How to change the front and rear differential fluid in your half-ton

How to change the front and rear differential fluid in your half-ton

Posted on 22. Jan, 2009 by in How To



This article was submitted by Tom Fraley aka MountaineerTom. Thanks Tom!

The Front

This is a really simple job to do. To make the job much easier, remove the front skid plate (5 bolts) and the bottom skid plate (4 bolts), if equipped. Once you get under the truck, you’ll easily see where all the skid plate bolts are located and how easy they are to remove. You will then have easy access to the drain plug and fill plug. The lower plug, of course, is the drain plug and the upper plug is the fill hole plug. In the picture below, you can see that they are already removed.

Front Differential

The drain plug is magnetic, so when you remove it, it will have some fine “fuzzy” looking bits of metal on the tip. This is normal. Just clean the magnet off before you reinstall it.

Front Differential Plug

Metal shavings stuck to the plug are normal

After all the fluid has drained from the front differential, just reinstall the drain plug, and fill the differential with SAE 80W-90 gear oil. (GM recommends part number 1052271 or equivalent) Synthetic can be used, but is not required according to the recommended fluids and lubricants in the owner’s manual. Fill the differential to within ½ inch of the bottom of the fill hole. The owner’s manual describes two ways of check the fluid level. To check the fluid level when the differential is at operating temperature, check to see that the fluid level is filled to the bottom edge of the fill hole. Basically, remove the fill plug and if a little bit of fluid runs out, it’s good. To check the fluid level when the differential is cold, check to see that the fluid level is ½ inch below the fill hole. And easy way to check this, is to use an allen wrench to find the fluid level. Use one that is a little more that ½ inch long on the short end of the tool to dip it into the fill hole. When the fluid is around the ½ inch mark on the allen wrench, the fluid level is good.Allen Wrenches

The easiest way to add the gear oil is to buy a hand pump that is designed to fit on top of the bottle of gear oil (in place of the cap). It allows you to pump the gear oil out of the bottle, through a tube, and into the differential. They can be found at most auto parts stores. Check with your dealer to verify the part number for the fluid, because the part numbers do change occasionally.

The Rear

Changing the rear differential fluid on the 1500 trucks also a pretty easy job. There are two ways you can do it though. You can just drain the fluid out and refill it, or drain the fluid out, remove the cover, clean out the inside and magnet, reinstall the cover and refill the differential. This guide covers removing the differential cover.

Rear Differential CVR

The drain plug is located on the bottom of the differential housing, and the fill plug is located on the passenger’s side of the differential. Both are 3/8’s drive square plugs. If you’re your just draining and refilling, these are the only two things that need to be removed.

Rear Differential Open

Drain the fluid from the differential. Take the cover off by removing the ten bolts securing it to the housing. The pictures below show what it looks like before and after cleaning it up.

Rear Differential Open

When cleaning up the cover, be sure to clean off the fine metal shavings off the magnet, which is normal. Also be sure to remove any pieces of the gasket that may have been left on the cover or differential housing.

Reinstall the cover and a new gasket (GM gasket part number 26066456 or NAPA gasket part number P27857). Some people use some gasket sealant, some don’t. I applied a very thin layer on both sides of the gasket to help it seal better and to make it a little easier to remove in the future. If you do use gasket sealant, don’t use so much that it will squeeze out into the inside of the differential when the cover is tightened down.

Rear Differential Mag

The easiest way to add the gear oil is to buy a hand pump that is designed to fit on top of the bottle of gear oil (in place of the cap). It allows you to pump the gear oil out of the bottle, through a tube, and into the differential. They can be found at most auto parts stores. GM recommends using synthetic SAE 75W-90 gear oil (GM part number 12378261) or equivalent meeting GM specification 9986115. The proper fluid level for the 1500 series is from 5/8 inch (15mm) to 1 5/8 inch (40mm) below the filler plug hole. (For the 2500 series, it’s 0 to 3/8 (10mm) below the filler plug hole.)

Allen Wrenches

Here’s a tip for measuring the correct amount of fluid in the rear differential. I used an allen wrench, like in the picture below, to measure it. I found one that the short end fell within the 5/8 to 1 5/8 range and “dipped” the short end in the hole until the fluid reached the required amount. Those measurements are from the owner’s manual of my ‘99. Other years may be different, so check your manual for the correct measurement. Also check with your dealer to verify part numbers for the fluid and gasket, because the part numbers do change occasionally.

