Automatic transmissions are among the most complex mechanical devices ever assembled into a mass-market consumer product. As a result, automatic transmission fluid (ATF) has a tremendously difficult task. It has to lubricate the myriad of moving parts, providing sufficient protection to components that will rotate literally billions of times over the life of the transmission. It also has to provide enough friction to the various clutches within the transmission. ATF also provides the vast majority of the transmission's heat dissipation, and serves as a means of transmitting hydraulic signals and power.

Yes, the demand on ATF is significant, but yet it's often neglected when performing routine maintenance. Transmission service is certainly not as easy or straightforward as oil changes, but considering the frustration and inefficiency of a transmission that isn't operating properly and the jaw-dropping cost of a rebuilt unit, we'll happily roll up our sleeves and crawl underneath our '96 GMC K2500 to show how it's done.

For this project, we'll need the appropriate fluid for the transmission. This is extremely important - do not attempt to fill the transmission with anything but what the manufacturer recommends. The GM Hydromatic 4L80E requires Dextron III, and we bought eight quarts since that's the amount our service manual states is required when dropping the pan. We'll also need a filter and a new pan gasket. Tools vary from job to job, but a set of sockets, a variety of extension bars, and a universal joint (or "wobble drive") will usually do the trick. You also may need a gasket scraper to clean the sealing surface of the pan and transmission case, and some solvent. Last but certainly not least, a large catch pan is required.

The type of fluid and filter change that we're about to show will replace only a portion of the system's fluid, and therefore is best suited to vehicles that are not long overdue for this type of maintenance. Before tackling this project, pull the transmission dipstick and perform a visual and olfactory check of the fluid's condition. A burnt smell or color indicates fluid that's far past overdue (or more severe problems), and it's recommended that the vehicle be brought to a service station for a flush of the transmission and cooler. If you do elect for a professional flush-and-fill, make sure that the filter is also replaced during the process. We prefer to perform this service on a regular basis (roughly every 25,000 miles), and therefore we don't worry about extracting every last bit of used fluid.

If your transmission pan has a drain plug, consider yourself lucky while you remove it to start draining the fluid. In this case, we're not that fortunate, so we start by breaking loose each of the pan bolts with the appropriately-sized socket. Don't remove any of the bolts at this time.

Often, it will be difficult to access one or more of the bolts, so a bit of creativity may be required. Don't rush things, because tough bolts only get that much worse if the corners get rounded off. In this particular case, a 3/8"-drive socket was too large to fit over the bolt head (the transmission mount was extremely close), so a slightly slimmer 1/4"-drive socket was used. Shortly after this photo was snapped, the bolt broke loose and I slammed my hand into the crossmember.

Some transmissions will utilize the pan bolts to secure various brackets, especially for the shift cable. If that is the case, remove only those bolts, and move the offending components out of the way.

Yes, I'm aware that my camera's flash still sucks, in large part due to the Ziplock bag that protected the body of the camera during these photography sessions.

Once all of the bolts have been loosened, remove them, leaving one in place at each corner of the pan. Once this is done, pick a corner to drop first (the front right corner looked to be the best bet in our situation). Remove that bolt, and loosen the others as required to start draining the fluid. As you can see, the fluid will start leaking from almost around the entire pan, so a drain pan at least the size of Montana the transmission will need to be in place to catch all of it. Keep the kitty litter handy to soak up any spills.

Once the flow of fluid stops, remove the remaining bolts and carefully lower the transmission pan. A substantial amount of fluid will still remain, so keep the pan as level as possible.

Here's what the valve body looks like. We recommend against poking around too much, as it's a rather delicate assembly. In this case, the filter is removed simply by pulling it down with twisting motion, being careful not to snag any of the wiring harness. We'd also recommend not dropping it into the drain pan and splashing ATF onto the lens of your digital camera (not like we would know anything about that).

We'll now let the transmission drain for a while as we tend to other tasks.

The fluid that we recovered gets measured so that we know how much to add during the refilling process. We recovered five quarts, and added an estimated half-quart to this amount to compensate for what was spilled (and soaked up by my shirt) during the process.

Next, we clean out the pan. It's entirely normal to find a fair amount of sludge and other wear debris, but significant amounts of ferrous material (such as that which is stuck to the magnet in the bottom of this pan) may indicate problems. This transmission has always made a racket in 1st gear, which can be an indication of planetary wear in this particular model (it can also mean absolutely nothing). We'll continue to keep an eye on things during future fluid changes, but there's not much use in obsessing over it at this point.  

At this time, you may choose to install a drain plug kit, or fit an aftermarket pan that includes one (some aftermarket pans also increase fluid capacity). We decided to forego that step, as leaks may result from modifying the pan.

Clean the sealing surface of the pan using some solvent (we used brake cleaner) and a Scotchbrite abrasive pad. If a cork gasket was previously used, it may be necessary to break out a gasket scraper, but be careful not to gouge the surface.

Place the new gasket in place. In this case, it turned out that the existing gasket was the reusable sort, and was of significantly higher quality than the cheap cork gasket that was supplied with our filter.

Also clean the transmission case, using extreme care not to damage the sealing surface or introduce dirt into the transmission internals.

Here's the replacement filter. Exciting, eh?

Take note of the filter's seal. It's obviously ideal to remove the old seal from the transmission if it didn't come out when removing the filter, but don't attempt this unless you're sure that you can remove it without damaging the transmission. While they usually pop out with ease, we've run into a couple of them that have been stuck in place and have added significant time to the project. If in doubt, leave the old seal in place, because once it's been damaged by a ham-fisted removal attempt, there is not other choice but to change it out.

Tap the new seal into place using a soft-faced mallet.

Gently press the new filter into place. Make sure that it is fully seated.

Gently lift the pan into place while attempting to keep the gasket lined up properly. This may take a few attempts. Install the pan bolts and snug them up, but don't tighten them yet. Feel free to make fun of the oily fingerprints that I've left on the outside surface of the pan.

Next, torque the bolts to the correct value (22 ft-lbs in this case, for those that are curious). If no tightening pattern is provided in your service documentation, simply go around the perimeter of the pan.

Before crawling out from under the vehicle, it's not a bad idea to clean off the exhaust if you've dripped fluid onto it during this process, or else you may get an unpleasant surprise shortly after starting it for the first time.

The correct amount of fluid is used to fill the transmission through the dipstick tube. We then start the engine, shift through each of the gears, and check the fluid level according to the manufacturer's recommendations (according to the dipstick, it should be done in Park and with the engine running). In this case, there are two ranges marked on the dipstick - one each for "hot" and "cold" - and so we don't need to warm up the transmission completely to perform a preliminary check. Once we verified that it was in the ballpark, we went for a drive to warm the trans to its normal operating temperature before we made a final observation of the level and checked underneath the vehicle for any leaks.

Since we're filling the trans through the dipstick tube, any pooling can hinder our efforts in obtaining an accurate fluid level measurement. Therefore, it's important to be patient while verifying that the transmission is indeed properly topped-off, as underfilling or overfilling may result in severe damage. Checking transmission fluid levels can be a significant challenge, which is why many manufacturers have removed the dipstick entirely in favor of measuring the fluid level electronically.

The old transmission fluild should be recycled, of course. Our local recycling center simply mixes it with waste oil, but you'll want to check with your location to see what should be done with it as to avoid contaminating a large amount of recycled material. At this time, we're unaware of any efforts to recycle automatic transmission filters, so we let the part drain for at least 24 hours and then dispose of it.


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