Coolant mixed with ATF: causes and transmission cleaning tips

The transmission is responsible for transferring power from the engine to the wheels. It is a very vital system in a car that needs proper maintenance and care.

The cooling system cools the engine and transmission with the help of fluid called coolant. The coolant circulates and carries heat away from the engine and transmission and dissipates it to the environment through the radiator.

Transmissions are one system of cars that work hard and generate lots of heat. As the transmission runs, the cluches, bands and gears generate a lot of frictional force and heat.

This makes external cooling of the transmission to be necessary, though not all transmissions are externally cooled. Water getting into the transmission can damage it in no time.

When this happens, the transmission will need a rebuild or a replacement unless the issue is identified and fixed before the damage begins.

A transmission oil that is mixed with coolant has the look of a strawberry milkshake.

It is possible that it is transmission fluid that is in the coolant system and not coolant in the transmission. That requires a different approach to fixing it.


How does coolant/water get into a transmission?

1. Through a bad oil cooler/heat exchanger: Many transmissions are cooled by the cooling system of the engine. The heat generated by the transmission is transferred to the coolant through the help of a heat exchanger which has two separate and interwoven chambers which causes the cooler coolant to carry heat away from the hotter transmission oil. That is why it is called an oil cooler.

 The oil cooler can cause coolant to leak into the oil when it ruptures internally. One main cause of the oil cooler's early failure is the use of water instead of an appropriate coolant solution. Another way the heat exchanger can cause coolant to leak into the transmission is through a worn gasket that causes coolant to leak from the coolant chamber to the oil chamber.

Oil coolers are either mounted directly on the transmission, mounted remotely and fed through cooler lines or integrated into the radiator and fed through cooler lines.

2. Intrusion through the vent valve: Many transmissions have vent vave used to release pressure. Flood water or a puddle of water that reaches the level of the vent valve can get into the transmission.

3. Intrusion through the transmission oil dipstick: A dipstick that is not water tight can let water get into the transmission when the engine is being washed or when flood water level is high to the point of the dipstick.


How coolant/water can damage a transmission

1. The clutch material of the transmission can absorb coolant and the coolant then damages the glue that binds the clutch materials.

2. A transmission fluid that is contaminated with water will no longer effectively lubricate the transmission.

3. Water in transmission causes corrosion to metal parts. 

4. As it is well known, water affects operation of electrical and electronics components negatively. Electrical components of the valve body that is inside the transmission can seize to function well due to the presence of water.


How to remove coolant/water from transmission

Removal of coolant from transmission entails removal of the entire oil that has been contaminated by coolant. By coolant, we mean either water used as coolant or coolant solution. Detection and prompt fixing of the issue is the only thing that can save the transmission. Success rate also depends on the amount of water that is in the transmission.

The first step is to fix the root cause of the water intrusion. This entails replacement of a bad oil cooler, oil cooler gasket, radiator, dip stick O-ring/dipstick or a bad transmission vent cover.A transmission flush that entails using a power flush equipment connected to cooler lines would be the fastest method but it is not safe for a transmission that has got some miles of operation.


Alex (not verified)

1 month 3 weeks ago

Just wanted to say thank you for the informative blog. Not many are able to explain the operation of the system, possible failures, and solutions as well as you have!