If you suspect that you have a leak from an axle seal or other area on your differential or vehicle, it can be difficult to determine what type of fluid you may be seeing on your driveway or on the chassis. Gear oil is typically black or dark brown due to use, but fresh gear oil is relatively clear or may have a slight brown or yellow tint. If your vehicle has a transaxle, it will typically use transmission fluid instead of gear oil, and this fluid is typically red and may be more easily identifiable. If you need a refresher on how to identify different types of automotive fluids, check out the guide in our How-To Hub.
As vehicles age, it’s likely they will experience leaks as gaskets, seals, and more fail, wear out, or suffer damage. Though you may be familiar with leaks around your engine, which could include oil, transmission fluid, or coolant, you may not be as familiar with leaks from around your axles. Depending on how your vehicle is built, it may have a differential at the rear, at the front and rear, or it may be equipped with a transaxle in the front that includes both the transmission and axle assembly. When you notice leaks, it’s important to pinpoint the source as quickly as possible to prevent a loss of lubrication that could quickly lead to axle or differential damage. Read on to learn more about what causes seals to leak and how to diagnose and repair this issue.
What is a Differential, Axle, or CV Shaft?
To identify the source of your leak or leaks, the first thing you need to know is where each of the parts of your drivetrain are located and how your particular vehicle is designed. If your vehicle is rear-wheel drive, it has a differential in the rear that connects to the transmission with a driveshaft. Four-wheel and all-wheel drive vehicles include a rear differential as well as a front differential that is driven by a transfer case and driveshaft to turn straight axles or CV shafts that drive the front wheels. And as previously mentioned, front-wheel drive vehicles feature a transaxle connected directly to the engine that drives the wheels using CV shafts.
The differential includes ring and pinion gears, carrier gears, and more that transfer the power input from the driveshaft to drive the axles and turn the wheels. In most cases, straight axles are used to connect to the center of the differential that include hubs and lug studs at one end and splines on the other that mesh with the internal differential mechanism. The tight tolerances inside the differential require specific weights and types of gear oil to work their best and limit the risk of damage and wear during normal operation.
Constant-velocity (CV) shafts serve the same purpose as axles, but include joints at both ends to allow the shaft to drive the wheels at a variety of angles without binding. These are used on vehicles with independent rear suspension, on the front differential of four- and all-wheel drive vehicles, and on the transaxle of front-wheel drive vehicles. The joints of the CV shaft are greased and sealed with a rubber boot to allow them to articulate as needed and minimize wear.
No matter what type of axle, differential, or transaxle your vehicle is equipped with, the axles, pinion shaft, or CV axle will include a seal to prevent gear oil or fluid from leaking. These rubber or silicone seals are designed to allow the shafts to spin freely while maintaining contact to seal the opening, but because they suffer constant friction, the axle seals may wear out over time and begin to leak. This wear can be accelerated due to excessive heat or allowing the vehicle to sit for a long period of time, which may cause the seal to become brittle, dry out, or allow the axle or CV shaft to stick to the seal and tear when the vehicle is moved again.
What Does Gear Oil Look Like?
What Causes Oil Seals to Fail?
Oil seals found on the engine, axles, differential, and more are typically engineered as a one-piece, press-in assembly that includes a metal structure for strength and rubber or silicone sealing surfaces. It is important that a seal be pressed or driven in straight to prevent warping or uneven contact that could lead to a poor seal, and keeping the right type of oil at the appropriate level can ensure that the seal receives some oil to keep it properly lubricated. Since we now know what seals are and where they can be located, now we can ask: “why do seals fail?”
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Seal Age and Wear
Like every part on your vehicle, time can take its toll on seals, and normal wear can cause damage that will prevent them from working as they should. Because they make constant contact with a rotating shaft, it is possible for the sealing surface to wear down and develop a gap between the seal and the axle or pinion shaft. When this happens, even if the gap is small, oil can begin to leak around the seal. -
High Heat or Pressure
Exposure to high heat can cause irreversible seal damage, which may lead to the rubber or silicone becoming brittle. Once the sealing surface has suffered damage or lost elasticity, it may no longer fit securely around the shaft and allow oil to drip or leak during or after operation. Excessive pressure or fluid levels that are too high may also cause the seal to suffer wear or blow out the seal and allow leaks to form.
• Vibrations or Alignment Issues
- Over time, it’s possible for other parts of the differential or axle assembly to suffer wear that may damage the seal. Vibrations may be caused by a bent axle or a damaged or worn axle bearing, which can lead to looseness and excessive movement. If the axle shaft or bearing is especially worn, it may also prevent the axle shaft or CV shaft from seating properly, which could lead to vibrations and premature seal wear. If this is the case, it may be necessary to not only replace the seal, but also to service or replace the CV shafts, axle shafts, bearings, or other parts that may be allowing excessive movement.
How to Find Oil Leaks
No matter where the leak may originate, it’s important to isolate the area of your vehicle it may be coming from. If you park on a concrete driveway, it may be very easy to see where oil or other fluids have pooled after you park your vehicle. No matter where you park, it may be beneficial to put down a piece of cardboard, not only to prevent staining, but also to help keep the area of the leak isolated and narrow down the source. Once there are drips on your cardboard, you can simply get underneath the vehicle and follow the pooling straight up, and hopefully this will lead you to the part that may be leaking. At this point, look for areas that may be dripping, oily, or wet. It may also be necessary to clean areas that may have older grease buildup and drive the vehicle again to see if leaks form in the area or if more oil drips from where the grease had been.
This method works well for engine, transmission, and differential leaks, but if your axle seals are leaking, it may be even easier to identify. When axle seals fail on straight axles that span the entire distance between the wheels, you may see grease or oil on the inside sidewalls of your tires. The only other fluid at the wheel end of a straight axle vehicle that could leak would likely be brake fluid, so basic inspection can usually tell you what type of fluid has been leaking.
