What is a Differential or Axle?
If you own a rear wheel drive or four wheel drive vehicle, then you are the proud owner of at least one differential.
Differential is a term commonly used when referring to your front or rear axle; you may have even heard them called “diffs” for short. The correct usage of the word “differential” actually describes one of the parts inside the axle – not the whole axle itself – but somehow the not-quite-correct name has stuck, so we’ll use it here too.
Your axles accept torque from the engine through a spinning propeller shaft. A set of gears inside the axle direct this torque at a 90 degree angle to each wheel. These gears also provide a gear reduction, which means that the wheels turn slower than the propeller shafts, but the torque is multiplied. Without this gear reduction, your engine would not have enough torque to move your vehicle effectively. Some axles also contain a limited slip or locking device that maximizes traction by making sure that both wheels are able to turn in slippery situations.
What is Differential Fluid?
Your differential fluid is under extremely high pressure as it squeezes between the gears, providing a barrier that prevents wear. There is a LOT of torque going through your axles, and the gears inside are under a lot of pressure! In order to last, they need to be properly lubricated. Every front or rear axle is filled with a special oil that takes care of this. The differential fluid lubricates the gears, bearings and other internal parts, and provides cooling to these components as well.
Why Does My Differential Fluid Need to be Changed?
Your axles contain a lot of moving parts that rely on being properly lubricated! Like most automotive fluids, differential gear oil eventually loses its ability to do its job properly. Gear oil contains several chemicals and additives that only last so long:
- Extreme-pressure (EP) additives that protect against wear. These additives can eventually react with the water that forms inside an axle from condensation, forming acids that may damage seals and other parts.
- Detergents that capture dirt and debris, suspending it so that the contaminants cannot cause scuffing or wear. Eventually the detergents in a fluid become “full” any cannot trap any more material.
- Corrosion inhibitors that prevent rust and oxidation inside the axle. These additives become less effective as they age as well.
- Anti-foaming agents that prevent the oil from frothing as the gears spin through it. These chemicals are super important, because foamed oil has air bubbles in it, which weaken the crucial fluid barrier between the parts.
- Demulsifiers that prevent water from combining with (and deteriorating) the gear oil. These additives can only handle so much water build-up, though, and eventually the oil needs to be replaced.
Replacing your differential/axle fluid regularly will ensure that the fluid never deteriorates to the point where your axle is becoming damaged. With proper maintenance, most drivers can expect their front and rear axles to outlast their vehicle!
Inexpensive Maintenance vs Costly Repairs
Sometimes we’ll hear from a customer who wants to replace their differential gear oil because they have an issue with one of their axles. This may be a chatter or shake on turning; a noisy pinion bearing; a differential gear whine; etc. Unfortunately, at this point it’s too late; changing the fluid will not fix anything. The damage has been done, and the axle probably should have been maintained better in the years prior. It’s important to remember that replacing your differential fluid on time is intended to prevent mechanical failures, and will not cure them.
We also provide very good quality axle overhauls and rebuilds in the event of a mechanical failure. However, these services can be very expensive; sometimes costing $2000-$3000, depending on how many internal parts need to be replaced. You could usually replace your differential fluid 10-15 times for that price, which just goes to show that a little preventative maintenance (in the form of regular differential fluid changes) can really save you money in the long run.
How Often Should My Differential Fluid be Changed?
This varies quite a bit from vehicle to vehicle. We usually recommend following the recommendations of your vehicle’s manufacturer, and their suggested service intervals will be listed in your owner’s manual. If you don’t have a manual, we’d be happy to look up the service schedule for you! While we always follow official service information from car manufacturers 95% of the time, there are occasions where we will recommend a service earlier, based on our experience in the industry.
Knowing how important your differential fluid’s job is, it makes sense that you want to keep that fluid inside your axles! Some of today’s axles do not hold much oil at all, and in the event of a leak, it’s easy for them to run low – causing extensive damage. There are several locations where an axle can develop a leak:
- The pinion seal, where the propeller shaft connects to the axle.
- The axle seals (also called wheel seals, or output shaft seals) where torque exits the differential via the axle shafts.
- The inspection cover seal. Some axles have a large service cover that can be removed for inspections, or to perform small repairs.
Repairing these differential leaks is usually a fairly simple repair involving replacement of a worn seal. However, a leaking seal can also be an indicator of larger problems inside the unit, such as worn bearings. Upon inspection of any leaks you may have, our expert technicians will determine if your leak is indeed a symptom of a larger problem or not.