How to Change Your Jeep’s Automatic Transmission Fluid
We use a 1999 Jeep Cherokee XJ to highlight some tips for doing a fluid and filter service on your Jeep’s automatic transmission.
For us, there really isn't a maintenance job we like doing less than changing the fluid and filter in an automatic transmission. Even if you're blessed with an automatic that has a transmission pan with a drain plug, it's frustrating. And if your tranny pan doesn't have a drain plug, be prepared for ATF to cascade all over your driveway and yourself. But it's something you need to do if you want continued good performance and extended life. So using a 1999 Jeep Cherokee XJ with a 4.0L engine, AW4 transmission, a swapped in NP231, and a factory exhaust downtube as an example, here are some tips to consider when changing your Jeep's automatic transmission fluid.
Draining Your Automatic Transmission Fluid
Unfortunately, not all factory automatic transmission pans come with a drain plug. The AW4 in this 1999 Jeep Cherokee did have a factory drain plug, but since a drain plug is a potential source of leaks that can drain an auto transmission to the point of losing function or permanent damage, the majority of auto transmissions come with no provision for easily draining the fluid. If you do have a plug in your factory or aftermarket pan, use a large-volume drain pan and remove the plug bolt and let it drain for a good, long while. Auto transmissions retain a lot of fluid up in their case and will drip for hours sometimes.
If your pan doesn't have a drain plug, then loosen all the bolts of the pan. Leave a couple bolts in one corner to keep the pan from falling and then use a flat-edge prying tool to gently break the pan seal in the most accessible corner of the pan. You want to angle the pan downward so the transmission fluid inside pours neatly out of it the same as if you were pouring water out of a baking pan and into a cup. Once you've gotten the majority of the fluid out of the pan, you can remove the last remaining pan bolts and carefully dump the rest into your receptacle. It sounds easy, but be prepared to make a bit of a mess.
Removing the Transmission Pan from the Vehicle
Sometimes, but not always, the transmission fluid level dipstick will be part of the transmission pan. In the case of this 1999 AW4, the dipstick inserts into a tube that's welded to the side of the pan. Make sure you remove the transmission dipstick first and then carefully lower the pan, making sure not to bend or break off any protuberances on exhaust, driveshaft, or crossmember components. Remember also that there will most likely be a good amount of residual ATF in the pan, so be careful not to dump a large amount on your driveway or garage floor.
Changing the Transmission Filter and Cleaning
Most automatic transmissions have a flat filter that strains out contaminants from the oil before they're pumped through the rest of the transmission system. Most of these, as is the case with this AW4, are bolted to the transmission valve body, although some insert with a loose press fit into a plastic bushing or with a rubber O-ring, or some combination of all. Also, some auto transmissions may utilize two internal filters, one being the flat filter and another a cannister filter that resembles a small, short engine oil filter. No matter what the case, after you remove the old filters, clean the mounting surfaces with a little aerosol cleaner and inspect for grooves or damage. When installing your new filters, lube any O-rings, bushings, or gaskets with some ATF to help prevent nicking and galling as it's inserted. Also, if you're installing a cannister filter it's always good practice, like with an engine oil filter, to add fluid to the inside of the filter so it's not running completely dry when you fire it up for the first time after the fluid swap.
You'll also want to carefully clean away any old RTV or gasket material from the pan mounting surface at this time. If at all possible, try to avoid using high-speed rotary tools that can fling small particles of RTV into the transmission where they could potentially block fine passages. It's best to use a sharp gasket scraper or razor blade, then transition to the rotary tool only if necessary.
Reinstalling and Sealing the Automatic Transmission Pan
Auto transmission pans can sometimes be a bugger to seal properly, and most synthetic transmission fluids like the ATF+4 required by most Jeep autos is quite expensive. So failing to get a good seal and realizing after filling your transmission up with $75 worth of new fluid that you've got to drop the pan and reseal it again won't be a welcome experience. For that reason we generally don't use cork or rubber pan gaskets since they can squirm out and fail to seal. In the past we've sometimes had good luck with Mopar Anaerobic gasket sealer, which stays in an almost liquid state unless acted upon by air. But it's somewhat expensive and our results with it aren't always stellar, so generally we use Permatex Ultra Black RVT silicone. As we said earlier, auto transmissions have many cracks, passages, and crevices and can drip fluid for hours, sometimes even days. It's not uncommon to be installing your pan and as you're positioning it in place to have a dribble of ATF spill out onto the perfectly degreased pan surface you've just prepared. The Ultra Black is super strong and generally seals even if a bit of transmission fluid inadvertently drips onto the transmission case pan seal surface, although we're not saying no surface prep is necessary for best results.
To clean our pan gasket we've found no better tool than a Snap-On Crud Thug. We've used this rotary wire wheel tool for decades now, and it's always amazing how quickly it strips automotive sealants away.
Reinstalling Your Automatic Transmission Pan
When it's time to reinstall your transmission pan, if you're using RTV you'll need to decide for yourself if it's better to apply a bead to the transmission case or the pan itself. In this case, we had to snake the pan up past the exhaust downpipe, and in doing so the rear of the pan almost contacted the transmission crossmember. We did a dry run before actually applying the RTV and discovered that, had a bead been laid on the transmission pan, it would've gotten messed up on the crossmember, so instead we applied the RTV directly to the tranny case and then positioned the pan underneath. We hung the pan just under the transmission and then using two of the pan bolts at opposite corners, loosely positioned them to ensure all the bolt holes would line up and we wouldn't need to slide the pan side to side, messing up our RTV bead. Once the two pan bolts located it properly, we installed the rest of the bolts finger tight and then torqued them to spec working outward from the center of the pan.
Once the pan was on and we let the RTV cure for a little bit, we added several quarts of ATF until it showed on the tranny dipstick and then gave the engine a quick fire before shutting it off. We checked the tranny fluid once more to make sure it was showing on the dipstick and then started the engine and put the transmission in Neutral. Most automatic transmissions require the gear selector to be in Neutral when checking the fluid level. With a safe level showing in Neutral, we gently shifted into Reverse, then Drive, then checked the fluid level again, adding as necessary and repeating the process until the level was in the middle of the hash marks on the dipstick tube. We test-drove the Jeep, checked for leaks, and called it job done.