This write-up covers the replacement of the pinion seal and requisite gear oil refill for my 2002 Mountaineer; this should be directly applicable to any 3rd Gen owner and to some degree any 8.8” differential. Expertise: Moderate. This job can be accomplished with common hand tools and a little help from the local parts store. A little ingenuity can also be quite advantageous. Obligatory Disclaimer: Do not do anything you are not comfortable with. I take zero responsibility for anything that may occur as a result of repairs or the act of repair you undertake. Proceed at your own risk! That said, this is overall a straight-forward project that can be completed by just about anyone on a Saturday, or an evening by someone who is proficient. Parts: Pinion Seal – I used Motor City Oil Seal S-3604. The key number is 3604, as you will most likely get a Timken seal like the sleeve. Readi-Sleeve – I used Timken part number KWK99181. Any sleeve that matches this one will work. Gear Oil –Two quarts of 75W-140 synthetic gear oil is needed, along with one 4oz. container of friction modifier for the limited-slip differential. I used Super Tech gear oil, but Royal Purple or Amsoil lubes can be had for only a few dollars more if you order ahead of time. The friction modifier additive is easiest and cheapest to come by straight from the dealership. Ancillary Fluids – RTV and blue Locktite are needed. Cost: Seal – $5 from the local parts store. Sleeve – $20 from RockAuto.com, before the 5% discount and shipping. Gear Oil – Royal Purple can be had online from retailers like Amazon for $15 per quart. Amsoil sells there version for about the same price. The quickest and cheapest option is Super Tech from the local WalMart, at just about $10 per quart. The Motorcraft Additive Friction Modifier should be ~$6 from the dealer. Ancillary Fluids – This is the sort of stuff a lot of folks have already. If not, account for $10 to pick them up from the parts store. “Specialty” Tools – Although this job only requires standard hand tools, there are a few that most likely are not in your average tool box. A 12-point 12mm wrench is needed to remove the driveshaft. A typical socket and ratchet will not fit on the bolts, so I have found a combination wrench to be the safest bet. A 1-1/16” socket is needed to remove the yoke flange retaining nut, and the typical tool box stops at one inch or less. A ½” to ¾” adapter is needed to get the differential drain plug off, and unless you work on heavy equipment you have probably never needed a tool of that magnitude. $20 at Sears or Lowes should get you these three tools, and the assurance that it has a lifetime warranty. A puller for the yoke flange will also be needed. If you do not have a use for having one hanging around the house, you can easily borrow one from the local parts store as a loaner tool. TOTAL: <$100 Tools, Necessary: 1-1/16” socket 12-point 12mm wrench ½” and ¾” ratchets, or adapter Breaker-bar (possibly with cheater pipe) Crowbar Paint marker Drain pan Chisel Tools, Rented: Three-pronged puller Tools, Optional: Ramps or jack stands Rubber mallet Filler hose Soup can Gloves Beer Part #1: Leak Verification Since you are here, one would hope that you already know what you are fixing. However, in the spirit of diligence, let us go ahead and verify that the proper issue is being rectified. Perhaps you have noticed a few drops of fluid in your parking space, in the general vicinity of the rear end. Or better yet, maybe you were doing a routine visual inspection of the undercarriage and noticed oil seeping from the differential. There are two typical locations for a rear end leak. The first is around the differential cover. This happens as the seal wears out over time, just like many seals throughout the vehicle do…especially if the fluid is not maintained. The second is the pinion seal. The driveshaft can wear a groove in the shaft, resulting in a leak path through the front seal of the differential. It is relatively easy to verify the origination of the leak. If the oil is only apparent around the differential cover, that is the culprit. If, however, the leak can be followed forward and up the differential to where the drive shaft meets, the pinion is at fault. A pinion seal leak will look something like this: Notice that the oil does leak down from the front of the differential to the back, possibly giving the appearance of a leaky rear cover. This is why it is good to verify. Part #2: Drain/Fill Plug Check The plugs on the differential cover can potentially be a critical road-block. To be sure that you are not stuck with your daily transportation dead in the driveway because you have no way to refill the gear oil, it is wise to first check that the plugs can be removed. Start with the fill plug since this is the highest point and if it is seized, no-harm no-foul. The smaller of the two plugs is for filling, the larger for draining: Get a ½” ratchet or breaker-bar and insert it into the fill plug: You should be able to get this loose by hand. A breaker-bar and/or cheater pipe may be helpful. Remember that the goal is simply to loosen the plug, not remove it at this point. Once you have successfully loosened the fill