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Whether your vehicle is front-, rear-, or all-wheel drive, a common component that all vehicles have is a gear differential. The differential is the housing that contains the axle's gear drive and connects to a drive shaft in order to provide power to the drive axle. Each differential, either in the front, back, or both in the case of four-wheel drive vehicles, has an input and output shaft to supply and distribute power. Each shaft has a rubber or hard plastic seal that is designed to keep gear oil from escaping but also protect the interior gear components from being contaminated from outside debris. In many cases, when leaking oil is found on the differential, it's caused by a damaged differential output seal or axle seal.
Like any other seal or gasket, the differential output seal is prone to wear and tear due to excessive exposure to the elements, age and exposure to gear oil, which is extremely thick and contains corrosive chemicals that eventually dries out the seal. When the seal dries out, it is prone to cracking. This creates microscopic holes in between the differential housing and the output seal shaft cover. While under power, the gear oil develops pressure and can escape from the holes in the seal, and leak onto the ground.
Over time, due to the facts above, the differential output seal can leak and allow fluid to escape. When this happens, the differential will not be lubricated so the bearings and gears can overheat. If these parts start overheating, there can be extensive damage to the differential that may render your vehicle inoperable until your differential is repaired.
Typically, the output seal will leak more while the vehicle is in motion; specifically when the axles attached to the differential are being propelled by the gears inside the differential. As oil leaks, the lubricity inside the differential is compromised, which can lead to significant damage of gears, axles and components inside the housing.
Similar to any mechanical component that loses lubrication, when the output seal is leaking fluid, there will be a series of warning signs or symptoms that should alert the driver that a problem exists. Some of the more common symptoms of a bad or broken differential output shaft seal include:
You notice fluid on the outside of the differential and axle: The most common sign that the output shaft seal is damaged occurs when you notice fluid coating the area where the output shaft connects the axle to the differential. Typically the leak will start from one part of the seal and slowly expand to penetrate gear oil through the entire seal. When this occurs, the fluid level will rapidly decrease inside the differential housing; which can lead to component damage.
Grinding noises coming from under the vehicle during turns: As gear fluid leaks, the metal components inside the differential develop excessive heat and can grind against each other. When this occurs, you'll typically hear a grinding noise coming from underneath the vehicle if you turn to the left or right. If you notice this type of sound, it means that metallic parts are actually grinding apart; causing significant damage.
Smell of burning gear oil: Gear oil is much thicker in viscosity than engine oil. When it begins to leak from the output shaft seal it can spill onto exhaust pipes under the vehicle. This is typically common with front differentials on all-wheel or four wheel drive vehicles. If it leaks onto the exhaust, it will typically burn as smoke, but is the leak is significant enough, it can ignite.
Any of the above symptoms can be avoided through routine service and maintenance. It's recommended by most automotive manufacturers to drain the differential oil and replace input and output seals every 50,000 miles. In fact, most output and input shaft seal leaks occur well after the 100,000 mile mark or after 5 years of wear and tear.
For purposes of this article, we'll focus on the best recommended methods for removing the old differential output shaft seal and replacing it with a new inner seal. However, every vehicle has unique steps for completing this process. As such, we'll focus on general instructions for removing and replacing the seal on most vehicles. For detailed instructions on how to complete that process, consult with your vehicle service manual or contact a differential specialist who can assist you with that task.
Part 1 of 2: The causes of a broken differential output shaft seal
Depending on the location of the differential, meaning if it's a front-drive differential or a rear differential, leaks from the output shaft seal can be caused by different circumstances. On front wheel drives, the transmission is commonly attached to a differential in one housing that is often referred to as the transaxle, while in rear wheel drive vehicles, the differential is powered by a drive shaft that is attached to the transmission.
