You change engine oil every few thousand kilometres, but have you ever even looked at your differential? BY BEN WOJDYLA
Changing the differential oil – it’s one of the most overlooked maintenance tasks on rear-wheel drive bakkies, SUVs and passenger cars. Because the differential is at the rear and under the car, it gets none of the star treatment that the engine up front does. However, if lubrication in the differential fails, you won’t be getting very far for very long. Fortunately, you only need to change this oil every 50 000 to 80 000 km (as always, check your owner’s manual).
The differential is a component in all cars and is designed to compensate for the difference in distance the inner wheels and outer wheels travel as the car goes around a corner. In a rear-wheel drive
car, the differential has its own housing and lubrication, a thick, dark oil. Front-wheel drive vehicles typically integrate the differential in the transmission housing and share the same fluid. The differential oil lubricates the ring and pinion gears that transfer power from the driveshaft to the wheel axles. If your car is fitted with a limited-slip differential, it also keeps all the moving parts in that assembly healthy.
Changing this oil is just as important as changing your engine’s oil, and for the same reason. Metal-to-metal contact wears down surfaces and creates heat from friction, which inevitably weakens the gears and leads to failure. Checking and changing the differential oil in a bakkie is actually pretty easy, and it’s only a bit more difficult in a car. In either case, this small procedure can save you a big headache down the road.
Prep the area
Depending on the design of your differential, this can be a very messy or a very tidy job. Some differentials have a drain plug; others require you to remove the housing cover. In either case, you’ll need a wide catch pan; a plastic dropcloth beneath that would be good insurance. Drive your vehicle for a few minutes to warm the oil, then change into your grungy clothes – you’ll probably get dirty.
Out with the old
It’s just changing oil, right? Nothing that complicated, but brace yourself, because old differential oil has the foulest smell in the automotive world. With that warning, remove the fill-hole plug at the top of the diff-casing, then unscrew the drain plug. If you don’t have a drain plug, unscrew the housing bolts, leaving a couple of bolts on top loosely attached to hold the cover in place.
Using a standard screwdriver, pry open the cover gently or the oil will gush out and cover you in that unholy stink. Be careful not to mar the surface of the differential housing. Let the oil drain completely, then remove the cover.
Assume that all the leftover oil in the axle is loaded with metal shavings. If you’re an oil-changing Boy Scout, you don’t have to worry about this, but the rest of us should take the time to wipe the remaining oil out of the housing, the gears and the wet side of the housing cover. Make sure to get it all, because there could be some shavings hiding in the nooks and crannies.
Clean the tip of the fill-hole plug too; most are fitted with a magnet to grab fine metal particles. Don’t go crazy with harsh cleaners – you wouldn’t want the residue to affect your new oil. Grab a razor scraper or light abrasive pad and clear off the mating surface of both the housing and cover. Wipe down both faces using a lint-free towel and brake cleaner.
Seal it up
Some cars have pre-made gaskets. If not, use a liquid gasket product designed for harsh conditions and oil exposure. Lay a single bead on the mating face of the cover and draw a circle around each mounting hole, then bolt the cover in place with just enough clamping force to flatten the bead. Let it harden according to the instructions, then tighten the bolts to your vehicle’s specs with a torque wrench.
Fill to the brim
Use the highest-quality gear oil you can afford to fill the differential. The weight and capacity will be listed in your owner’s manual; your differential could hold as much as 3 litres. Be sure to read that manual, though, because some limited-slip differentials require a secondary friction-modifying additive.
Fill the differential directly from the bottle if you’ve got clearance, but if space is tight, you can get a pump or extension hose to make the job easier. The bottom of the plug hole is the maximum fill line, so when oil starts dripping out, you’re finished. Install the plug, torque it to spec, and you’re good for tens of thousands of kilometres.
What’s the Diff?
A differential allows your car to take corners without drama. If both drive wheels rotated together, they’d jump-skitter because the outside tyre travels further than the inside. There are many variations on the design, but they fall into three categories: open, limited-slip and torque-vectoring.
The oldest type of differential. Its simple, rugged design is widely used. The powered pinion gear meshes with the ring gear, which in turn transmits power to both axles through a second set of gears. The only weakness? When slippage occurs, power is directed to the wheel with the least grip.
To overcome this problem, a clutch system is installed into the differential. The action of the mechanism locks the left- and right-side axles together if wheel slip happens.
This is the latest and greatest type of differential. This design employs the full array of sensors in a car to determine which wheel should be getting the most power, and uses electronic controls and clutches to direct it there.
Differential oil may have a noxious odour – due to its high sulphur content – but it’s just a heavy version
of motor oil. Pour the used oil into a large jug and recycle it. Some cities have collection depots.