If you're like most people, you worry a lot about your car's engine oil. This makes sense, considering oil bathes and lubricates the power plant of the vehicle. But it's the job of the transmission to parcel out the engine's power to the wheels, which means transmission fluid -- the magenta-colored lifeblood that coats gears and torque converters -- warrants just as much attention as engine oil.
Most cars on the road today have automatic transmissions and hence automatic transmission fluid, or ATF. Pressure changes within the ATF cause the transmission to switch gears. Even on a relatively simple drive, from your house to the office, let's say, the transmission and the fluid do quite a bit of work. Fluid temperatures soar to 175 degrees Fahrenheit (79 degrees Celsius), which seems hot to us but is perfectly normal for ATF. In fact, if fluid temperatures remained at 175 degrees Fahrenheit, ATF would mimic the Energizer Bunny and keep going and going for 100,000 miles (160,934 kilometers) or so. Unfortunately, fluid temperatures rarely stay in the optimal range. Numerous driving conditions -- stop-and-go driving, hauling a heavy load, driving long distances or up and down mountains -- can heat transmission fluid beyond acceptable limits. At these higher temperatures, ATF begins to break down, and your transmission begins to shift gears roughly, slowly or both.
This is why most manufacturers recommend that you change your ATF and filter every 20,000 to 25,000 miles (32,187 to 40,234 kilometers). Repair shops, quick-lube chains and service departments can do this for you, if you want to shell out the bucks. But it's not impossible to do it yourself as long as you have a little bit of knowledge and a few simple tools. And in a down economy, doing some of your own auto repairs and maintenance can save a fair amount of money in a year.
So let's get started. On the next page, we'll cover the tools and materials necessary to change the fluid in a car with an automatic transmission. Changing the transmission fluid in a manual transmission works a little differently. You should consult your owner's manual or, better yet, take your car to a good mechanic.
Tools Needed to Change Transmission Fluid
Before we get to the tools, let's discuss two different approaches to changing automatic transmission fluid (ATF). The first involves pumping out the old transmission fluid before adding the new. This approach has the advantage of removing more of the old fluid, even the ATF that collects in the nooks and crannies of the torque converter, but it's a bit more complicated. The second approach, known as the "drop-the-pan" method, doesn't require any pumping. As a result, it's much easier to do, although it leaves behind some of the old fluid to mix with the new ATF you add.
This article will explain the "drop-the-pan" method, which results ultimately in a partial fluid change. Don't let this discourage you. A partial fluid change still extends the life of your transmission. And you can always have your mechanic do a more thorough ATF flush at major service milestones.
Your mechanic most likely uses a transmission flusher, which completely flushes the cooler, torque converter, pump and lines without dropping the pan. Without access to such a high-end piece of equipment, you're going to need some tools and materials. The first thing you need is a transmission filter service kit. The service kit includes a transmission filter and a pan gasket. You're also going to need a catch pan and 3 to 6 quarts (3 to 6 liters) of automatic transmission fluid. All transmission fluid varies from car to car. Read the sidebar to learn more about the different types of ATF.
You can find the rest of the required tools in your garage. Here's what you'll need:
- Socket wrenches
- Longneck funnel
- Two old milk jugs
- Jack stands or car ramp
- Wheel chocks
- Brake cleaner
- Several clean shop rags
You'll use the milk jugs for measuring, not collecting. You can get by without them, but it's handy to know how much fluid gravity pulls from your transmission. When you go to add new ATF, you'll have a ballpark idea of how much fluid to add.
Collecting the right tools is just the beginning. Up next, we'll look at some necessary prep work before you start draining the fluid.
Preparing for a Transmission Fluid Change
Before you lift your car, you should inspect your transmission to diagnose its health. Your car will need to be running to do this, so start the engine and let it run until it gets warm. With the gearshift in park and the emergency brake on, check the transmission fluid level using the dipstick located at the rear of the engine. It will be sticking out of the transmission or, if you have front-wheel drive, out of the transaxle.
Now, pull out the dipstick, wipe it on a clean rag and reinsert it into the tube, making sure it's seated completely. Pull the dipstick out again and look at the film of fluid on the end. Some dipsticks indicate add levels, and some show full levels for cool, warm or hot fluid. If your transmission is in good health, the automatic transmission fluid (ATF) should not be low. It should also have a pinkish to reddish color and smell like petroleum.
If the fluid level is low and you're in between service, add transmission fluid and keep checking every two weeks. If your transmission fluid is consistently low or appears burned and dirty, you could have a more serious transmission problem. Take your car to a mechanic so the transmission can be thoroughly inspected.
If the fluid level and quality looks good and it's been 20,000 miles (32,187 kilometers) since your last transmission service, you should proceed with the fluid change. To do this, you'll need to raise your car. You can use jack stands, but ramps make the job easier. Ramps offer good stability and lift your car 7 to 9 inches (18 to 23 centimeters) off the ground. If you opt for jack stands and do the work on a gravel driveway, consider placing a piece of plywood under the stands to prevent them from digging into the ground. Either way -- jack stands or ramp -- have wheel chocks ready to keep the rear tires from rolling.
Fluid changes should only be done when the engine and transmission are at normal operating temperatures. With your vehicle raised, let the engine idle for a few minutes, then turn it off. When the transmission fluid is still warm, but the vehicle has cooled down, you're ready to remove the old transmission fluid. Keep reading to learn how.
