The automatic transmission in your car is responsible for transmitting the engine’s power to the wheels, making them turn. It does this by “automatically” choosing the correct gear ratio, based on your driving. But to do this effectively, it needs lubrication — transmission fluid, to be exact.

What is automatic transmission fluid?

Automatic transmission fluid is oil-based, with lubricating, cooling and hydraulic qualities. It has special additives blended in to help it do its job. These additives help the fluid do all this:

• Lubricate better
• Reduce wear
• Inhibit rust and corrosion
• Clean and protect metal surfaces
• Improve viscosity
• Prevent seals from leaking
• Reduce oxidation
• Flow better at low temperatures
• Maintain thickness at high temperatures
• Pour easily at all temperatures

Different automatic transmissions require different types of transmission fluid. Be sure to check your owner’s manual before you add any to your vehicle.

Automatic transmission fluid is usually red but can be other colors. Whichever hue it is, it’s typically a different color than other car fluids. This makes it easier to identify a fluid leak under your car.

How does automatic transmission fluid work?

As the transmission operates, the fluid circulates through it, lubricating it and absorbing heat.

Because the fluid is sealed inside your automatic transmission, it’s not exposed to the damaging byproducts of combustion the way engine oil is. As a result, automatic transmission fluid needs to be changed much less frequently than oil.

Still, over time, automatic transmission fluid will become contaminated, use up its additives and lose its lubricating qualities. When this happens, your transmission can overheat or have other problems.

Check your owner’s manual for a maintenance schedule. The manual may also have instructions on how to check your transmission fluid, including the location of the dipstick if your car has one. If there is no dipstick, and no information on checking the fluid, ask a mechanic for recommendations.

Signs your automatic transmission fluid is bad

If your transmission fluid is getting old or is contaminated, you’ll be able to tell eventually. Let’s start with a couple of symptoms that apply only if you have a dipstick, and then a few more general ones.

Transmission fluid is low

Pull the dipstick and check it. If the fluid level is below the full mark, it needs to be topped up.

Solution: If the fluid is clear, you can top it up with fresh automatic transmission fluid. Check the level frequently while you do this, as you don’t want to overfill it. One thing to watch out for: If the fluid is low, you might have a leak. Look under the car and the hood for signs of transmission fluid. If you have a leak, you’ll need to get it fixed by a mechanic.

Transmission fluid is dark or contaminated

While it’s normal for your automatic transmission fluid to darken as it ages, it should not totally lose its color. If the fluid on your transmission dipstick is brown or black, smells burnt or has lots of dark particles in it, get the car to a shop immediately.

Solution: A mechanic can check your transmission fluid and diagnose any problems. It may be that the fluid has gone too long without being changed, or there may be a problem with the transmission itself. The sooner you deal with this, the better.

If you wait too long to change the fluid, it can actually be more problematic to flush it — the mechanic may recommend keeping the old fluid in and letting the transmission go as long as it can. But this is a bad scenario, as eventually you’ll need to shell out for a new transmission.

Transmission fluid is leaking

Whether you have a transmission dipstick or not, a leak is a big problem. Check for puddles under the car or on parts under the hood.

Solution: Get your car to a repair shop as quickly as you can. If you can top up the fluid in the meantime, do so. There are many possible sources of transmission leaks, including gaskets, seals, fluid lines or a loose transmission pan. The leak can usually be repaired, along with any other damage the leak may have caused.

Your transmission isn’t working properly

Problems with your transmission fluid can cause your automatic transmission to have problems shifting, staying in gear or overheating. You may feel the transmission slipping or hear odd noises while you drive.

Solution: Get the vehicle to a mechanic immediately. These drivability problems can be caused by low, dirty or old fluid, or some other internal cause. Having the problem taken care of right away could potentially save you the cost of a new or rebuilt transmission.

Keep your automatic transmission fluid fresh

The continued good health of your automatic transmission depends on maintaining your transmission fluid. Checking the fluid regularly is the first step.

Having your automatic transmission fluid changed or flushed at the manufacturer’s recommended intervals will also help. This will also prevent you from voiding your powertrain warranty, which normally covers transmission problems for several years. Check your owner’s manual for whether you need a transmission fluid change or flush.

Doing these two things should keep your automatic transmission operating at peak efficiency for many years to come.

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.