Skip the auto shop - Our mechanics make house calls

Q: Why would transmission fluid leak?

asked by on

Why would transmission fluid leak?

First, we should acknowledge that the transmission, whether automatic or manual, is actually designed to hold the fluid without leaking. However, there are many factors which can cause a transmission to leak.

Drying or hardening seals can cause leaks. Failing pressure lines in an automatic transmission system can crack, dry rot, or be damaged by something hitting them while driving. Automatic transmission systems use a cooler unit usually included in the radiator and these can fail due to vibration or corrosion of the housing over time. If an external or additional cooler is used, it is often in front of the radiator and condenser and subject to damage from debris on the highway. Areas known for leaks include faulty components, misaligned or faulty gaskets, and loose drain plugs or lines. Temperature changes and the difference in metal and gasket types can lead to leaks over time. Modern electronically-controlled transmissions have electrical connections that go into the transmission case or through the transmission pan. I have seen porous casting in the past causing internal or external leaks as well. In older vehicles that used vacuum modulator valves to control shift points, the diaphragm would sometimes rupture, causing the fluid to be leaked into the engine through the vacuum line that was used to signal or control the valves’ operation.

As with any system, a regular check of fluid level and condition will help to avoid costly repairs. If you suspect a leak, have the vehicle inspected as soon as possible. Transmission fluid serves several purposes. It acts as the pressure agent to actuate clutch packs to control the different gear ranges. It also lubricates and cools the transmission. Loss of fluid causes aeration of the fluid, known as cavitation. Like all hydraulic systems, the transmission system depends on the ability of the fluids to not compress if the level get low. If they get air bound, parts can slip and damage will ensue. Keeping the level full and the fluid cool will extend the life of the transmission.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Brakes catch first thing

For an issue like this, a certified mechanic will need to inspect the vehicle and determine exactly where the problem is located. However, it sounds to me like you have a problem with a sticking caliper or a fluid leak....

Q: My jaguar s-type has steering liquid leak. When you top up with ATF all the noise goes away and the vehicle is drive-able. But th

Hello, thanks for writing in about your Jaguar S-Type. This is a common problem with power steering pumps. Over time, they can develop leaks for a variety of reasons such as bad gaskets, leaking hoses, or excessive pressure in the...

Q: Car not shifting out of drive

The shift linkage connections have plastic bushings that go bad and the linkage binds or will not shift at all. The bushings on some models can be replaced and the whole cable has to be replaced of the bushings are...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...