How to Prevent Premature Seal Failure
Though seals are bound to fail over time from age and use, proper maintenance can help ensure the longest life from factory-installed and new seals alike. The most important step you can take to prevent seal and gasket failure is to keep the right type and amount of fluid in the system. In the case of your axle seals, ensuring that you use the appropriate weight and type of gear oil as recommended by your manufacturer in the differential, and periodically changing the gear oil, is the best way to prevent not only wear to the seals, but also damage to the internal gearing of the differential and the axles.
Most manufacturers recommend changing the gear oil in your vehicle every 30 to 60 thousand miles, but many drivers forget about this maintenance task until a problem arises. If the gear oil in your differential wears out or oxidizes, it no longer possesses the lubricating properties necessary to prevent premature wear or overheating that could damage the gears and the seals. This is also true if the gear oil level is low, as there will not be sufficient volume to reach all of the lubricating points of the differential, including lubrication of the seals. If you have neglected your gear oil for too long, you may notice metal particulates or even a silver shimmer from the fluid when it is drained that is the result of wear inside the differential. You might also notice clunking, grinding, or whirring noises when accelerating or coming to a stop. If you’d like to know more about how to change your differential gear oil, check out our helpful video.
How to Replace a Leaking Axle Seal
Though there is no one set procedure to replace an axle seal on any vehicle, the process is similar across most makes and models that share similar drivetrain designs. In any case, replacing an axle or CV shaft seal will involve disassembling the wheel end and removing the shaft to access the seal, and may also require specialty puller or installer tools to make the repair. If you need to rent the tools required for your axle seal repair, check out the available options in our Loaner Tool Program. This program allows you to pay a small deposit for axle sockets, ball joints and tie rod tools, bearing and seal drivers, hub pullers, and more, and then return the tool for a full refund when you’re done.
How to Replace a CV Axle Seal
To replace a CV axle seal, it is usually necessary to disassemble the wheel end to allow enough space to pull the CV axle from the differential or transaxle. In some cases, you may be able to separate the lower ball joint from the spindle or knuckle and make enough room to break the CV axle loose and move it out of the way, but if this is not sufficient, more steps may be necessary. These can include removing the axle nut socket and using an axle or hub puller to press the CV axle out of the hub, as well as completely separating the knuckle from the ball joints and tie rods or disconnecting or removing the shocks or struts depending on the way the vehicle is designed.
Removing a CV shaft for the first time can be a daunting task, and should be addressed carefully to prevent damaging or separating the joints internally. Most CV shafts are designed with a snap ring at the end of a splined shaft that is intended to be “popped” into place and retained securely. To remove most CV shafts, it may be necessary to use a pry bar or a CV shaft removal tool to apply torque and leverage and disengage the retaining ring. This can sometimes take several tries, but once the CV shaft begins to move outward from the transaxle, it can be fully removed.
Once the CV shaft has been removed, you should be able to access the axle seal and bearing. Using a seal puller, you should be able to contact the seal and use leverage to begin to unseat it. It may be necessary to move around the seal, prying it outward a little bit each time, until you work the seal out of position.
When you have the seal removed, clean the oil or fluid from the opening and lubricate the new seal with fresh fluid before reinstalling. It is important that the seal be driven straight, so an appropriately-sized seal driver may help you apply even pressure across the seal. If the seal becomes crooked or stops before being driven in fully, you may need to apply pressure to other areas of the outside perimeter of the seal and continue to work your way around it until it is fully seated. Finally, apply fresh fluid to the sealing lip of the seal before reinstalling your CV shaft to lubricate it, and then finish reassembling the rest of the wheel end by reversing your disassembly procedure.
How to Replace an Axle Shaft Seal in a Straight Axle
The steps to remove an axle shaft and replace a seal in a straight axle are similar to the procedures to replace a CV axle seal, though the design and retainer style may differ. You will still need to disassemble the wheel end, including removing brake calipers and rotors or brake drums and possibly backing plates, and then you are ready to begin the axle disassembly.
Straight axles may be held in place by bolt-on retainers at the end of the axle shaft, metal clips inside the differential assembly, or a combination of attachment methods. This may necessitate the removal of the differential cover to access the ends of the axle shafts and remove the retainers. Once the axle shafts are free, they can be slid out of the end of the axle assembly to access the seal.
Removing and reinstalling the seal will be largely the same as the procedure detailed for CV shaft seal replacement. Using a seal puller tool, work the seal out of the housing until it is free. Then you can clean any gear oil from the inner surface before reinstalling the new seal. Coat the outer edge of the new axle seal in gear oil to lubricate it, then begin driving the seal into the housing. Do your best to drive the seal in straight to avoid damaging the edges, and continue driving the seal until it bottoms out in the housing. Then, you can lubricate the sealing lip of the seal with gear oil and reinsert the axle shaft. Be sure to reinstall any retainers, clips, or other attaching hardware before reassembling the brake system parts and wheel end assembly. If you removed the differential cover to disengage the axle shaft, you will also need to use a new differential cover gasket or RTV sealant to reinstall the cover and refill the differential with gear oil.
Diagnosing and Replacing a Leaking Axle Shaft Seal
Using the procedures detailed in this article, we hope you are better equipped to handle diagnosing and replacing your own oil seals at home. Though these repairs can be difficult for those that are not well-versed in DIY maintenance and service, they typically require only basic hand tools and some specialty equipment that can be rented from your local O’Reilly Auto Parts. If you’re still not comfortable diagnosing and repairing your own oil leaks, our parts professionals are always available to answer any questions you may have. We’d also be happy to recommend a local repair shop to complete your repair.