Removing Old Transmission Fluid
There's a universal truth about newer cars: They often make do-it-yourself repairs and service jobs more challenging than you'd expect. The same can be true when it comes to changing transmission fluid. First, you have to find the transmission pan, which serves as the reservoir for the automatic transmission fluid (ATF). Because you know the location of the transmission fluid dipstick, you have a good clue. The pan will be sitting directly beneath the dipstick. Unfortunately, some cars require that you remove the oil pan before you can get to the transmission pan. In other cars, you must first remove parts of the exhaust system.
Once you've exposed the pan, it's time to drain the fluid. Some transmission pans come with a drain plug. If you're lucky enough to have such a pan, you can simply remove the plug to drain the ATF. Most transmission pans, however, don't have plugs. The only way to drain the old ATF is to remove the entire pan. Before you do, place a large catch pan under the transmission pan. Then, begin loosening the bolts that attach the pan to the transmission. Be prepared: As soon as you loosen the pan bolts, fluid will leak out around the edges.
Don't remove all of the bolts, as you'll have an overflowing pan of oil and, very likely, a mess. Instead, remove bolts from all but one side, tap the pan with the mallet to break the seal and tilt the pan away from you, letting it pivot on the side with the bolts. The fluid should spill over the lowered side of the pan and into the catch pan. Once the bulk of the fluid has drained, remove the other bolts and the pan. Pour any remaining fluid into the catch pan. Then, to find out how much fluid came out of your transmission, transfer the drained fluid into empty one-gallon milk jugs. You're not looking for an exact measurement -- just a general idea of how much to replace.
Finally, remove the old transmission filter, which is attached to the transmission with bolts, clips or the filter's O-ring seal. You can pull off a seal-attached filter by twisting and pulling on the part. Don't worry about cleaning the filter. It has done its job, and is ready for the trash.
The transmission pan, however, will need some cleaning before you add new ATF. That's the topic of the next section.
Adding New Transmission Fluid
Before you add new transmission fluid, you should clean the pan. Use a rag to wipe away the residue from the inside of the pan. Don't be alarmed if you find metal debris -- even healthy transmissions will leave small amounts of metal in the pan. Excessive metal debris, however, could indicate a more serious issue. If your pan has a particle magnet, which keeps metal away from the moving parts of the transmission, you should clean that, as well.
Next, remove any traces of the gasket from the edges of the pan. You should be able to peel away most of the gasket by hand, but some sticky residue will remain. Use a knife or flat-head screwdriver to scrape off this residue, being careful not to scratch the surface of the pan. Brake cleaner will remove any stubborn remnants of gasket material. You can also apply brake cleaner to the pan bolts.
While the pan and pan bolts dry, install the new filter. Make sure it's mounted in the same position and that the O-ring is properly positioned. Reattach any bolts or clips that secure the filter to the transmission. Then you can reinstall the pan, using the gasket -- either rubber or cork -- recommended by the manufacturer. Many rubber gaskets are self-sealing, but some rubber gaskets and most cork gaskets are not. If your gasket is of the latter variety, apply fastening cement before you install the gasket. Start every pan bolt by hand for two turns before tightening. You don't want to overtighten the bolts, as this will flatten the gasket and cause leaking. If no recommended torque is given, make the pan bolts "screwdriver tight."
Now you're finally ready to add new transmission fluid. Most vehicles allow you to do this through the dipstick tube, using a longneck funnel. Pour a little less than the amount of automatic transmission fluid (ATF) you collected in the milk jugs. Next, start the engine and let it run idle for at least one minute. With the parking and service brakes applied, move the gear selector through each position, ending in park. Recheck the fluid level and add fluid to bring the level to an eighth-inch below the add mark. Run the engine until it reaches normal operating temperature, and then recheck the fluid level -- it should be in the hot region.
A word of caution about adding ATF: Overfilling can be just as bad as having too little fluid. So go slowly, check the dipstick often, and in no time you'll be back on the road with a happy and healthy transmission.
Keep reading for more car maintenance tasks you might want to try.
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- Erjavec, Jack. Automotive Technology: A Systems Approach, 4th Edition. Thomson Delmar Learning. 2005.
- Freudenberger, Bob. "Saturday Mechanic: Changing Your Transmission Fluid." Popular Mechanics. June 2003. (Oct. 28, 2009). http://www.popularmechanics.com/how_to_central/automotive/1272521.html
- Plueddeman, Charles. "Auto Repair: Do It Yourself, but Do It Right." MSN Autos. (Oct. 28, 2009) http://editorial.autos.msn.com/listarticle.aspx?cp-documentid=1088044
- Rise, Donald. "How to Change Transmission Fluid Properly." Auto Tips & Advice. Aug. 18, 2009. (Oct. 28, 2009) http://autotips.plentycar.com/how-to-change-transmission-fluid-properly-part-1/
- Sclar, Deanna. Auto Repair for Dummies. Hungry Minds, Inc. 1999.
- Yahoo! Autos. "How often should the automatic transmission fluid be changed?" (Oct. 28, 2009) http://autos.yahoo.com/maintain/repairqa/transmission/ques123_0.html
- The Unofficial DMV Guide. "Replacing Transmission Fluid." (Oct. 28, 2009) http://www.dmv.org/how-to-guides/transmission-fluid.php