Most vehicles with a rear solid axle are true workhorses. They pull hard, and most of their owners treat them well too. Since they are expected to put so much torque to the ground when heavily loaded, they rely on the solid rear axle to keep going instead of CV joints.
However, most truck and similarly designed car owners ignore the ever so important rear differential.
The reason is rear differentials aren’t regular service units. Under normal circumstances, they will keep slaving away faithfully without calling for attention. Nonetheless, changing the rear differential fluid frequently is a great way to keep it working for years to come.
What Could Make My Rear Differential Leak?
Even though your rear differential looks all solid and well sealed up, it has a couple of gaskets, seals, and joints that could come loose over time. If this happens, the gear oil within the differential will start streaming out.
Some of the top causes for a leak include:
A Clogged Differential Vent Tube
Your differential has a small vertical tube that vents the inside of the entire assembly. Its role is to release excess air and or pressure from within the differential.
The vent is on the upper side of the pumpkin (the enlarged section of the differential) or the differential tube. Ensure that it is clear of any debris. If it is not present and there’s just a stub, consider having it replaced.
A clogged vent will increase pressure in the system making it easier for the otherwise unpressurized differential gear oil to leak.
Damaged Rear Pinion Seals
The pinion is the front section of the differential pumpkin that connects to your driveshaft. This joint needs a seal to keep the oil from leaking.
If the seak is damaged, oil will drip down the differential. Mostly, this will start off as a slow leak. You won’t have a pool of differential fluid under the car but the entire center of your diff will be messy and oily.
A Leaking Differential Cover
The backside of your differential has a bolted-on cover. The joint between this cover and the differential has a seal to keep the oil in.
You will have a leak if the seal is damaged or the bolts weren’t torqued to spec. Either way, the oil will seep out and drain to the bottom of the cover by gravity. The lower section of the diff will be darkened with oil and there might be some drops dripping from the bottom end.
Perforated or Cracked Rear Differential Tubes
Any dark streaks of oil along the bottom of the differential tubes means that you have cracks or perforations.
Any physical damage, for instance hitting a rock when offroading, could damage the tube. Moreover, rusting over time could lead to a rot that punches through to the interior of the differential.
If this happens, you will see some moisture and fluid seeping from within the differential tube at the point of failure.
Differential tubes have a sticker at the bottom that gathers moisture and actively prevents rusting. Always check yours to ensure that it is intact.
Leaky Axle Seals
An axle seal sits at the end of the differential. They should be bone-dry under normal circumstances.
However, if they are cracked, they will start leaking and stain everything they are in contact with. Over time, the slow leak will develop into something bigger that can drain a substantial amount of oil from your differential.
How Do You Know if Your Rear Differential is Leaking?
So far, we’ve touched on how a smear of oil on the differential definitely hints on a leak. We are going to look at other indicators that could indicate that your differential oil has been leaking for a while.
Nonetheless, it is often wise to inspect your undercarriage and the differential often since by the time the other signs start manifesting chances are your differential is almost running dry.
A Pool of Oil Under the Rear Section of the Car
After parking for a while, it is always wise to inspect the area directly beneath your car before driving off. This is a great way to tell if you are leaking any fluid.
A puddle of differential oil leak is similar to that of an engine oil leak. This is because they are both a type of gear oil.
The only way to be sure that it is your differential leaking is by relating it to the rear of the car, exactly beneath your diff. If you peep under the car, you will see where it’s dripping from and even see sections of the differential covered in grime and oil.
Be keen when inspecting any leaks towards either of your tires. It could be a differential gear oil leak at the axle seals or brake fluid leaking from a faulty brake system.
Strange Noises and Vibrations from the Rear of Your Vehicle
A normal well-lubricated rear axle runs smooth and silently. You barely notice its there. However, as the oil seeps out and drops to very low levels, the gears remain exposed and unlubricated.
This will either cause vibrations when driving or a loud rattling and whirring noise as you drive.
If it gets to this point, stop pushing your car. Don’t drive it as you will cause more damage to your differential. Get it inspected first before filling it up. There could be damaged parts or metal shavings that need to be taken care of before sealing up the differential oil again.
An Unusual Burning Smell From Your Car
Another hint that you should check your differential oil is a thick burning smell. When differential oil is exposed to the environment, it thickens up. As the differential turns and gets hot, the oil will burn easily.
The result is a strong burning oil smell that you can trace to the back of the truck or car.
How Do You Stop a Rear Differential Leak?