The output seals on front-wheel drive vehicles can become damaged due to excessive heat, hydraulic fluid deterioration or excess pressure. Damage can also occur to the seal due to exposure to the elements, old age or simple wear and tear. On rear-wheel differentials, the output seals are usually damaged due to old age or over exposure to elements. They are supposed to be serviced every 50,000 miles, but most car and truck owners don't complete this service.
Under most circumstances, a slow leak coming from the differential output seal won't cause any problems with driving. However, since the oil supply has no way of being replenished; without physically adding it to the differential, eventually it can cause serious damage to internal components inside. It's when the oil leaks for a significant period of time that most of the symptoms are displayed such as:
- Grinding sounds coming from under the vehicle while turning
- Smell of burning gear oil
- Clunking sounds coming from the vehicle during forward acceleration
In each instance above, damage is being done to the internal components inside of the differential.
- Warning: The job of replacing the differential output shaft can be very complex depending on the type of vehicle you have. It's always recommended to fully review the manufacturer's service manual before attempting to complete this job. As we've indicated above, the instructions below are general steps for replacing the output seal from the typical differential. If you do not feel comfortable completing this job, always defer to an ASE certified mechanic.
Part 2 of 2: Preparing the vehicle for replacing the differential output shaft seal
According to most service manuals, the job of replacing the differential output shaft seal may take anywhere from 2 to 5 hours to complete. On some vehicles that have solid rear end housings, the inner seal is referred to as the axle seal, which is typically located on rear wheel drive vehicles and inside the rear hub of the vehicle. To remove this type of output seal, you'll have to remove the differential housing and detach the axle from inside.
On front-wheel drive vehicles, the output seal is also commonly referred to as a CV Joint seal. This should not be confused with the CV boot, which covers the CV joint housing. To remove the common output shaft seal on a front wheel drive differential, you'll need to remove some brake hardware, and in many cases, remove struts and other front end components.
In general, the materials you'll need to remove and replace the seal; after removing ancillary components will include the following:
- Can brake cleaner
- Clean shop rags
- Drip pan
- Limited slip additive (if you have a limited slip differential)
- Seal removal tool & installation tool
- Screwdriver(s) flat and Phillips head
- Socket set and ratchet
- Replacement differential output seal
- Replacement rear end oil
- Plastic gasket scraper
- Torque wrench
After collecting all these materials and reviewing the instructions set forth in your service manual, you should be ready to complete this job.
Part 2 of 2: Steps for replacing the differential gasket
According to most manufacturers, this job should be completed within a few hours, especially if you've collected all the materials and have a replacement gasket. Although it's not required to remove the battery cables to complete this job, it's always a good habit to complete that step prior to working on the vehicle.
Step 1: Raise vehicle on jack stands: In order to remove any differential output seal (either on the front or rear of the vehicle) you'll have to remove the wheels and tires in order to pull the axle from the differential. This is why you'll need to raise the vehicle on a hydraulic lift or place the vehicle on jack stands. It's always best to use the hydraulic lift if you have access to one.
Step 2: Remove the wheel: Anytime you replace a leaking output shaft seal, you'll have to remove the wheels and tires first. Using an impact wrench or star wrench, remove the wheel and tire on the axle that has the leaking differential output shaft then place the wheel aside for now.
Step 2: Preparing the axle for removal: In most cases, you'll have to remove the axle from the differential in order to replace the differential outer seal. In this step, you'll follow the instructions listed in your service manual for removing the following components.
- Spindle nut
- Wheel bearings
- Brake caliper
- Emergency brake (if on rear axle)
- Shock mounts
- Tie rod ends
On front-wheel drive vehicles, you'll also have to remove steering components and other front end suspension parts.
- Note: Due to the fact that every vehicle is different and has different attachments, it's vital to follow the instructions in your service manual or to have this job completed by an ASE certified mechanic. A good rule of thumb is to write down each step of removal as installation after replacing the broken seal will be completed in the opposite order as removal.
