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Knowing Your Automatic Transmission

Torque Converter

Automatic transmissions are complex, wonderful things. They are usually a mystery to most do-it-yourself mechanics. It’s very likely that if you’re working on your car from your garage at home or in the driveway, you aren’t going to be doing any major transmission repairs on your own. On the other hand, knowing how it works and some of the simpler things that can go wrong is a good thing.

In this article, we’ll look at how most automatic transmissions operate and how to check fluid levels and do other basic tasks with their maintenance. In another article, we’ll discuss diagnostics and basic repair procedures.

Torque Converters

Most modern automatic transmissions use a torque converter to transmit engine torque to the transmission itself. This converter is a sort of pod-shaped round cylinder that sits in the bell housing of the transmission. When it’s turned by the engine, its internal blades agitate the transmission fluid within, creating a vortex of movement within the pod. This in turn catches blades on the other side, spinning them and the attached output shaft for the transmission’s gearing. The purpose of the torque converter is to smooth the transitions between engine torque output and transmission shifting.

If there isn’t enough fluid in the torque converter to create a smooth spin, the transmission will slip because the converter won’t be able to smooth things.

Transmission Fluid

Automatic transmission fluid (ATF) accomplishes two vital tasks for the transmission. First, it fills and powers the torque converter’s work. Second, it lubricates the transmission and its gears. Both are equally important to the smooth operation of the transmission.

Checking transmission fluid levels is fairly easy. In most vehicles (check your owner’s manual), fluid levels are checked with a dip stick located in the engine compartment, usually with a yellow finger loop to designate it as the transmission’s dipstick. Fluid levels are usually checked with the engine warm and running and the transmission in Park. This has fluids at a specific level, since the transmission will have the torque converter full, but not be circulating fluids in most of the gearing. This means that the transmission pan will not be completely full. Make sure your park brake is set and the transmission is in Park!

Pull the dipstick, wipe it clean with a clean shop towel and replace it, making sure it’s seated firmly. Then pull it again and check the levels. They should be at or very near the FULL mark. There is usually an “ADD” mark a bit below that, so if fluid is above that ADD line, you probably do not need to add any.

Other Auto Trans Components

Other components of the automatic transmission you should become familiar with are its diagnostic port (especially true of electronic shifting transmissions), locations for its filters and regulators, and the locations of sensors that tell the car’s computer (electronic control module) what the transmission is doing. These are different for every transmission, so consult your owner’s manual or an automotive repair manual specific to your vehicle or drivetrain.

Flushing the Transmission

Most automatic transmissions have a maintenance interval that requires a fluid flush and (in most cases) a filter replacement. This is a relatively straightforward job that most DIY mechanics can perform. On most vehicles, it will take about 30-45 minutes to accomplish.

Start with the drain. On the bottom of most auto trans pans is a drain plug. Drain the transmission’s fluid into a proper catch pan or container for recycling. Be sure it drains completely, which can take some time. Pull the dipstick to make it a bit quicker. As the fluid drains, look it over to see if there are any visible flakes, dirt, or other things in it that could indicate excessive wear or other problems.

Once drained, remove the transmission’s bottom pan. It will need to be thoroughly cleaned. Some will have a magnet attached to the bottom which is there to attract any metal flakes shed by wear in the transmission. If it is excessively covered in flakes, you may have a serious issue inside the transmission. Be sure that in cleaning the pan, you’ve thoroughly removed the old gasket.

Clean off the gasket from the transmission as well, if any residue is left. Then remove the filter(s) and replace those using the procedures in your repair guide. For most vehicles, a simple removal and replacement is all that’s required; on others, they may need to be pressurized or pre-filled first. Now that you’re looking at the transmission, check for other issues you may not have noticed before, such as signs of leaking, clogged inlets for the pump, etc. Repair as necessary. Then replace the pan, using a new gasket, and refill the transmission’s fluid (usually done via the dipstick shaft). Be doubly sure that the fluid you’re using is exactly what is specified for the transmission. Different weights and viscosities affect the transmission’s performance and longevity, so be 100% sure that what you’re using is what the transmission requires.

In a future article, we’ll talk about transmission slipping and adding fluid or other repairs you can do to remedy it.