As with most automotive maintenance questions, following the factory-recommended intervals for changing your transmission fluid is generally your best bet. But in the case of automatic transmissions, there may be some exceptions. (Note: While manual transmissions may also have service internals, the process is comparatively simple, so we’ll only be discussing automatics here.)
Related: We Answer Your Manual Transmission Questions
Why Does the Transmission Fluid Need to Be Changed?
It mostly comes down to heat. Heat is the biggest killer of your transmission, partly because it’s the biggest killer of your transmission fluid. The fluid is not only a lubricant, but it also serves to prevent corrosion and actuate the clutches inside the transmission that perform the shifts. When heat breaks down the fluid, it loses some of its capabilities.
Excess heat can be generated by stop-and-go driving, hauling heavy loads or towing a trailer. For vehicles operating under “severe service” conditions, you should probably have the transmission fluid changed more often — as might be called for in the maintenance schedule. Also, the clutch surfaces wear over time, and that creates debris. That debris is largely trapped by a filter that’s replaced when the fluid is changed.
Furthermore, it’s possible that the fluid level can get low, which can cause shifting problems. However, if the fluid is low, it’s likely because it’s leaking out somewhere, as it doesn’t just “burn off” as engine oil sometimes does. You can usually tell if transmission fluid is leaking, as you’ll see pink or red puddles on the ground — or maybe dark puddles if the transmission fluid is bad.
In any case, a transmission-fluid change, if done correctly, can address all of those issues.
When to Change Transmission Fluid
In the past, the factory-recommended interval for changing the automatic transmission fluid was typically between 30,000 and 100,000 miles, but some newer vehicles have what’s referred to as “lifetime fluid.” However, there’s some question as to whether never changing the transmission fluid may shorten that “lifetime.” Thus, there’s something to be said for at least checking the fluid (as some can leak out) and probably changing it at about 100,000 miles. If you plan to keep the car well past that point, it’s a small price to pay if it saves you from having to replace the transmission — which would be a big price to pay.
On another note, some folks question whether it’s a good idea to change the transmission fluid in an older car if it hasn’t received its previously scheduled fluid changes. (If you bought the car used and don’t know whether the fluid was changed at recommended intervals, it may be logged in something like a CarFax report.) However, if the transmission is shifting properly and the fluid is vivid red or pink without metallic flecks in it — as “good” fluid normally is — you’d probably be fine changing the fluid as a preventative maintenance measure since it implies the transmission itself is OK.
The concern is that if the fluid appears to be past its prime — dark-colored, smelling burnt or with metal flecks in it — then it may be best to leave it alone. Why? Because it probably looks that way due to wear in the transmission. That bad fluid may have a “sticky” quality to it or metal flecks that act to aid friction, and if it’s changed, the new fluid might cause the transmission to start slipping. That’s because there are friction clutches in the transmission that are activated when changing gears, and if they’re very worn, the stickiness of the old fluid or grainy metallic flecks might be all that’s preventing those worn clutch surfaces from slipping.
This theory often comes into play when someone buys a used car with high mileage and an unknown service record. It’s always best to check the transmission fluid before buying a used car, and if it’s at all questionable, take a pass — replacing the transmission is a huge expense, often costing more than a 100,000-mile car is worth.
Of course, it’s always possible that the seller just replaced the transmission fluid themselves in an effort to correct a slipping transmission (or to just make it appear as though the maintenance has been kept up); in that case, you’re reliant on the results of a test drive. Any questionable transmission behavior (such as slipping, rough shifts or a delay in gear engagement) is a good reason to look elsewhere.
Checking Your Automatic-Transmission Fluid
While older cars with automatic transmissions almost always have a transmission-fluid dipstick (similar to the engine-oil dipstick but usually located near the back of the engine), many newer vehicles have no transmission dipstick at all. Your owner’s manual should tell you whether you have a transmission-fluid dipstick, where it’s located and how to check the fluid level. The last might include a bit of a procedure, which often involves parking the car on a level surface, having the engine warmed up and running, applying the brakes and shifting slowly through all the gears before putting the transmission in Park and checking the level.
What you should be checking, however, is not just the level, but also the condition of the fluid. “Good” fluid is usually vivid red or pink; “not good” fluid is usually dark or has metal shavings in it — which you can usually both see and feel with your fingers. If the fluid is foamy, it may mean that the fluid level is too high (likely because somebody added too much at some point, possibly due to not following the directions in the owner’s manual for checking the level), which is bad and means some fluid needs to be removed. If it’s too low, make sure to add the specific kind needed for your car; there are several types, and they differ in properties. Also be certain you don’t overfill it.
Some newer cars don’t have a transmission-fluid dipstick. This may be the case for cars with lifetime fluid, which might be advisable to check and change at about 100,000 miles regardless. In those situations, it’s probably best to take the car to a trusted mechanic at either a dealership or independent service facility to have the fluid checked or changed, as it can be a tricky procedure. However, if you’re an avid do-it-yourself type and want to try it, you may find an internet search for, “How do I check [or change] the automatic transmission fluid on a [year/make/model]” to be helpful.
One more thing: Some shops might recommend a “flush” rather than just a “change.” That’s partly because a change doesn’t replace all the fluid; in fact, it may replace only a third to a half of it, as the rest is held in the torque converter and internal passages that don’t drain out. A flush, on the other hand, is intended to replace all of the old transmission fluid with new fluid. While that sounds like a good thing, sometimes it might not be. A theory here is that flushing can dislodge debris that’s trapped in some of the internal passages, allowing it to clog up other passages downstream. It could also blow out old seals that are otherwise doing their job.
Furthermore, if the transmission fluid is dark or contains metallic particles (and is thus somewhat bad), it’s possible the friction qualities of that old fluid might be all that’s preventing the transmission from slipping, and you’re now getting rid of all of it. As a result, you’ll often hear stories of a transmission acting up soon after a flush has been done.
More From Cars.com:
- Why Is My Car Leaking Oil or Another Fluid?
- What Is a Dual-Clutch Transmission?
- What’s the ‘L’ in Your Shifter and When Should You Use It?
- What Fluids Should I Top Off?
- More Service News
The Bottom Line
A modern automatic transmission is an extremely complicated device — far more so than an engine — and many mechanics wouldn’t think of taking one apart to try and fix it. (They’d more likely remove it, send it off to a specialist to repair, then reinstall it.)
There are also many variations in the design of automatic transmissions, making viable generalizations difficult — which is why you’ll hear numerous theories and opinions about them, but few absolutes. Yet one generalization that’s certain is that you want yours to last as long as possible, because replacing it — regardless of design — is an expensive proposition.
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