Step 4: Remove the axle: Once all hardware has been removed to permit you to remove the axle from the differential, pull the axle from the differential. In most cases, this does not require a special tool to remove the axle from the vehicle. As the image indicates, you can see how the supper control arms are still attached to the axle. This makes installing this part much easier after replacing the damaged seal.
The image above shows the bolts that secure the CV joint to the front differential on a standard front wheel drive vehicle. You'll have to remove these bolts to remove the axle from the differential as well. This step is not common for rear wheel drive applications. As indicated multiple times above, always refer to your service manual for exact instructions.
Step 5: Removing the damaged differential outer seal: When the axle has been pulled from the differential, you'll be able to see the output seal. Before you remove the broken seal, it's a good idea to stuff the inside of the differential with a clean rag or disposable shop towels. This will protect the inside of the differential from being exposed to elements or become dirty.
To remove this seal, the best method is to use a seal removal tool, which is displayed in the image above or large flat blade screwdriver to slowly remove the seal from its housing. The critical thing to remember is that it is important to not scratch the inner portion of the differential.
Remove the seal completely, but keep it to match with the replacement part you've purchased before you attempt to install the new seal.
Step 6: Clean the inner seal housing of the differential and axle housing: The most common source of new leaks appearing from recently completed outer seal replacement jobs is due to lack of cleaning by the mechanic. It's important to remember that the two parts that attach together must be clean and free of debris in order for the seal to properly do the job.
Using a clean rag, spray some brake clean on the rag and clean the inside of the differential first. Make sure to remove any excess seal material that may have broken during the removal.
Next, clean the axle male fitting that is inserted into the differential gear box. Spray liberal amount of brake clean on the male fitting and gear section of the axle and clean all grease and debris.
In the next step, you'll be installing the new differential output seal. The tool above is a seal installation tool. You can find them at Harbor Freight or a specialty tool supply store. They are very good for installing seals in differentials, transmissions and virtually any input or output shaft.
Step 7: Install the new differential output seal: Using the tool displayed above you'll install the new seal by following these guidelines.
*Remove the rag or paper towel you inserted to the inner portion of the differential
Using fresh gear oil, place a thin film around the entire circle housing where the seal will be installed. This will help the seal install straight.
Place the seal on the differential
Place the seal installation tool flush on the new seal
Using a hammer, hit the end of the installation tool straight until the seal has popped into place. In most cases, you'll actually feel the seal "pop" when it's correctly installed.
Step 8: Lubricate axle ends and reinstall into differential: Using fresh gear oil, generously lubricate the axle gear ending that will attach to the inner gears inside the differential. Carefully place the axle in the gears, making sure they align straight and not forced. As you can see from the image below, make sure you line up the axle correctly. Many people tend to mark the axle with the hub during removal as a resource.
Tighten all bolts and attachments that you had to remove in earlier steps in the reverse order of removal before proceeding to the final steps.
Step 8: Fill differential with fluid: After the axle has been installed and all suspension and steering hardware installed as well, fill the differential with fluid. To accomplish this step, refer to your service manual, as each vehicle has different procedures for this step.
Step 9: Reinstall wheel and tire: Make sure to install the wheel and tire and torque the lug nuts to the recommended torque settings.
Step 10: Lower the vehicle and retighten all bolts to the differential. Once you've completed the process of replacing the differential output seal, you might want to consider replacing the other one on the same axle (especially if it's a front wheel drive).
Some other components on front-wheel drive vehicles that you should remove and replace during this service will include CV boots; as they commonly tear at the same time as the output seal on front wheel drive vehicles. A good road test of 15 miles is recommended after replacing this component. Once the test is done, crawl underneath the vehicle and inspect the differential housing to ensure no fresh fluid is leaking.
When you have finished this task, your differential output seal repair is done. If you've reviewed the steps in this article and are not confident completing this project or need an extra set of professional hands to help fix the problem, contact YourMechanic and one of our local ASE certified mechanics will be happy to assist you replace the differential output seal.
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