Monday, September 25, 2023 Detailed Auto Topics
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Some of the most common problems with automatic transmissions begin with leaks

A transmission leak most often appears as small red fluid spots under a vehicle.  These spots are one early symptom of a transmission problem. Leaks never take care of themselves and normally get worse. Quick attention can help this problem remain small.  Left unattended, they may cause the transmission to fail.

Leaks are an early sign of transmission problems

automatic transmission leaks are no laughing matterThe fluid is literally the life-blood of an automatic transmission

Most automatic transmissions hold eight to fourteen quarts of fluid.  The automatic transmission fluid lubricates, serves as a hydraulic fluid and helps clean and condition the seals of the transmission.

When a leak develops, fluid is lost. With less fluid, the pump has trouble maintaining pressure and less protection is available to the transmission. Modern transmissions can sense a loss of fluid pressure. When this happens they may re-direct fluid from the lubrication circuits to the hydraulic circuits in order to keep the vehicle moving. This does keep the vehicle in motion, but can also cause severe damage. This is one reason why leaks are so dangerous. They can result in a burned up transmission.

Pressurized leaks

Diagnosing transmission leaks is easier with a bit of understanding. Some leaks will occur with the engine running.  These are normally pressure related leaks. Seals and connections that are pressurized when running are the first place to look.  This type leak may be less noticeable when the vehicle is parked and may not leave spots under the vehicle.  A physical inspection may be needed to learn there is a problem.  Examples might be, the front pump seal, the cooler lines, side covers and sensors.

side view of typical leak points on an automatic transmission

Cooler line leaks  

Cooler lines and hoses connect the transmission to the cooler and are a common area for pressurized leaks. Leaks in these areas often show up after someone has tampered with the lines. When flushing a transmission, they often remove these lines and leaks may result. Replacing a radiator involves disconnecting the cooler lines and may also cause a leak. A front collision, even minor in nature, may bend the lines or move the mountings, causing transmission fluid leakage. Pressurized leaks will account for a significant loss of fluid, compared with gravity type leakage.

Fluid level or gravity leaks 

Other points may tend to leak when the vehicle is parked over night. Common examples are the pan gasket, the fill tube seal and the shifter lever seal.

Front view of an automatic transmission showing potential leak points

When the engine is running and for a while after stopping, fluid remains in the torque converter and various drums and clutches of the transmission. This has the effect of lowering the fluid level in the pan. When the engine stops running, gravity very slowly pushes these fluids back the pan of the transmission.

fluid level rises after engine is off and the transmission sits

It is normal for the fluid in the torque converter to drain back to the pan, when the vehicle is not driven.  This causes the fluid level to rise where it may reach higher points in the transmission. This is why some leaks only occur, or are much worse when the vehicle sits for a long time.  If the vehicle is driven every day, fluid is pushed back to the converter and the level stays lower.  Under these conditions, the leaks may not show or may be far less severe.

Because the pan is usually the lowest point, many leaks appear to originate there. In reality, they may be leaking from higher up and running down to the pan. This type of mis-diagnosis results in many pan gaskets needlessly replaced and leaks that persist.

Tightening the pan and side cover bolts is also a very bad idea. Over-tightening bolts will only crush the gasket and perhaps bend the pan. Not only will it not stop the leak, it may create a leak that did not previously exist. The threads in the aluminum transmission case can also be damaged by over tightening bolts.

Front wheel drive transaxles

Front wheel drive transmissions are prone to more leaks than rear wheel drive transmissions are. They usually also have axle seals and side covers that may leak. A front wheel drive transmission also includes the differential. This makes leaks even more critical, due to the high load on these components. A lack of lubrication will quickly damage the transmission and differential.

In our experience, stop-leak never repairs a leak and very likely may create a much larger problem

While it might seem inviting to add stop-leak to a leaking automatic transmission, this should never be done. Stop-leak may work by swelling the seals. Unfortunately internal seals may also swell and a complete rebuild may become necessary.

Automatic transmission stop leak is not a good idea

The best way to prevent automatic transmission leaks is with a proper transmission service. New fluid contains seal conditioners that help keep seals pliable and prevents leaks. Having a professional perform the service also prevents additional damage from improper service.

Many transmission leaks are very easy to repair, when properly diagnosed. Once the source of the leak is found a new gasket or seal is usually all that is needed. Best is to have a specialist that knows automatic transmissions advise you.

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