The approach you use to stop the leak depends on what is leaking. Let’s look at a couple of solutions and where they work the best.
Using Some Rear Differential Sealer
A sealer additive is a great way to fix microscopic leaks leading to smears on your differential. It works perfectly for any leaks from partially damaged seals.
The procedure is as simple as adding a high-quality differential sealer through the differential fluid fill plug.
Remember that this isn’t always a sure fix. It might work or it might not. If it works, it might hold for a while. You still have to replace the aging seals as a permanent fix.
Replace the Rear Cover
If the rear cover was the problem, you will have to pull it out and reseal it. It could be as easy as tightening up the bolts if the seal is still intact.
Sometimes, you might have to get a near seal or an entirely new rear cover if the damage is too much.
Resealing the Axle Sides
The axle sides are the closest to your wheels. Their seals rarely go bad. However, if they do, you have to pull out the axles and replace them.
Even though the replacement seals are cheap, getting to the worksite is not easy. You might spend some good hours at it – or pay a neat sum in labor to have a professional replace it for you.
Replacing the Pinion Seal
The last serviceable section of your differential is the pinion seal. This is a bit easier to replace than the axle seals.
Drop the driveshaft to expose the pinion seal and get a perfect fit for your vehicle. After installing it, ensure that you torque up the driveshaft to spec to create a perfect seal.
A Rotten or Rusted Through Differential
If your car sat for long and the differential tubes are rotted through, you have very few options short of replacing the differential.
Cleaning up the damaged spot and using a welding trick, or even JB weld could help. However, the integrity of the seal depends on the size of the damage and how good you were at welding.
In my opinion, it would be better to invest in a replacement tube or complete diff instead of a slap-dash fix that might not hold for long.
How to Refill Your Differential Oil
After repairing the leak, you will have to replace the oil in your differential. You will have to drain the remaining oil before adding in a fresh batch. Here is a step-by-step guide to changing your differential oil.
Note: Some differentials don’t have a drain plug. You will have to open the backplate and manually drain all the oil from the diff’s pumpkin.
- Jack up the vehicle and stand it up on some jack stands. Ensure that it is level. Meaning that you have to use four jack stands for an all-around raise.
- Identify, clean, and loosen the drain bolt on your differential. It’s often located under the pumpkin (the large section of the differential)
- Place an oil collection pan under the drain and loosen the bolt fully
- Remove it and allow the oil to flow. Give it some time until it drains completely
- Wipe any smears off the bottom of the shaft and replace the drain bolt. Remember to torque it to spec and don’t strip it.
- Locate and open the gear oil fill bolt. It’s often to one side of the differential pumpkin
- Use a pump to slowly fill up the differential with recommended gear oil
- Add the manufacturer-specified amount of oil or keep on pumping until the oil starts spilling out of the fill hole
- Clean the inside of the fill hole bolt. Sometimes, they have a magnet that collects any metal shavings created as differential spins
- Replace and tighten the fill hole bolt. After that, clean off the differential to make it easier to note future leaks
Pro Tip: Some people prefer draining their differentials faster by opening up the rear cover plate. This will work just fine if you can’t find the drain plug or can’t open it. Just remember you will have to scrape out the old seal and apply a new one when replacing the cover.
While removing the backplate might sound more complicated, it actually lets you clean up the differential better.
With the plate open, you can clean out residual oil removing metal shavings that pool at the bottom of the pumpkin or on the cover plate. Don’t use any special cleaners. A clean rag dipped in clean diff oil should do.
How Often Should You Service Your Differential?
If your differential isn’t damaged or leaking, you can keep on driving until your vehicle hits anywhere between 30,000 and 60,000 miles. The duration depends on your manufacturer’s specification and how hard you push your truck.
Even if it’s not due for servicing, it will do you no harm to inspect your differential often or every time you drive offroad.
Personally, I check my differential and the car’s undercarriage at least once a week.
How Do I Know if My Differential Fluid is Low?
First, you can use the usual signs of a leaking differential to identify the possibility of a low level.
If you are leaking differential oil, the levels will definitely be very low.
However, if there are no leaks, you will have to physically check the levels. The only problem is there is not dipstick or a level indicator. You have to do it manually.
- Identify the fill hole. Normally to one side of the differential pumpkin
- Use some brake cleaner fluid and a brush to clean up any grime from the bolt
- Give it some minutes to dry before loosening and removing it
- Clean and dry your finger. Choose one that fits into the fill hole
- Dip your finger into the hole and note the oil level
- You should feel the oil at or just below the fill hole level
Can You Top Up Differential Oil?
