If you don't have a drain plug, unscrew the housing bolts, leaving a couple of bolts on top loosely attached to hold the cover in place. Using a standard screwdriver, pry open the cover gently or the oil will gush out and cover you in that unholy stink. Be careful not to mar the surface of the differential housing. Let the oil drain completely, then remove the cover. Assume that all the leftover oil in the axle is loaded with metal shavings. If you're an oil-changing Boy Scout, you don't have to worry about this, but the rest of us should take the time to wipe the remaining oil out of the housing, the gears, and the wet side of the housing cover.
Make sure to get it all, because there could be some shavings hiding in the nooks and crannies. Once the cover is shiny, run a magnet around the inside to pick up stray metal shavings.
Clean the tip of the fill-hole plug too; most are fitted with a magnet to grab fine metal particles. Don't go crazy with harsh cleaners—you wouldn't want the residue to affect your new oil. Grab a razor scraper or light abrasive pad and clear off the mating surface of both the housing and cover. Wipe down both faces using a lint-free shop towel and brake cleaner. Some cars have pre-made gaskets. If not, use a liquid gasket product designed for harsh conditions and oil exposure, such as Permatex Ultra Black.
Lay a single bead on the mating face of the cover and draw a circle around each mounting hole, then bolt the cover in place with just enough clamping force to flatten the bead.
Let it harden according to the instructions, then tighten the bolts to your vehicle's specs with a torque wrench. Use the highest-quality gear oil you can afford to fill the differential. The weight and capacity will be listed in your owner's manual; your differential will usually hold as much as 3 quarts.
Be sure to read that manual, though, because some limited-slip differentials require a secondary friction-modifying additive. While a drain plug exists in some rear differentials, other rear differentials have a housing cover that needs to be removed instead.
Have a wide catch pan ready with a plastic drop cloth underneath it. Turn on your vehicle and drive it around the block for a couple of minutes so that the oil can be warmed up. Then, change into some work clothes and get ready to get dirty. Of course, you could just hire a mechanic and save yourself the trouble of getting dirty.
Plus, going to a professional will ensure the job is done right so that you have more safety and less risk of mistakes being made. Note that the cost above only applies to a single differential housing.
If you have all wheel drive, you may need to double the cost depending on your maintenance interval. When driving your car through a turn, or corner, the inside drive wheel — closest to the inside of the corner — travels less than the outside wheel does.
If the wheels were spinning at the same speed, then the car would easily fishtail around the corner, but having a differential in between the two drive wheels prevents the car from doing that as it allows the wheels to move independently. Inside the differential is a cluster of gears that spin the wheels independently, however, these gears need lubrication in order to move more freely and not heat up too much due to the constant friction, hence the need for differential fluid in the housing.
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