04 Causes of Allison Transmission Leaking from Vent with Solutions

The vent located on the top of the transmission is a small tube made of plastic.

The vent is designed to flow hot air to maintain the transmission’s temperature.

Sometimes the vent leaks fluid and causes transmission issues.

In this article, I will assist you in resolving the Allison transmission leaking from the vent issue.

Throughout this article, I have discussed everything you need to know about transmission leaking from the vent and how to fix the problem.

Cause and Solution to Allison Transmission Leaking from Vent

Allison transmission leaking from the vent is caused by 03 most common and 01 rare issues, including pump cavitation, overfilled transmission, transmission fluid overheating, and disassembling the transmission vent.

Fixing these issues will stop transmission fluid from leaking from the vent.

01. Pump Cavitation

Pump Cavitation

A pressure difference in the pump causes the liquid to turn into vapor.

A leak in the pump or deficient fluid levels can cause pressure differences in the pump.

As a result, the fluid in the pump becomes aerated and can leak from the vent.

Transmission fluids sometimes become extremely viscous at extremely low temperatures, around -25⁰C.

It causes fluid to become trapped in the pump, preventing it from draining properly.

When the transmission warms up, trapped fluid escapes through the vent.

As a result, tiny drops of fluid are discharged.


If there are leaks, fix them.

It is a pervasive problem in the 2001-2004 models, so the models need extra care.

You can remove the factory vent setting and add an external tube.

Set the outer tube of the vent as a mounting bracket in front of the transmission.

This can help you eliminate the problem.

02. Overfilled Transmission

Overfilled Transmission

An overfilled transmission fluid also causes leaking from the vent.

If you fill the transmission with more fuel than recommended, excess fluid will flow through the vent and cause leaks.


Check the fluid level and remove excess fluid from the transmission.

A suction gun can remove excessive amounts of fluid from filter tubes with a hose.

If the fluid level seems normal, check your dipstick.

The dipstick may show the wrong level.

03. Overheated Transmission Fluid

Overheated Transmission Fluid

Overheated transmission fluid is another reason for transmission leaks from the vent.

Several problems, including leaks, contaminated fluid, or low fluid levels, can cause transmission fluid to overheat.

When the fluid overheats, it may enter the vent and cause leaks.


If you find transmission fluid overheating, stop the vehicle on the roadside and wait for the transmission and engine to cool down.

You should take your vehicle to a repair shop for a proper diagnosis to resolve the issue.

04. Disassembling the Transmission Vent

When the transmission vent is disassembled, the configuration and location of the vent can be changed, causing transmission leaks.

Occasionally, this is the cause of the problem, but it is rare.

Disassembling the vent can cause seal rings to leak and a gasket to blow out, which pressurizes the fluids to flow through the vent. 


You should consult a professional mechanic to solve the problem.

Watch this video to learn how to fix Allison transmission vent leaks.

Final Thoughts

A blocked transmission vent causes leaks, so you should check your transmission fluid level and quality every time you change the fuel.

Hopefully, after reading this article, you are now aware of the causes and solutions to Allison transmission leaking from vents.

Now you can solve the issue by yourself by following my instructions.

To learn about Allison Transmission’s other problems, check out the following blog posts:

Leave a Comment