Automatic Transmission Fluid and its Purposes
Automatic transmission fluid, or ATF, is one of the most complex fluids in a car. It's also one of the most confusing because there are so many different types, some of which are manufacturer-specific and incompatible.
ATF has three primary roles: The first is to act as a hydraulic fluid to actuate the bazillion moving parts inside the transmission that enable it to shift gears all by itself; the second is lubricate and cool the internal parts of the transmission, much like motor oil does in the engine; and the third is to regulate the friction of the clutches to assure smooth shifts.
In some cars, ATF is also used as power steering fluid. When this is the case, the power steering usually (but not always) will use the same kind of ATF as the transmission does. Some manual-transmission cars also use ATF as a gearbox oil.
Using the wrong fluid in an automatic transmission can cause impaired transmission function, gradual transmission damage, or sudden and catastrophic transmission damage, depending on how bad was the mismatch. If the ATF you use is "wrong enough," it's possible to instantaneously transform a perfectly good car into a candidate for the junkyard. Repairing or replacing an automatic transmission on a used car will often cost more than the car is worth.
Even choosing an inferior grade of the correct ATF can cause immediate and noticeable reductions in the transmission's performance such as late, irregular, or rough shifting. Automatic transmissions are finicky beasts.
The moral of the story is that even more so than is the case with most automotive fluids, choosing the right automatic transmission fluid is absolutely critical. So how does one do that? Let's talk about a few ways.
How to Choose the Right Automatic Transmission Fluid
Method 1: Read the Manual
Either your vehicle's owner's manual or its service manual should have information about the correct ATF for your car. Many, however, provide only the OEM's part number for their own fluid. In that case, you can either use the manufacturer's fluid (not a bad idea in any case to avoid compatibility problems), or cross-reference the fluid with an aftermarket manufacturer's line.
Method 2: Ask the ATF Manufacturer or a Retailer
Most automatic transmission fluid manufacturers have Web pages that allow you to input your vehicle's information to determine which of their fluids is the correct one for your vehicle. Here are the fluid finders for Redline (my personal favorite), Valvoline, and Pennzoil, just to get you started.
Method 3: Let a Professional Mechanic Do the Job
Replacing ATF is not an expensive job on most cars. It's also one of those jobs that's much easier if you have a lift. Having your dealership or an independent shop change your car's ATF is a perfectly-reasonable option that probably won't cost very much money.
Method 4: Buy Your ATF From the Dealership
You'll pay a little more, but at least you'll know you have compatible fluid.
When to Check and Change Automatic Transmission Fluid
If your car has an ATF dipstick, it should be checked whenever you check the other fluids under the hood. I don't have any automatic transmission cars at the moment, but when I do, I check the ATF weekly. At a minimum, I suggest you check it monthly, assuming your car's transmission has a dipstick. Look at both quantity and condition. Wipe the dipstick with a clean white cloth and check for discoloration, and sniff it to determine if it smells burnt.
That being said, ATF in general tends to last longer than most automotive fluids, and synthetic ATF lasts even longer. In fact, many new cars come with "lifetime" synthetic ATF pre-installed, and many don't even have dipsticks. If this is the case for your car, then you don't need to change the ATF unless the transmission starts giving you problems, or the warranty runs out. "Lifetime," in automotive terms, usually means the lifetime of the warranty, not all of eternity.
Other cars do have a manufacturer-specified service interval for the automatic transmission fluid to be change. This information can be found in your vehicle's owner's manual.
My own preference is to leave synthetic ATF alone for as long as the transmission is operating properly unless the manufacturer has a stated service interval. It's not unusual for synthetic ATF to last for more than 100,000 miles (about 161,000 kilometers). For conventional ATF, I always believed in changing it at about 50,000 miles (about 80,000 kilometers) unless the manufacturer specified a different interval.
I'm not going to provide step-by-step instructions for changing automatic transmission fluid because it's my opinion that the internal filter screen should also be changed when the transmission fluid is changed, and that job is probably more involved than the average visitor to this site wants to get into. If you want to give it a try, I suggest you consult your vehicle's service manual. I'm warning you, though, it's often a messy job (though usually not an especially difficult one). Many transmissions lack drain plugs, so the pan has to be removed. It's definitely a job you want to do on a lift.
The exception would be if your car has an external, spin-on transmission filter. Some gasoline-engine cars, like many older Saturns, had them; and many diesel-engine cars have them. In almost all cars with spin-on transmission filters, there are two bolts on the transmission that are used for fluid changes. One is on the bottom of the transmission and is used to drain the fluid, and the other is near the top and is used to install the new fluid. If your car has that arrangement, then it's a much easier job, not unlike an engine oil and filter change.
One note of advice: Whenever you have a situation where you have a fill hole on the top and a drain hole on the bottom, open the fill hole first to make sure the screw or bolt comes out. You don't want to drain the fluid and then find that you can't refill it because the bolt in the fill hole won't budge.