Transmission Seal Leak Symptoms, Plus Causes of Failure

Transmission seals play a crucial role in preventing transmission fluid from leaking. There are two types of transmission seals in a vehicle, depending on their purpose. Without a proper seal, your vehicle can leak transmission fluid and develop serious issues.

What Is a Transmission Seal?

There are two types of transmission seals. There are static seals and dynamic seals. Static oil seals are used between two non-moving parts, while a dynamic oil seal is used between a fixed and a moving part. The o-ring is one example of a static seal. O-rings are installed around parts like the oil filters and the front gearbox cover.

There are also dynamic seals, which typically seal the gap between the transmission case and a rotating shaft. Dynamic seals are more complex, since they have to prevent fluid from leaking between a moving and non-moving component.

A vehicle typically has two or three dynamic oil seals. One example is the input shaft seal. It prevents fluid from passing through the gap between the input shaft and the transmission housing. The input shaft is the part that connects the transmission to either the torque converter or clutch disk, depending on the transmission type.

Another dynamic oil seal located at the transmission’s output shaft is called the rear transmission seal or driveshaft seal. The output shaft is the part that transfers power to the wheels. Front-wheel drive vehicles have two transmission seals because they have two output shafts, one for each axle.

How Do Transmission Seals Work?

Static seals are relatively simple. They seal fluid by using a thin strip of synthetic rubber that’s wedged between the gap of two conjoined parts. Since rubber is elastic, it can stretch to fill the small gaps between two separate parts. Dynamic seals are a little more complex since the input and output shafts can rotate thousands of times a minute. How does a dynamic transmission seal keep fluid inside when the shaft is constantly moving?

Dynamic seals have a more complicated shape, with one or two lips depending on their design. The outer lip fits into a recess on the transmission case, while the input or output shafts are inserted through the inner lip. A thin coiled spring, known as a garter spring, is fitted along the inner lip. As the input or output shaft spins, the garter spring exerts pressure onto the sealing lip. This constant pressure keeps the rubber sealing lip in contact with the revolving shaft, preventing transmission fluid from seeping through.

Why Do Transmission Seals Fail?

Both static and dynamic seals deteriorate over time because most oil seals are typically made from synthetic rubber. This material typically hardens or softens after several years of exposure to oxygen, heat, or light. When this happens, the seal can start to fail. Dynamic seals might also wear out faster because they’re constantly in contact with a moving part.

Another reason why dynamic seals fail is because of excessive play between the rotating assembly and the inside of the transmission. Front-wheel drive vehicle axle seals can leak if the inner side of the axle rests on the inside of the transmission. The output shaft seal of a transmission can also leak if there’s excessive tailshaft play caused by excessive bearing play. 

What Happens If a Transmission Seal Breaks

The transmission seal doesn’t directly contribute to the transmission’s role of transferring power to the wheels and applying gear ratios. However, keeping the transmission seal working properly is very important because leaking transmission fluid can cause serious transmission issues.

Leaks are often discovered while your vehicle is parked because fluid droplets can accumulate under your vehicle’s gearbox. Automatic transmission fluid often appears crimson in color. Meanwhile, the fluid in manual transmissions is semi-transparent or honey-colored. Your vehicle will likely exhibit the following symptoms if it’s leaking transmission fluid:

Grinding Gears

Since transmission fluid prevents metal on metal contact between components, a lack of fluid can result in grinding noises.

Gear Slipping

When the engine’s RPM increases and the vehicle doesn’t speed up normally, then it could be because the fluid level is low and gears are slipping.

Excess Vibrations

Grinding gears and gears slipping can also cause the transmission to create more vibrations.

Slow Acceleration

Your vehicle’s transmission will struggle to shift gears without enough transmission fluid. Expect gear shifts to be slower and clunkier. As a result, your ability to accelerate will be hampered.

Delayed Gear Shifts

Reduced transmission fluid can result in slower gear shifts, which can lead to slower acceleration.

Overheating Transmission

The transmission can accumulate too much heat if there isn’t enough fluid to cool it. This can lead to transmission damage or even transmission failure.

Burning Smell

Overheating transmission fluid may produce a burning smell.

Signs of a Leaking Transmission Seal

Your vehicle will give off signs if the transmission seal is leaking. Here are some symptoms for you to check:

Fluid Leaks 

You can check if the transmission seal is leaking by looking beneath your car. If you notice dark spots, then it’s likely transmission fluid that leaked out.

Brown-Colored Dipstick

You can check the transmission fluid level by looking at the dipstick. A brown-colored one indicates the transmission fluid needs replacing. And if the level is low, there’s a chance a leak drained most of the fluid.

Illuminated Transmission Light

An illuminated transmission light could signal different issues. One of which is an overheating transmission caused by the transmission fluid.

A transmission leak should be repaired as soon as possible. A leak can starve your transmission of the fluid it needs for cooling and lubrication. If a leak is allowed to persist, it can cause the transmission to run low on fluid. This can result in severe damage. At that point, the transmission will need to be rebuilt or replaced.

How To Get a Quality Replacement Transmission Seal

We know it isn’t always easy to find the time to look for quality replacement parts for your vehicle, but a faulty transmission seal is a big red flag you can’t just ignore. Thankfully, you can get a great part with a guaranteed fit here at CarParts.com.

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