How to Add Fluid to a Rear Differential

by Eli Laurens
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Differential gears on vehicles turn the angle of drive energy ninety degrees to rotate the wheels. These gears are housed inside a casing at the center of the rear axle, and you must remove the case plate to replace the fluid. The main gear and the related hardware have to be lubricated with a heavy gear oil that's designed specifically for differentials. You should be able to drain and refill a differential in about 30 minutes.

Step 1

Park the vehicle on level ground and set the parking brake. Locate the differential under the back of the vehicle and place a large drain pan underneath it. Remove the rear plate bolts with a socket wrench. As the bolts come loose, gear oil will drip out, and slowly prying off the cover with a screwdriver will release the plate's seal. Fluid will come gushing out at first, and it will take a few minutes to drain entirely. Be sure you allow all the old fluid to drain out, as it may contain small metallic particles from the gears that can compromise the effectiveness of the new fluid.

Step 2

Scrape the old gasket from the cover plate and the differential casing with a screwdriver, being very careful not to damage the surfaces of either one. Wipe the interior of the cover with a dry, lint-free cloth. Apply a bead of RTV sealant to the lip of the cover, then press the new gasket into the correct position over it. Place the cover plate in position on the differential casing and hand-tighten the bolts. Once the cover is in place, tighten all the bolts until some of the sealant oozes out the sides of the connection. Wipe off the excess sealant, and allow the cover to dry for at least an hour.

Step 3

Turn the differential's refill bolt with a socket driver in a counterclockwise direction. The refill bolt is usually on the passenger side, in front of the axle about halfway up on the differential casing. The small plug will unscrew and reveal the refill opening. Pour the correct differential fluid through this hole until it seeps back out. When the oil reaches the bottom of the refill hole, the differential is full. Place the plug bolt back in the hole and tighten the bolt clockwise.

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