Water or coolant mixed with transmission fluid – is the gearbox ruined?

A silent killer of automatic transmissions or gearboxes is the water or coolant that is mixed with the automatic transmission fluid (ATF). This problem used to be seen in older vehicles with neglected engine cooling systems, but is becoming common in newer 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 contaminated fluid be cleaned before damage occurs?

Virtually all automatic transmissions in circulation today use cellulose-based paper-lined plates called clutches or frictions. These clutches act as brakes to move and stop different components within the gearbox. When the gear lever is placed in gear or in reverse, it is the friction that is applied.

The paper that covers the clutch plates is a very delicate material that is glued to a steel column. Before the paper sticks 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 long time under normal operating conditions.

Clutch material is hygroscopic. This means that when the clutches are exposed to moisture, the paper material will displace the ATF by water. That moisture reaches the steel plates causing oxidation and breaks the glue that binds the paper to the plate. A study by International Lubricants Inc on the effects of exposed water on automatic transmission clutches states: “Testing indicated that water added at levels as low as 600 mg/kg migrated to the surface of untreated paper frictions and contributed to the paper’s loss of coating and erratic twist transfer properties.” In layman’s terms, that means less than a tablespoon of water or engine coolant in a transmission can cause a failure.

How did the water get there?

There are three ways water can get into a transmission:

1. Through the engine radiator. From the 1950s until now, most automatic gearboxes are cooled by the same water-based system that prevents the engine from overheating. There is a separate tank in the radiator for transmission fluid that allows the coolant to take the heat away from the ATF without mixing the two fluids. When a leak occurs between the ATF and the engine coolant tanks in the radiator, the fluids will mix with each other. It was more common in older vehicles that had cooling systems eroded due to neglect, but some of today’s newer vehicles use materials that fail due to cooling system pressure problems.
2. Exposure to deep water. Driving through large puddles during rain storms or driving off-road 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 enters through the rod. Most vehicles have a dipstick where ATF is checked and added. Moisture can easily enter the transmission if water was sprayed on the dipstick during engine cleaning, or in some cases, water draining from rain or a car wash drips onto the dipstick. GM and Chrysler have bulletins regarding this problem on some models of their vehicles. The authorized businesses will have access to consult for this type of bulletin. A telltale sign of this problem is moisture or rust around the dipstick tube.

Replace or rebuild?

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

Long story short, if water gets into a transmission, there is no way to avoid an expensive repair. Flushing the fluid will only cost you extra money and may make the inevitable failure happen sooner. Maintain your engine cooling system regularly and ask a professional transmission mechanic if your car is common for this type of failure. If so, bypass the radiator with an external oil cooler.