plug, move on to the big-ol’ drain plug. This plug is designed for a ¾” ratchet! A ratchet or breaker-bar that size is quite expensive, and something that most would never have a second use for. A ½” to ¾” adapter is much easier and cheaper, and the path I chose to take: Put whatever form of ¾” wrench that you have on the plug: If you are lucky (or Arnold Schwarzenegger) you might be able to bust the plug loose. If all you manage to accomplish by cranking on the drain plug is herniation, all is not lost. With a floor jack you should be able to get the plug loose like I did: Be very careful doing this; at first I was afraid the breaker-bar might actually blow apart at the joint. I jacked the breaker-bar one pump at a time. The rear of the vehicle would actually raise, then the weight would overcome the plug and it would lower back down. After a few iterations of this the vehicle stopped moving and the plug was then manageable by hand. Part #3: Drive Shaft Removal Now that we know where the leak is coming from, and we know that the gear oil can be replaced, we can actually get started on the repair. There are a couple choices to make at this point with regard to being under the vehicle. The first option is to work with it on the ground…not my preferred method. You can either use jack stands or ramps on the rear tires to gain elevation and much needed head room. The jacks are useful because you can then put the transmission in neutral and rotate the driveshaft around, making for easy access to all four driveshaft bolts. However, this puts the strain of removing the flange nut on the emergency brake and will most likely not be enough. The ramps do not allow for the driveshaft to be freely rotated, but the weight of the vehicle takes the strain of removing the flange nut. I prefer ramps, so I loosened the driveshaft bolts that I could reach prior to driving up the ramps, then removed the rest: Regardless of which method is used, always keep safety first. Remember that on ramps there is nothing to prevent the vehicle from rolling back, other than gravity and the emergency brake. With a jack, ALWAYS use jack stands prior to working on the vehicle. With the vehicle adequately secured, it is time to crawl under there and get to work! Start by removing the four 12mm bolts at the rear of the driveshaft: As previously mentioned, these bolts are 12-point. Due to the way the driveshaft sits, I have never had enough space to get a socket on two of the four bolts. As such, a combination wrench is recommended. If the wrench does not provide enough leverage, supplement it with a cheater pipe or a swift blow with a mallet…working on stuff is just not as much fun until you get to hit it with a hammer. Once the bolts are loose, a 6-point 13mm socket is useful for speeding up the removal process: With all four bolts removed the driveshaft can now be broken loose from the place it has probably never left since leaving the factory. Over the years the driveshaft and flange have become quite happy together, so a crowbar is typically needed to break them apart: The driveshaft weighs about 30 pounds, most of which is still on the other end connected to the transmission. Still, please be cautious and do not knock yourself unconscious when you pry it off: Now head to the other end of the driveshaft, at the transmission: The driveshaft simply slides out of the transmission: Be careful removing the driveshaft, as the pinion seal on the transmission is easy to knick. Now is as good a time as any to verify that the transmission pinion seal is not also leaking. If it is, it is easier to replace it now as long as the driveshaft is off. You can notice that mine is a little dirty, but was not leaking with any consistency to worry about: The larger section of the driveshaft right at the transmission is a damper and accounts for a lot of the driveshaft weight. As such, be even more careful not to knock yourself unconscious when the driveshaft comes free: It is always good to have your wife available to administer first aid should you decide to give yourself a quick nap (or to assist in taking pictures). Part #4: Differential Drain Now we can migrate back to the rear end. Before yanking the yoke flange off, it is best to let the differential drain so that fluid is not running out of the end of the differential you will be working on. Slide a drain pan under the differential and remove the ¾” drain plug that you worked loose earlier: About two quarts of fluid should drain out. If your differential is like the vast majority out there, it is most likely long overdue for a fluid exchange. The gear oil will probably be a brownish color, and smell like…well, to be honest it smells like something only appropriate for a mature audience, so just be prepared when you catch a heavy whiff of it: Once the old oil is drained out you will want to cap off the source of that unholy smell right away. Lightly apply blue Locktite to the drain plug threads: Now the plug can be reinstalled, just cinching it down as tight as you can: That wraps up the business end of things, and you can get back to the task at hand. Part #5: Yoke Flange Removal With the driveshaft out of the way and the gear oil