Truck/SUV Maintenance:

Changing Differential Fluid
In A Rear-Wheel Drive Car

(And Fixing A Leaking Cover Pan )

By Bruce W. Maki, Editor

In This Article:

The cover pan is removed from the differential, cleaned and reinstalled. Differential is filled with new gear oil and checked for leaks. Discussion of maintenance interval.

Related Articles:

Skill Level: 3 (Intermediate) Time Taken: About 2 Hours


This 1999 GMC Jimmy had been leaving small puddles of dark oily fluid under the back end.

The red arrow points to the rear differential, which has that characteristic round bulge in the middle of the axle.


I jacked up the SUV and placed jack stands under the axles directly below the leaf springs.


Raising Vehicles Safely:

Always use proper jack stands when working under a raised vehicle.

Jack stands must be placed under a sturdy part of the vehicle, such as the axle or a level portion of the frame.

Make sure the jack stands are sitting on a solid surface, such as concrete.

Follow the jacking recommendations in your vehicle's owner's manual. In this article I placed the floor jack under the differential, but some car manufacturers advise against this practice since it could damage the differential.



I dropped the spare tire to give me more room to work.

While the spare tire was off the car, I took a few minutes to

  • check the air in the spare,
  • clean the dirt off the backside of the rim, and
  • spray white lithium grease on the cable winch mechanism that is used to lower and raise the spare tire.


The differential cover pan (inside the red circle) was all wet with gear oil.

This vehicle has an anti-sway bar (red arrow) that is placed just a few inches behind the differential. This suspension component complicates an otherwise straight-forward job.


I used a ratchet and a 13 mm socket to remove the bolts.


At the top of the differential, there were 3 metal brackets that held the brake lines in place.

These brackets and brake lines make it more difficult to remove and replace the cover pan.

Brake lines are not easy to move out of the way, since the brake lines are quite stiff. Bending a brake line can impair proper brake function and possibly cause a line to leak.


Two of the cover bolts couldn't be removed with a socket because they were behind the anti-sway bar. I used a 13 mm wrench to remove these bolts.


Then I pried on the cover pan with a flat-blade screwdriver...


... and this thick oil started pouring out.

By the way, I had a big plastic oil drain tub underneath the differential to catch the fluid.


When the oil had mostly drained out, I slid the cover pan down and out. This wasn't easy because the anti-sway bar was so close to the differential.


A view of the inner workings of the differential. These hypoid gear mechanisms are trouble-free... as long as the oil level is kept up and changed occasionally.

The oil will continue to drip for hours, and can drip onto the mating surface when the cover pan is re-installed, interfering with the silicone gasket material.

I sprayed some automotive brake parts cleaner into the differential to remove the thick gear oil. Then I used paper towels to mop up the liquid. I don't want any solvent left behind to dilute the new gear oil.


I used a razor scraper to scrape off the old gasket from the differential case.



Cleaning Up The Differential Cover Pan:


The cover pan had most of the gasket still stuck to it. This thing was really greasy and dirty.


I scraped off the gasket with the razer scraper.


I put the cover in a dish pan of mineral spirits and used a parts cleaning brush to rinse off as much dirt and oil as possible.

In a mechanics shop we would use a "parts washing tub", which is a big basin with a brush that has solvent pumped through it. I don't have one yet.


I used a wire wheel on a bench grinder to clean up the mating surface of the cover pan.

Then I sprayed the pan with brake cleaner and wiped it with a paper towel.

Cleaning the mating surfaces is important. If a wire wheel is not available then a wire brush can be used... it's just slow and tedious.


I cleaned up the differential case with a Roloc abrasive disc mounted in a die grinder.

But... I couldn't reach behind the anti-sway bar, so I cleaned those spots with 220-grit sandpaper.


The differential case after being cleaned.

I hosed it down again with brake cleaner to wash out the grit and debris.



I applied a thin bead of automotive RTV silicone around the rim of the cover pan.



It's my understanding that the silicone should go around each bolt hole.



I slipped the cover back into place. This wasn't easy with the anti-sway bar and the brake lines in the way.

This difficulty made me wonder if a better approach would be to apply the silicone to the differential case instead of the pan. I've always applied the silicone to the pan, but this was the first time I'd done a differential service on a vehicle with an anti-sway bar adjacent to the differential.


I aligned the bolt holes and installed the bolts. It's important to get ALL bolts started before fully tightening ANY of them.

I tightened the bolts with a 13mm socket, and used a box-end wrench where a socket wouldn't reach.

It's important to NOT OVERTIGHTEN these bolts, or the silicone will be squeezed out.


