Rick's Free Auto Repair Advice

Transmission fluid leak

Causes of a transmission fluid leak

Every transmission has seals and gasket that can leak as they wear out. A worn transmission seal or gasket usually only drips small droplets, not puddles. If you see a puddle of red or brown transmission fluid on the ground, chances are you’ve got a more serious problem where engine coolant has entered the transmission.
Water or ethylene glycol engine coolant that enters an automatic transmission can destroy the internal components. There are only two ways water and coolant can enter the transmission; someone added it to the transmission or the transmission oil cooler located inside the radiator has failed.

Symptoms of coolant in the transmission

• Transmission fluid blowing out of the vent tube
• Puddle of transmission fluid on the ground after driving
• Transmission fluid is cloudy, strawberry milkshake colored
• Water floating on top of the transmission fluid sample
• Gaskets are swollen or wrinkled
• Shudder during gear engagement or during gear changes, especially when shifting into overdrive.

Transmissin fluid and coolant mix

Transmission oil cooler failure

It may sound strange that the transmission oil cooler is located inside the radiator, but transmission fluid is often hotter than engine coolant. So even if the engine coolant is very hot, it’s still cooler than transmission fluid. So car makers locate a small transmission oil cooler inside the transmission. If you neglect the recommended coolant changes and corrosion begins inside your cooling system, it can attack the transmission oil cooler, causing a leak.

Depending on the size of the leak and the temperature of the coolant, transmission fluid can either leak into the coolant or coolant can leak into the transmission.

transmission oil cooler in radiator

Transmission fluid in the coolant

When transmission fluid leaks into the coolant it attaches to the surface area of the radiator and engine coolant passages, insulating the radiator fins and reducing cooling effectiveness. The transmission fluid mixes with the coolant when it’s circulating, but rises to the top when the engine is off.

Coolant in the transmission fluid

When coolant leaks into the transmission, it turn the normally red transmission fluid into a strawberry color or can turn it milky white. Since coolant doesn’t lubricate like transmission fluid, it can’t protect against internal wear. In addition, the water in the coolant swells the spacer plate gaskets, clutch facings and band friction materials. It also rusts internal steel components.

Coolant in the transmission requires a complete teardown and rebuild. You can’t simply flush it out and continue running the transmission.

How to detect coolant in your transmission

In some cases, coolant won’t show the normal telltale signs of a strawberry color or white milky deposits. In that case, you should obtain a test kit. They’re available from several companies. The Acustrip is $25 and uses test strips. A kit is good for 10 tests.


Accustrip test strips for detecting coolant in transmission fluid

Find the ACUSTRIP 711254 Series at acustrip.com/

Or, order the Gly-Tek TestKit and test a sample of the fluid. The kit costs $102 and contains enough supplies to conduct ten tests.
The test takes less than five minutes and shows results within fifteen minutes.
Obtain a test kit from this company
Nelco Company
1047 McKnight Rd South
Saint Paul
Phone: 651-738-2014
Fax: 651-738-944
©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat


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