drained, the yoke flange can be accessed for removal. There is a large 1-1/16” nut that holds it onto the spline. The proper procedure would be to discard the nut, and replace using XXXX torque to set the preload. Since doing that requires torquing that will just about rip your arms off, I chose the typical method of returning the original nut to its original position. To accomplish this, the nut, yoke flange, and spline need to be marked with a paint marker: I went a bit overboard with the marker, but it is better than rubbing off a small marking and not being able to see it when you go to put things back together. With the marking done, the nut can now be removed. It helps to keep track of how many turns you get the nut off, but in reality the torque of putting it back on will not go unnoticed if you try and leave it looser than you found it. Toss the 1-1/16” socket onto the breaker bar, and throw the cheater pipe on the end while you are at it for good measure, since this one is almost as bad as the drain plug: Spin the nut all of the way off, and the flange should now be ripe for the pickin’: Although the differential has been drained, it does not hurt to slide the drain pan under the flange to catch any remaining fluid that may drip…you really do not want to be rolling around in that stuff: Flange removal requires the use of a puller, which you most likely got as a loan-a-tool from the local parts store: In this case, some assembly required is an understatement. It may take more time to figure the tool out than it does to do the rest of the job, so a degree in engineering might come in handy here. Once you get the contraption figured out, get it secured to the flange something like this: As the center of the puller is driven down by the socket, it pulls the flange off of the spline: After a bit of wrench spinning, you should end up with the flange in your hand and the differential looking something like this: You can see in the picture that there was still a bit of fluid dripping from this end, all the more reason to keep the drain pan nearby. Part #6: Pinion Seal Removal The next step is to fix the source of the leak. It is always good to know what you are looking at and how it works, so the next two pictures should help. The pinion seal is an interference-fit with the front of the differential, and looks like this: When installed, the flange interfaces with the seal like this: Removal of the old seal is best done with a slightly dull chisel or screwdriver. It is important that the implement is not sharp, as you do not want to gouge the differential itself; that would create a whole new set of leakage issues. Slide the chisel up against the seal, then work it loose by working around the seal with the chisel and a hammer: Again, remember to be gentle so as not to scare the surface of the differential! Once the seal is off, the differential should look similar to this: There may be some sealant still stuck around the mating face where the pinion seal was seated. Gentle scraping and some cleaner should be used to get that surface as clean and smooth as possible. Part #7: Pinion Seal Installation The repair continues with the installation of new parts. Here is a comparison of the old and new pinion seals: I have often found installation of these seal quite aggravating. Since the seal must be forced into place, it is very difficult to keep it straight and prevent it from popping out on the opposite side you are driving in. The trick is to use something to distribute the load around the entire seal, and whack it forcefully to get it started good. Here is my make-shift pinion seal installation tool: The open end of the soup can fits nicely into the groove of the seal, making it perfect for driving the seal into the differential: Spread a little RTV around the leading edge of the pinion seal, then seat it by hand into the differential: With the seal as even as possible, put the can around it (still on the differential, unlike the picture) and smack it with a mallet: Once the seal is well seated, you can work your way around the seal hitting it to get it all the way onto the differential: Here is a parting shot of the courageous can that so willingly dedicated itself to the cause: Part #8: Flange Sleeving Time to get back to the yoke flange that was removed and address one potential cause of the leak. Here it is, after cleaning: The outer edge of the center of the flange is the contact point with the pinion seal. As it rotates millions of times, a groove can be worn into the surface that can cause the leak. The easiest, and cheapest, solution is to sleeve the flange so that it has a smooth surface again: Put RTV around the lip of the flange for the sleeve to slide onto: Place the sleeve onto the flange, ensuring that it is straight: The sleeve comes with an installation tool that fits over the sleeve: Gently tap the sleeve all the way down onto the flange: That should leave you with a renewed and rejuvenated surface like this: Go ahead and clean up any excess RTV that pushed through, giving a nice finished product: Part #9: Flange Installation With the yoke flange once again ready for action, it can be reinstalled: Start by smearing RTV onto