When does the axle seal need to be replaced in a car?

Updated: March 15, 2020
In a car, an axle seal is the part that seals the connection of a CV axle (driveshaft) to the transmission or rear differential. The job of an axle seal is to keep the transmission fluid or differential oil from leaking out. In some all-wheel drive vehicles, one of the front CV axles is connected to the transfer case (unit).
Axle Seal Front CV axle (driveshaft) seal.
A transfer case seal might also be referred to as an axle seal. When should an axle seal be replaced? Should both axle seals be replaced at the same time? Is it safe to drive with a leaking axle seal? Let's start with the first question. The axle seal replacement is not a part of regular maintenance, the seal only needs to be changed if it leaks.

How do you know if an axle seal is leaking? Gear oil or transmission fluid drops on your driveway are some of the early signs. For example, in a 2008-2011 Ford Escape, a leaking left axle seal in the automatic transmission is one of the common problems. Early symptoms include reddish-brown fluid drops behind the left front wheel, where the left front driveshaft connects to the transmission. An axle seal leaks more during highway driving, so the oil drops on the driveway may not always be present. As more fluid is leaked out, the transmission fluid level drops and the transmission will start slipping. If not caught in time, the transmission may even get permanently damaged. A leaking axle seal of the rear differential or transfer unit might cause a gear oil (sulfur) smell coming from under the vehicle.
Of course, if you have your oil changes done regularly, your mechanic can spot an axle seal leak and inform you before any major damage is done.
Leaking transmission (axle) seal Leaking axle seal
When a car is lifted on the hoist, an axle seal leak can be identified by oil splashed around the area of the seal. See the photo. The low transmission fluid level is another reason to suspect a leak.

Is it safe to drive with a leaking axle seal? It depends on the severity of the leak. If the leak results in significant fluid loss, the leaking component (transmission, differential or transfer unit) could eventually get damaged. If the leak is very small and the repair is not possible at the time, keeping the transmission fluid or differential oil level topped up can help avoid the failure of the component.

Should both axle seals (left and right) be replaced at the same time? No, it's not necessary. If one axle seal is leaking, it doesn't mean that the other seal will start leaking soon.

How expensive is the axle seal replacement? Even though a new axle seal is not a very expensive part, the repair cost may vary from $150 to $650, depending on the complexity.
For example, in the Ford Escape mentioned above, a leaking axle seal is recommended to be replaced as a kit with an axle bushing and inner CV joint; this may cost up to $600-$650.

In many cars, an axle seal is covered by the powertrain warranty, check your vehicle's warranty brochure to verify. In a typical front-wheel drive car, replacing an axle seal might cost $150-$250 for one seal. In some vehicles, a small amount of transmission fluid leaks out when the seal is removed. This means that the transmission fluid level will need to be topped up after the seal replacement.

Is it difficult to replace an axle seal as a DIY project? In a typical front-wheel drive car, the hardest part is to remove the CV axle (driveshaft) to get to the seal. Without a hoist and proper tools it's quite difficult, especially if the seal is leaking on the side of the longer driveshaft (the right side in most cars).