Filling The Differential With Gear Lube:

I let the silicone dry for about an hour. I don't want to take chances with the gear oil affecting the silicone, or vice-versa.

This small plug covers the fill hole for the differential fluid. The plug has a square recess that fits a standard 3/8" ratchet or extension.

Many vehicles have a fill plug on the cover pan. Some plugs are just rubber grommets.


I used a 3/8"drive ratchet with a short extension to remove the fill plug.


The usual method of filling the differential involves sticking a quart bottle of gear oil into the fill hole and squeezing the bottle to encourage the thick oil to flow.

I used 80w-90 gear oil, which is usually the lubricant used for differentials and manual transmissions.

Check your owners manual for the exact specifications.


I also tried this method: A pump device that threads onto a quart bottle of oil.

This was kinda slow and tedious, but it might be the best way for some vehicles.



The clear flexible tube easily fit into the fill hole. But... you need to make sure the tube doesn't fall out while you're pumping the oil.


The normal method of filling a differential is to keep adding fluid until it drips out of the fill hole (arrow), then replace the plug.

After I replaced the fill plug I lowered the car from the jack stands and drove it out of the garage. I figured I was done.


But Wait, There's More :

After I changed the fluid in the differential it didn't leak. For one whole day.

By the second day there were more drops of oil appearing beneath the rear end of the car. I put the car back in my garage to examine the differential again.

There was a wet streak running about 3/4 of the way up the cover pan. I suspected that the cover pan itself was leaking, and not the silicone gasket material.

To find the leak, I sprayed the cover pan with brake parts cleaner and dried it with compressed air. Then I had the owner drive the car around the block. I saw a small spot of fluid about 3/4 of the way up the cover pan. After twenty miles of driving the wet spot had almost reached the bottom of the differential. I had found my leak.


After I carefully pried the brake line out of the way, I sanded down the metal with a die grinder and a sanding disc.

Using the corner of a file, I poked through the cover pan (arrow).


I mixed up some JB Quik (fast-setting JB Weld epoxy) and applied it to the clean metal. You can see a dimple where the hole was. After this patch, the leak was fixed.

JB Weld seems to stick to bare steel, but it will adhere better if there is a hole so the goop can flow around the edge of the metal and clamp on. Sometimes I have actually drilled extra holes so the JB Weld can surround the metal.

Why did the cover pan rust? I suspect that dirt got built up between the brake line bracket and the cover pan. Any dirt build-up will hold moisture and allow the steel pan to rust faster. Heavy steel parts don't seem to have major rust problems if water can drain away quickly.

A thorough undercarriage washing would be a good idea, especially when winter is over, or after driving on a lot of dirt roads and puddles.

To prevent this problem from happening again, I bent the brake line bracket slightly to keep it away from the cover pan.


Maintenance Interval:
How Often Should The Differential Lube Be Changed?

The short answer is: Whatever the vehicle manufacturer recommends. But don't expect to find the service interval in your owner's manual. You may need to call a dealer or look in a manufacturer's service manual.

For example, the factory service manual for my Dodge Dakota says to change the rear axle fluid as often as every 12,000 miles under "severe service" conditions such as extensive high-speed driving in hot weather. But for normal usage Dodge doesn't mention changing the rear axle fluid any time during the first 120,000 miles.

Some people suggest that differential fluid be changed after the first 12,000 miles, such as these articles:

My approach is: Do it when it's convenient. If I'm doing other work on my vehicle, and I've already got the car raised up, it wouldn't hurt to change the differential fluid. But how often, you ask? My educated guess is... maybe every 60,000 miles, more or less.

I will agree with AMSOIL in their recommendation to change the differential fluid after the first 12,000 miles, because most of the wear will occur early in the life of the gear mechanism.


Limited-Slip Differentials:

These units often employ clutch packs that are similar to the clutch packs in automatic transmissions. To perform properly, these clutching mechanisms rely on certain additives in the fluid. Limited-slip rear-ends may need to be changed much more frequently than ordinary differentials. Consult your dealer or manufacturer.



More Info:

Tools Used:

  • Flat-Blade Screwdriver
  • Socket, 13mm
  • Box End Wrench, 13mm
  • Bench Grinder with Wire Wheel
  • Pneumatic Die Grinder with Abrasive Disc
  • Dish Pan

Materials Used:

  • Automotive RTV Silicone Sealant
  • Brake Parts Cleaner
  • Mineral Spirits
  • JB Weld (2-Part Epoxy)

Related Articles:

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© Copyright 2007 Maki Media Group LLC

Written June 2, 2007