the spline (not the threads!): Slide the flange back on, aligning the white marks if you can, and apply blue Locktite to the threads: Now you can He-Man the 1-1/16” nut back on there with the breaker bar: Once you feel like the nut can go no farther, check your progress; it can probably go farther: Get the white marks lined back up, then catch your breath. Part #10: Differential Breather Check Another potential culprit for the leak is the differential breather tube. It comes out of the top of the differential and runs up into the rear passenger side quarter panel. If this is plugged up or damaged, it could force the fluid out the front of the differential instead of venting any positive pressure. It is easiest to get in there by first removing the spare tire. Head into the back hatch of the vehicle: Remove the floor plate and lower the tire with a ¾” socket: Leave plenty of slack in the hoist line and slide the tire out of the way: This should give a clear view to the differential. The breather hose is outlined on the right side of the picture, and connects to the top of the differential just out of view: Once you crawl under there, you can see the connection: There is a plastic fitting on the end of the hose that simply slides out of the differential: In my case, the line had actually broken a few inches from the fitting: If this happens close enough to the fitting, you can still use the hose that is in the vehicle and just slide it down a few inches. Just take the fitting off the broken hose: Then clean up the end of the remaining hose. This is the best point to go ahead and blow into the hose and make sure there are no obstructions. You should be able to breathe relatively easily into the hose. If you cannot, replace the hose. Now you can reinstall the fitting: With the hose a little shorter, you need to follow the hose up into the quarter panel and pull it down a little bit to make up the difference. Here is a look up into the quarter panel: Looking further up the quarter panel reveals the check-valve, circled: With the extra slack pulled down the breather hose, the fitting can now be slide back into the differential: That should cover any potential causes of the leak! Part #11: Differential Fluid Fill Leaving the spare tire out of the way for the time being makes refilling the differential that much easier. This will require two quarts of 75W-140 and 4oz of friction modifier for the limited-slip. Double check your axle code and verify if you have limited slip, otherwise use the fluid specified in the owner’s manual. Please ignore the third quart of oil in the photos; my 2002 owner’s manual still called out the amount needed for a solid rear axle, not the independent rear. Get the rear end ready by removing the already-loosened fill plug with the ½” ratchet: Pull the seal off of the oil bottle and chop a good sized chunk off of the tip: It is then easiest to put a foot of tube on the end of the bottle; I used ½” clear hose: Before trying to squeeze the oil into the differential, start by setting the bottle down and getting the tube well situated in the fill hole: Then you can invert the bottle and let gravity do the work, assisting with an occasional squeeze: Do not squeeze too much, as that could pop the tube off and make a mess. With the first quart empty, give the friction modifier bottle the same treatment: The friction modifier is thicker, and in a smaller bottle, making it difficult to transfer the fluid easily: I pulled out the closest sharp object I had and made a quick breather hole in the bottle: This made it flow much quicker, and squeezing the fluid out much easier. Swap the cap from the first bottle of gear oil onto the second bottle, then continue filling the differential slowly this time: The differential will not hold the entire bottle, so you want to go slow and be ready with the drain pan for when fluid starts flowing back out of the fill hole. Keep the fill hole plug on standby, and be ready for when you see this: Notice how clean, clear, and not stinky the new stuff is?!? Toss the fill plug back on there as quick as you can: Snug the fill plug back down with the ½” ratchet, and you are almost done. Part #12: Driveshaft Installation The only remaining step is to put the driveshaft back where you found it. That should be pretty simple at this point. Slide the weighted end back into the transmission, being careful not to damage the seal: Work your way down to the other end of driveshaft and hold it in place with one of the 12mm bolts: You can then push up on the driveshaft and get it seated, running the other three bolts in: Crank all four bolts down, and you are ready to go for a test drive! With the vehicle back down on all four tires, start it up and just run it into forward and reverse a time or two. It is even better if you can have someone else do that while you look underneath to watch and make sure that nothing is loose or leaking. Once the stationary check is complete, go for a spin around the block. Pay attention for any new noises, which there should be none of if you followed the steps laid out here. Get back home, grab a beer, and enjoy the fact that you just saved hundreds of dollars on a repair bill!