You could theoretically top up your differential oil. However, this could be pointless. Normally, your differential oil will just get dirty. The levels shouldn’t dip unless:
- It is almost time to service your differential
- You are having a microscopic leak that you haven’t noticed
If you not your gear oil level is dropping, inspect all the possible leak points in your differential. Organize any repairs or take mitigation measures if you notice any problems.
If everything looks to be in great condition and the drop was very small, you could decide to replace the entire differential oil. After all, your typical differential needs less than 5 quarts of oil.
Can You Drive With a Leaking Differential?
You can still keep driving with a leaky rear differential as long as you have enough oil to keep the gears well lubricated.
However, this is bad practice as you don’t know when the oil will dip too low for your differential’s liking.
You should service and repair the differential as soon as you notice a leak. Since there is no electronic or visible gauge for the oil in there, chances are you’ve already lost a good amount by the time you notice the leak.
If you don’t notice the leak in time, your car will start whining or howling as you drive. The noise gets worse as the car picks up speed.
The same could also happen if your gear oil is dirty and contaminated.
Have your car fixed and repaired as soon as possible. Stop using it until you can fix the problem and replace the gear oil.
What Causes Rear Differentials to Go Out?
A rear differential (or a front one in this case) is a very simple and robust piece of engineering. It’s a strong tube with a couple of gears in it. All they need is some oil and they’ll keep turning for ages.
Low Oil Levels
Things go wrong when this oil runs low or ‘goes missing.’
The gears will grind, break and even lock. The extent of the damage depends on how long you run your vehicle without adequate differential oil.
Aging Seals or Rust
Aging seals at different joints in the differential will cause leaks. Leaks will then cause the low oil level problem discussed above.
Rust can also be a problem for cars that sit for long in humid or wet places. Rust can eat through the tubes causing holes. These holes will create structural compromise or just create holes through which the differential oil leaks.
Pushing the Differential too Hard
Car manufacturers spec out a differential such that it’s tough enough for normal use. This means you shouldn’t have problems if:
- You don’t overload your vehicle
- You don’t tow more than you should
- You drive responsibly
- You don’t do huge engine modifications that affect torque and horsepower output
Any misuse that pushes your car beyond specifications not only voids your warranty but also puts the drive train at risk. Overloading, drag racing, performing burnouts, or drag racing will strain your differential.
Spinning your tires very often when offroading in mud could also strain your differential.
Note that a modification that changes the engine’s output without upgrading related parts could be problematic.
For instance, giving your drive train more torque than it was designed to handle could either snap drive shafts or damage the gears in your differential.
What Happens if Your Rear Differential is Damaged?
Normally, your differential will go out slowly. You will see the signs of damage. A catastrophic failure will only happen if you ignore them.
It starts with howling, grinding, and vibration. If you ignore this, your rear differential will completely lock up.
A locked-up differential can’t transfer power to the rear wheels. It could also lock the wheels from spinning. This will render your car totally undrivable.
If it happens when moving, your car might spin out as if someone suddenly pulled the hand brake.
Don’t let things get this far. It is very dangerous. Moreover, fixing a totally blown differential is more expensive than repairing a faulty diff that still spins freely.
How Much Differential Oil Do I Need?
Most differentials will need around 3 quarts of oil. After all, the differential isn’t that big and there is no reservoir for extra oil.
You should consult your vehicle’s manual for specifics. The manual should tell you what type of oil you need and how much of it you should use.
Where is the Differential in My Car?
Well. A differential is hard to miss. It’s a solid chunk of metal assembly running across your rear or front axle. It has a huge pumpkin-like protrusion towards its center.
Ensure that you are looking straight into the undercarriage and are not obscured by plastic or metal plates designed to protect your vehicle’s undercarriage.
Check this too: Wheel Bearing Noises Vs. Rear Differential Noises
Don’t worry if you can’t see it. Not all cars have a differential. Most unibody vehicles use CV joints to transfer power to the wheels. So do almost all front-wheel drive vehicles.
Most trucks or body on chassis SUVs use differentials. Some also have differentials on the front wheels so be sure to inspect your vehicle thoroughly when preparing to service.
Bottom Line
Taking good care of your differential in your truck or any other rear wheel drive vehicle is important. It is a great way to keep your vehicle moving.
Those occasional checks might sound like a nuisance but will come in handy. They will help you identify any problems and sort them before it is too late for your differential.