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24 Responses to “How to change the front and rear differential fluid in your half-ton”

  1. Rod

    31. Aug, 2009


    How much of each fluid will I need if I drain and fill the front and rear differentials on my 2004 Yukon.

  2. beerryunuri

    02. Feb, 2010


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  3. waleed

    12. May, 2010


    Where can I buy SAE 75W-90 gear oil (GM part number 12378261) fluid. I have to buy it online. No GM near where I live. THANKS!

  4. terrance

    04. Aug, 2010


    how much gear oil will i need.

  5. Graphic Nature Vinyl

    20. Aug, 2010


    I purchased my oil from NAPA. I used Royal Purple 75w/90 for both front a rear, then I purchased Auto Track 2 fluid from the dealer. I bought 5 qts or the 75w/90 to be safe, I will probably return 1

  6. Jim

    04. May, 2011


    A good tip I heard is to crack open the fill plug before draining the oil. That way if the fill plug is stuck to tight to open you can still drive your rig to a shop so they can get the job done. Otherwise you have an empty diff. and no way to fill it.

  7. Sean

    08. Jun, 2011


    That’s what happend to me, Good thing I had a welder to tack the fill plug, and get it out. I decided to use all Redline fluid, redoing the all the fluids to Redline this weekend. its about $300 but the dealer wanted $700 to do them all.

  8. Mike @ Conversion Vans

    31. Dec, 2011


    Anything different about the 2500 series trucks? I’m picking one up and want to do all the basic maintenance as it’s got over 100K and I don’t think it’s ever been changed. This looks like a job I can do in the Driveway, but I’m wondering if it’s even worth the time and mess… seems like a pretty cheap process even to pay someone else. Should I be worried about the wear inside a 100K truck enough to remove the cover and clean it out or should I just drain and fill? I just watched a video on Youtube of a guy who did this whole thing and the oil looked dirty and the pan was rusted, but the gears looked fine.

  9. jack

    13. Jun, 2012


    2008 Denali fill front diff 1/8″ from top of fill plug, Per owners manual.

  10. Name (required)

    24. Jun, 2012


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  11. Armando

    02. Aug, 2012


    i drive a 2007 GMC Sierra 1500 crew cab. i plan doing a rear diff. fluid change tomorrow, i know the owners manual says 1/8″ from the fill plug but once i am done i will let you know in quarts what is required. i always find it easier that way!!!

  12. Wayne Roller

    01. Sep, 2012


    If you switch your front dif fluid to synthetic (I recommend this especially in cold climate), make sure the plastic vent connector is white. If black, the o ring is not compatible with synthetic lub and will leak over time. Easy to change and only costs about $25. It is simply threaded into the housing.


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    13. Jun, 2013


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  18. Jon

    19. Nov, 2013


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  19. Jon

    19. Nov, 2013


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  20. Youtube.Com

    04. Jan, 2014


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  21. Justin

    08. Feb, 2014


    Don’t forget the additive for the rear diff! This must be added in addition to the 75W90 fluid.

  22. Will

    01. Mar, 2014


    Nice write-up.

  23. James W Cilecek MD

    21. Mar, 2014


    Removing frozen plugs. Use WD 40, then tap drain or fill plug slightly. Repeat several times over a couple days before attempting to use the 1/4 ” drive. Use heat around the opening with either a solder gun or iron to expand the metal around the fitting. Clean off WD 40 first as it’s combustible. Repeat these measures until you are able to remove the plugs. Use an adapter to convert the 1/4″ drive to 1/2″ drive with a longer ratchet or fixed wrench possibly with an extension. The more length you have, the more leverage and power you’ll have….and an easier time removing these plugs. If all else fails, use LOCKTITE FREEZE and RELEASE and spray on the fitting, but not on the surrounding areas. It causes the fitting to contract and go down to -40F…. and also applies penetrating oil too. Let stand for 1/2 hour. Repeat a day later if necessary. This is a new product and works wonders in removing frozen fittings and spark plugs especially if they are in an aluminum head. You’ll succeed. Go slow and repeat these steps without applying too much torque on the frozen fitting. After removing, clean the threads well and coat with chassis semi-synthetic grease before installing and don’t tighter too much as these levels should be checked every year to spec.. Use GM Synthetic from GM in rear and use Valvoline or Castrol Synthetic in the front, but with the proper vent as mentioned above. Amsoil is also excellent for both.. Car Doc.


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