Transmission Fluid

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What is this?

Changing automatic transmission fluid is best left to a mechanic equipped with a transmission flusher, like this Wynn's Transerve II+, which replaces the fluid more thoroughly than you could achieve with gravity alone.
Changing automatic transmission fluid is best left to a mechanic equipped with a transmission flusher, like this Wynn's Transerve II+, which replaces the fluid more thoroughly than you could achieve with gravity alone.

 

Transmission fluid is a slippery liquid that acts as a lubricant for all of the moving parts inside your transmission. In an automatic transmission, this fluid also serves as a coolant and a viscous fluid that transmits power from the engine to the transmission.

A variety of fluids are used for different transmissions. Automatic transmissions use something called — shockingly — automatic transmission fluid. Manual transmissions use a variety of oils: regular motor oil, heavyweight hypoid gear oil or even automatic transmission fluid in some cases. Your owner's manual will tell you what your transmission calls for.

Should I do this service when it's recommended?

Yes, definitely, regardless of whether you have a manual or an automatic transmission.

Manual: Most manufacturers recommend that manual transmission fluid be changed every 30,000 to 60,000 miles. Under heavy-duty use, some manufacturers suggest changing transmission fluid every 15,000 miles.

Automatic: Service intervals for an automatic transmission vary from every 30,000 miles ... to never. The typical service interval is 60,000 to 100,000 miles. Changing it more often does no harm.

Why do I have to do this?

Manual: In a manual transmission, the problem is not so much the fluid degradation, but rather fluid contamination. This contamination occurs over time as the synchronizers, bearings and gears in the transmission wear out. The resulting metal particles then float around in the lubricant. And we all know that oil with microscopic particles of metal in it does not lubricate as well as clean oil. So if these contaminants are not drained out, they will shorten the life of your transmission.

Automatic: Because more heat is generated in an automatic transmission, automatic transmission fluid actually degrades and breaks down with use.

In addition, like in a manual transmission, automatic transmission fluid will also become contaminated with worn bits of the transmission. If these contaminants are not drained out, they will shorten the life of your transmission.

What happens if I don't do this?

If you don't change the transmission fluid on schedule, you'll be lubricating your transmission with metal shavings and other contaminants. This will shorten the transmission's life. The result could be a hefty boat payment to your mechanic. In other words, changing your transmission fluid at the correct interval is a good investment.

Is there any maintenance required between intervals?

Automakers have gotten better in recent years about labeling or color-coding dipsticks. The ring on the right is red to match the automatic transmission fluid, the level of which this dipstick measures. The yellow ring is clearly marked "engine oil."

Automakers have gotten better in recent years about labeling or color-coding dipsticks. The ring on the right is red to match the automatic transmission fluid, the level of which this dipstick measures. The yellow ring is clearly marked "engine oil."

Yes. It's important to regularly check the transmission fluid level between service intervals. Letting your car run low on transmission fluid can cause the transmission to shift improperly — or not at all. It also can harm the internal parts of your transmission, which will not be properly lubricated. Unfortunately, you may not hear any noises or have other clues that your transmission is low on fluid, until it's too late. So it's important to get it checked.

Here's how you can check your transmission fluid level:

Manual: Checking the transmission fluid in a manual transmission can be difficult. A few thoughtful manufacturers have included a dipstick, but that's the exception rather than the rule. If you own a car with a manual transmission, we suggest that you ask your mechanic to check the fluid level when your car is up on the lift during an oil change. It takes just a minute.

Automatic: If you own a car with an automatic transmission, your car will have a dipstick for this purpose. Be careful not to make the common mistake of confusing the transmission dipstick with the crankcase dipstick.

Can I Do This Myself?

It depends on where you rank on the Car Talk do-it-yourself scale:

There are two kinds of screwdrivers?
I've successfully hung pictures around the house.
I've changed my own oil.
I've fixed things on my car that involve removing more than five bolts.
I've built a working nuclear reactor out of wood.


For most cars, checking the automatic transmission fluid consists of pulling the transmission dipstick out while the engine is warmed up and running and with the transmission in park. We suggest that you check your owner's manual, however, since some manufacturers may have a different procedure.

Of course, always check your fluid level if you notice a leak of any kind.

Car Talk Tip: Unlike engine oil, transmission oil doesn't burn up. So if you're low on transmission fluid, you almost certainly have a leak.

changing automatic fluid photo courtesy of Wynn Oil Company;
color-coded dipstick photo by Joe Wiesenfelder, Cars.com

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