Water or coolant mixed with transmission fluid: Is the gearbox ruined?

A silent killer of automatic transmissions or gearboxes is water or the mixture of coolant with automatic transmission fluid (ATF). This problem used to be seen in older vehicles with sloppy engine cooling systems, but it 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 checked or replaced completely.

Can contaminated liquid be cleaned before damage occurs?

Almost all automatic transmissions on the road 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 put into gear or reverse, it is the frictions that are applied.

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

The clutch material is hygroscopic. This means that when the clutches are exposed to moisture, the paper material will displace the ATF through water. This 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 water exposed to automatic transmission clutches states: “Testing indicated that water added at levels as low as 600 mg / kg migrated to the surface of the raw paper frictions and contributed to the loss of paper, coating and erratic torque 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 that water can enter a transmission:

  1. Through the engine radiator. From the 1950s until now, most automatic gearboxes are cooled with the same water-based system that prevents the engine from overheating. There is a separate tank in the radiator for the transmission fluid that allows the coolant to remove the heat 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 eroded cooling systems due to neglect, but some of today’s newer vehicles use materials that fail due to pressure problems in the cooling system.
  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 to prevent a failure is to check for water in the ATF after a vehicle has been in this type of scenario.
  3. Moisture ingress 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 engine cleaning or, in some cases, water that runs off from rain or a car wash drips onto the dipstick. GM and Chrysler have bulletins related to this problem on some models of their vehicles. Qualified stores will have access to verify these types of newsletters. A tell-tale 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 the 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 when exposed to moisture. These computers or mechtronics cost up to $ 2,000 and that does not include rebuilding the rest of the unit. When parts cost this much, it is often a better decision to replace the gearbox entirely.

In short, if water gets into a transmission, there is no way around an expensive repair. Flushing out the fluid will only cost you extra money and can make the inevitable failure happen sooner. Check your engine’s cooling system regularly and ask a professional transmission mechanic if your car is a common one for this type of failure. If so, bypass the radiator with an external oil cooler.

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