How To Change Your Truck’s Differential Fluid

Difficulty: Medium
Cost: Less than $50
Time Required: 1 – 2 hours
Special Items Needed: 1/2″ Socket, 3/8″ drive, RTV silicone, diff gasket, gear lube
It’s a good idea to drain and fill your differentials every 15k-20k miles. This does a couple of things. First, it lets you take a look at your gears to make sure everything is good, and it gets contaminants (and water) out of your diffs.

Note: If you have a limited slip system on your axle, you must add an additive. Check out your manual for specifics.

Differentials are a little different depending on the type of axle. My 95YJ has a Dana 30 axle on the front and Dana 35 on the rear.

There are a couple of ways to change your diff fluid. Most oil change places will pump the old lube out and replace it with new. You can do the same with a hand-style pump that you can get at AutoZone, etc for about $10.

However, this may not take all the contaminants out of the very bottom, or water, since oil will float on top. This also doesn’t give you an opportunity to check your gears for bad wear.

Therefore, taking the diff covers off, draining the fluid, replacing the cover, and refilling is recommended. This also gives you a chance to paint over any trail damage to the diff covers.

I started out on the front diff. There is a bolt in the cover that can be removed with a 3/8″ socket drive. It’s easier to do this before you take the cover off, so you can get some muscle to it, in case it is tight.

To remove the bolts around the edge of the diff cover, you use a 1/2″ socket. Make sure to have an oil drain pan under the diff as you take the cover off to keep from spilling the fluid onto the ground.

After removing the diff cover and draining the oil, make sure to get all the small pieces of metal and oil out of the very bottom of the diff. Clean off the areas where the cover and diff come together. I used some sandpaper and a razor blade. I also took the diff cover and put a nice coat of black rust preventative paint on the cover to cover up some prior injuries.

After cleaning, painting, etc, it’s time to put the cover back on. Some people say to use a gasket, some say just use RTV. I decided to use both. I used RTV Blue and put a thin bead on the side of the gasket against the diff.

I then placed a larger bead on the cover including going around the bolt holes. Place the diff cover on, and tighten down the 1/2″ bolts. 30-35 ft.lbs. should be enough. Give the RTV a few minutes to set up, then refill.

A fill hose can be made by taking a short length of hose, about 2 1/2′ and attaching it to a funnel. (Check out the picture.) Take the hose and stick it in the fill hole, and start filling. The front diff takes a little over 3 pints, the rear takes 2.5 pints.

I used Mobil-1 synthetic 80w90, but the manual calls for 75w90. You can either use the hose or just use the bottles (they squeeze). I did a little of both. Fill the diffs to 1/2″ below the fill hole. On to the rear!

The rear diff was the same, except the fill hole has a rubber plug instead of a bolt. AND, it was harder to fill up. Besides that, they are the same. After filling, etc, wipe the area down really well, and wait for a few minutes to check for any leaks around the seals. Congrats! You’ve just changed your diff fluid and saved yourself some money.

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