipstick and wipe it down with a cloth. Next, put the dipstick back in and pull it out one more time. You should now be able to accurately read the result.

If the line is halfway between the max and min lines, you don’t need to do anything. However, if it is looking a little low, you will need to top up the transmission fluid. 

Here is a YouTube video, demonstrating exactly how to check and top up your vehicle’s transmission fluid:

How Do You Know If Transmission Fluid Is Overfilled? – Signs & Symptoms 

We have looked at what transmission fluid is, how to tell if the transmission fluid is low, and how to fill it back up, but what about if your car has too much transmission fluid? Just like when there isn’t enough fluid in your car’s transmission, there are several tell-tale signs of overfilled transmission too. 

While you can always check your transmission fluid levels by using the method above, there are several symptoms of too much transmission fluid that drivers should be aware of.

While accidentally adding an extra quarter or even half a quart won’t cause catastrophic damage, if you overfill my one quart or more, you may notice some worrying side effects. 

The signs of overfilled transmission fluid include: 

• Overheating. 
• Pressure buildup. 
• Leaking transmission fluid. 
• Excessive wear of transmission system. 
• Poor acceleration.
• Difficulty changing gear. 
• Weird noises. 
• Complete transmission failure. 

If your car has too much transmission fluid, your car will be giving you several signs that there is a problem. To learn more about the consequences of overfilling the transmission, let’s take a closer look at each of these points. 

Overheating

When too much fluid is added to the transmission, this can cause seals to break inside the gearbox if excess fluid leaks inside. Also, transmission fluid acts as a coolant for the transmission, when it is unable to circulate due to incorrect levels, this can lead to overheating issues and expensive damage. 

Pressure build-up

When there is too much transmission fluid in the system, pressure can begin to build to dangerous levels. If left unchecked, overfilled ATF can cause enough excessive pressure to damage seals, break moving components and cause the transmission fluid to begin leaking into places it is not supposed to go. 

Leaking transmission fluid

As we just mentioned, too much transmission fluid can cause leaks. When this happens, you may go from too much to not enough transmission fluid in a short space of time and this can lead to big problems for your car.

When fluid is leaking, your car’s transmission will not perform as it should as you may notice several problems when it comes to gear shifting. 

Excessive wear on the transmission system

Transmission fluid acts as lubrication for the whole of the gears system and there is such a thing as too much lubrication!

When there is an excess of fluid in the transmission system, the gears will not engage correctly and they may keep slipping. Too much fluid can cause the bushings and bearings to become damaged and may lead to the premature deterioration of other parts of the transmission. 

Poor acceleration

If you are noticing your car is not accelerating up through the gears as it should and there is not as much power, as usual, this is a sign of too much transmission fluid.

This happens because the excess fluid causes the foam to form around the gears, preventing the parts from engaging correctly. If the gears can’t shit up effectively, your vehicle will not pick up speed as it should.

Weird noises

Too much transmission fluid can cause strange noises to come from under the hood. This happens because when the gears keep slipping, this can cause the parts to grind instead of fully engaging. Metal on metal will make a strange and unpleasant noise. 

Complete transmission failure

Due to everything we have mentioned above, if the excess transmission fluid issue is not dealt with properly, all of these side effects will soon begin to take their toll.

When you continue to drive with fluid leaks, slipping gears, overly lubricated parts, damaged seals, bearings, and so on, it is only a matter of time before it becomes too much for the transmission and the whole system fails. 

What to Do If Transmission Fluid Is Overfilled – Easy Fixes 

As you can see, even if you add just one quart over the recommended amount of transmission fluid for your vehicle, big problems can occur. To be on the safe side, the wisest thing to do in this situation would be to drain the transmission system and refill it with the correct amount. 

Flushing the system may seem like a dramatic approach but you don’t want to risk continuing to drive with too much transmission fluid. To help make the process as easy as possible for you, here is a step-by-step guide on what to do if the transmission fluid is overfilled:

• Step 1 – Park your car on a flat surface. Leave the engine running but make sure the vehicle is put into park.
• Step 2 – You now need to take a transmission to check if there actually is too much fluid in the system. Remove the transmission dipstick, wipe it with a cloth, and then put it back in. Now, remove the dipstick again to get an accurate reading. If the fluid line is above the maximum marker, you know there is too much fluid in the transmission. 
• Step 3 – If there is too much fluid, you will need to remove some. There are a couple of ways you can do this, you can either siphon out the excess fluid with a hose or drain the transmission by opening the drain port and catching the fluid underneath in a large bucket. Both methods can be tricky and messy and it is important that you use all the correct equipment to successfully complete the task. 
• Step 4 – If you are completely flushing the transmission of old fluid, make sure it is totally empty before you add in the correct amount of transmission fluid. Be careful not to overfill it again!
• Step 5 – Once you have refilled the transmission fluid, do the dipstick test again to make sure that you have the correct amount now. 

Here is a YouTube video, demonstrating how you can drain the transmission fluid using tubing and a jerry can: 

FAQs

Do you leave the car running while adding transmission fluid?

Yes, the engine should be left on when you add transmission fluid to your car. However, it is essential that the transmission is left in “park” while you fill the fluid. The handbrake should be activated to keep you safe and the transmission should be warm for best results. 

Can you mix old transmission fluid with new?

While it is possible to mix old and new transmission fluid, you will not achieve the best results this way. Debris and metal particles can build up in the transmission over time, to achieve the best viscosity, it is best to flush the old fluid before adding new.

With all fresh fluid, your transmission will be able to perform at its best.

Is half a quart of transmission fluid too much?

You should always aim to add the correct amount of transmission fluid to your vehicle. Too much or too little ATF can cause issues, including overheating, slipping gears, and complete transmission failure.

However, adding half a quart should not cause any major problems, just be sure to keep a close eye out for any of the tell-tale signs of too much fluid in the transmission. 

Does changing transmission fluid help shifting?

Over time, debris can begin to fill up in the transmission fluid and this can impact its viscosity and performance. Too much and too little ATF can make shifting gears difficult, as can old fluid.

It is important to periodically check how much transmission fluid is in your car using the transmission dipstick. If the shifting issues do not resolve after changing the transmission fluid, it would be wise to take your vehicle to a professional for diagnosis. 

Conclusion 

When you overfill the transmission by one quart, you may find your car begins to experience several different problems. Overfilling the transmission fluid can lead to overheating, ATF leaks, shifting issues, and complete transmission failure.

It is possible to avoid the consequences of too much transmission fluid by draining the excess fluid from the system. This can be done by either using a siphon or draining the fluid out of the drain port in the transmission fluid pan.

Whatever method you decide to use to fix the overfill, make sure you check the fluid level again with the dipstick at the end. Low transmission fluid levels can be just as problematic as too much!

Here are some other related topics for you to check out:

Did You Overfill Your Transmission Fluid? Here is How To Remove and Fix

Transmission Hot Idle Engine: Meaning, Causes & Fixes

How Much Transmission Fluid Do I Need?

What are the symptoms of an overfilled transmission?