Truck Maintenance Tip: Don’t Forget the Differentials

The engines and drivetrains found in today’s trucks are incredibly powerful and wonderfully complex. In order to keep everything running smoothly, periodic oil changes are required. Failure to perform this maintenance on your truck can result in expensive repairs costing thousands of dollars. Everyone knows how important it is to change the engine oil on a regular basis, but for many owners, the differential’s get scant attention, if at all. That’s too bad, because changing the oil in a differential is important. Like an engine, metal-to-metal contact in the differential creates heat from friction that over time wears down parts. There’s no doubt about it, failure to keep fresh oil in the differential will inevitably lead to a breakdown that could leave you stranded on an outing.

At this point, it’s important to understand what a differential does and where it’s located. If you look at your rear axle, you’ll see what looks like a large pumpkin in the center. That’s your differential. The ring gear and drive pinion inside the differential perform two important functions. One, they transfer power from the driveshaft to the rear wheels. Two, they allow the inside and outside of your wheels to spin at different speeds during turns. This difference in speed is needed because the outside wheel travels farther than the inside. There’s also a front differential on four-wheel drive trucks, but it typically doesn’t get as much of a workout as the one in the rear.

How often should you change the oil in your rear differential if you haul and tow a lot? Every 30,000 miles? Every 60,000 miles? The answer is neither. For those who work their trucks hard, the oil in your rear diff should be changed every 15,000 miles. Why? Because of the work loads we place on our trucks. Hauling around a 3,000-pound camper or towing a 15,000-pound fifth wheel is hard on a rear differential. If you don’t believe me, take a reading of your rear differential with a temperature gun after driving for a couple of hours on the freeway. You’ll be shocked at the number. As for the front differential, unless you use four-wheel drive a lot, the oil in it can be changed every 30,000 miles. If in doubt, consult the maintenance manual for your truck.

When it comes to changing the gear oil in your differential, you have two choices on how to get it done. You can do the work yourself or you can take it to a shop. Doing the work yourself, of course, can save you big money. The AAM axles on my truck take a total of 6 quarts of oil (2 quarts front and 4 quarts rear). At $20 a quart, the total job will cost you $120. That’s much cheaper than the $300 or $400 a shop will charge you. The choice is yours, unless you have a special lifetime warranty that requires that you take your truck into the dealership for servicing.

Changing the Differential Fluid

If you’ve never changed the oil in your differential it’s actually pretty easy. To do the job, you’ll need an oil pan, a drop cloth, gloves, and the required tools to remove the cover and reinstall it. If you’re lucky, your differential will have a drain plug on the bottom. If it doesn’t have a drain plug, then you’ll need to remove the entire cover to change the oil. Differential oil smells pretty bad so be prepared for the stench. To remove the cover, unscrew the mounting bolts, leaving a bolt on top partially removed to hold the cover in place. To remove the oil, use a putty knife or flat-bladed screwdriver to gently pry open the cover. Let the oil drain completely, then remove the cover.

Thoroughly cleaning the differential after draining the oil is important. If this is your first oil change, you should assume that the leftover oil is filled with metal shavings. Because of this, you’ll want to thoroughly wipe down the inside of the housing and the cover with a rag to ensure that all shavings are removed. Before reinstalling the cover, you’ll also want to ensure that the mating surfaces of the housing and cover are also cleaned using a lint-free cloth and brake cleaner. And speaking of brake cleaner, some owners like to use it to clean the insides of the differential. If you do this, make sure the brake cleaner is fully evaporated before refilling the differential with oil. The last thing you want is any residual amounts of brake cleaner messing with the viscosity of your oil.

Reinstalling the differential cover and refilling the differential is pretty simple. Most differentials have a gasket, you’ll want to reuse it. If not, then you’ll need to use a liquid gasket product designed for extreme heat and oil exposure like Permatex Ultra Black. Don’t skimp when buying the oil for your differential. Use a synthetic gear oil like Royal Purple. Most newer trucks, including my Ram 3500, call for a synthetic GL-5 SAE 75W-90 gear oil, though some owners prefer to use a heavier 75W-140 oil for the rear differential. Refer to your owner’s manual if in doubt. If you’ve got the clearance, fill the differential directly from the bottle. If not, use a hand pump like the Amsoil G2039. Hand pumps like these are pretty cheap and can be bought any auto parts store. Fill the oil to the bottom of the plug hole until the oil starts dripping out. Reinstall the fill plug and torque it to spec. The job is done.

AFE Aftermarket Differential Cover

The procedure outlined above is primarily for differential’s without a drain plug. The process takes longer and is messier if your truck’s differential doesn’t have a drain plug, but it doesn’t have to be that way. You can buy an aftermarket differential cover that has one. This is what we did. We bought an aluminum AFE Power diff cover (Part. No. 46-70010) for my 2013 Ram 3500 pickup truck. This particular cover is very functional. Not only does it have a drain plug on the bottom and a fill plug on top, but it also has an overfill plug on the side. We also purchased the optional deep reach magnetic fill plug and optional oil level sight glass for the overfill port.

As you’d expect, installing the AFE Power differential cover was quick and easy. For the job, you’ll need to use the enclosed O-ring which is basically a rubber gasket that is installed on the back of the cover. Some aftermarket covers may call for the use of the OEM gasket, but not this one.

The AFE Power diff cover is a great addition to our truck. Not only does it look great, but it keeps the oil cooler through the use of cooling fins and a larger reservoir (this particular cover holds 5 quarts compared to the 4 quarts OEM). These features are nice, but honestly, the main reason we bought it was for the drain plug. Having it will make future oil changes fast and easy since removing the cover will no longer be necessary. With rear diff oil changes occurring every 15,000 miles, this time-saving feature will be welcome. We bought our AFE Power differential cover on Amazon.com for $230. This price does not include the oil sight glass and deep reach magnetic fill plug.