Water or Coolant Mixed With the Transmission Fluid: Is the Gearbox Ruined?

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A silent killer of automatic transmissions or gearboxes is water or coolant mixing with automatic transmission fluid (ATF). This problem used to be seen in older vehicles with neglected engine cooling systems, but is becoming common in later model cars and trucks that have followed their maintenance schedules. The result is always the same: the transmission must be completely overhauled or replaced.

Can the Contaminated Fluid be Cleaned Out Before Damage Occurs?

Just about every automatic transmission on the road today uses cellulose based paper lined plates called clutches or frictions. These clutches act like brakes for moving and stopping different components inside the gearbox. When the shifter is placed into drive or reverse, it’s the frictions that are being applied.

The paper that lines the clutch plates is a very delicate material that is glued to a steel backbone. Before the paper is glued to the plate, it has the strength and consistency of a graham cracker. Once the material is bonded, it becomes much stronger and can last a very long time under normal operating conditions.

The clutch material is Hygroscopic. This means when the clutches are exposed to moisture, the paper material will displace ATF for water. That moisture reaches the steel plates causing rust, and breaks down the glue that bonds the paper to the plate. A study done by International Lubricants Inc about the effects of water exposed to automatic transmission clutches states, “The testing indicated that water added at levels as low as 600 mg/kg migrated to the surface of untreated paper frictionals and contributed to loss of the paper coating and erratic torque transfer properties.” In laymen’s terms, that means less than a tablespoon of water or engine coolant in a transmission can cause a failure.

How Did Water Get There?

There are three ways water can enter a transmission:

1. Through the engine’s radiator. From the 1950’s to now, most automatic gearboxes are cooled with the same water based system that keeps the engine from overheating. There’s a separate tank in the radiator for transmission fluid that allows the coolant to take heat away from the ATF without mixing the two liquids. When a leak occurs between the ATF and engine coolant tanks in the radiator, the fluids will mix with each other. It was more common in older vehicles that had eroded cooling systems due to neglect, but some of today’s newer vehicles are using materials that are failing because of pressure problems in the cooling system.
2. Exposure to deep water. Driving through large puddles during rain storms or off road driving can expose the transmission’s breathing system to moisture. The best chance of preventing a failure is to check for water in the ATF after a vehicle has been in this type of scenario.
3. Moisture entering through the dipstick. Most vehicles have a dipstick where ATF is checked and added. Moisture can easily enter the transmission if the dipstick was sprayed with water during an engine cleaning, or in some cases, water draining from rain or a carwash drips onto the dipstick. GM and Chrysler have bulletins pertaining to this problem on some models of their vehicles. Qualified shops will have access to check for these types of bulletins. A tell tale sign of this problem is moisture or rust around the dipstick tube.

Replace or Rebuild?

It depends on how much water mixed with the transmission fluid, how long the vehicle was driven with the contaminated ATF, and the type of gearbox your vehicle has. Metal and electronic parts inside the transmission will quickly corrode when exposed to moisture. If there is too much damage inside of the gearbox, the cost of the parts to rebuild the trans will exceed the cost of replacing the unit with a remanufactured product. Some manufacturers like Nissan and Chevy have computers inside of the gearbox that will fail when exposed to moisture. These computers or mechtronics cost as much as $2,000 and that does not include rebuilding the rest of the unit. When parts cost that much, it’s often a better decision to replace the gearbox entirely.

To sum things up, if water gets in a transmission, there’s no way to get around an expensive repair. Flushing the fluid out will only cost you extra money and can make the inevitable failure happen sooner. Service the engine’s cooling system regularly and ask a professional transmission mechanic if your car is common for this type of failure. If it is, bypass the radiator with an external oil cooler.

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