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