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Both automatic and manual transmissions use rubber or plastic seals to keep fluids inside. Leaky seals need to be replaced, but they may also indicate a more serious problem with the transmission.



What are Transmission Seals?

The input shaft seal, also called the front transmission seal, prevents fluid from slipping past the input shaft. The input shaft inserts into the torque converter or clutch disk, depending on whether the vehicle is equipped with an automatic or manual transmission.

The output shaft seal, sometimes called the rear transmission seal or driveshaft seal, prevents fluid from escaping where the driveshaft yoke slips into the transmission. Front-wheel drive vehicles have two “rear” transmission seals, one for each axle.

How do Transmission Seals Work?

The seals have two important dimensions, the outside diameter and the inside diameter. A seal with the proper outside diameter fits tightly into the recess on the transmission. The inner diameter of the seal has an interference fit along with a tiny spring to keep it snug against the input shaft or the driveshaft.

When the input shaft or the driveshaft is inserted into the transmission, the seal locks fluids inside. The seal is designed to prevent leaks regardless of how fast the input shaft or driveshaft spins.

How are Transmission Seals Made?

Transmission seals are made of rubber and plastic. Some seals are reinforced with steel. Most seals have a very small spring around the inside diameter to help produce an interference fit with either the input or the output shaft.

Manufacturers produce these seals in bulk using heavy, factory grade machinery.

Why do Transmission Seals Fail?

  • Oxidation causes rubber and plastic seals to lose their flexibility, resulting in cracks and eventual fluid leaks.
  • Transmission seals may fail prematurely, or not seal at all, if they are installed improperly.
  • Bearing wear inside the transmission damages the seals, causing leaks.

What are the Symptoms of Seal Failure?

  • Automatic transmission fluid (ATF) spots or puddle under the vehicle. ATF is red, reddish, or brown and has a distinctive smell.
  • Gear oil or motor oil spots or puddles under the vehicle.
  • The transmission may slip or make excessive noise as fluid loss accelerates.

What are the Implications of Seal Failure?

  • If left unchecked a small leak may develop into a large leak.
  • Large leaks may result in damage to the transmission if they are not diagnosed and repaired in a timely manner.

The seals on your vehicle’s transmission are an important part of keeping your drivetrain in good working order. Avoid damage to major components by catching leaks early.

If you have questions or concerns about transmission seals or any of your vehicle’s components, come into your local Pep Boys where we can answer any question, help you find any part, or perform any vehicle service you might need.