by Chris Weis
Mike Powell / Photodisc / Getty Images
It may seem like a pretty long shot, but engine coolant or antifreeze can get into the automatic transmission fluid. Transmission fluid temperature is regulated within the engine’s cooling system. The fluid passes through a small tank inside the engine’s radiator. Engine coolant surrounds the tank to help keep the fluid inside at a constant temperature. Any rupture of the internal radiator tank can allow the coolant to mix and contaminate the transmission fluid. Engine coolant can also become contaminated with transmission fluid, as the pressures of either fluid fluctuate. The extent of damage to either circuit depends on the severity and longevity of the internal leak.
It is a fact that water and oil do not mix, and antifreeze is mostly water despite the additives involved. However, the drive pump comes close to fully combining the two fluids. Automatic transmission pumps are made up of gears that mesh to compress and drive fluid, which is basically hydraulic oil. Due to the tight tolerances between the gears, the gears are capable of crushing the various coolant and oil particles into a foamy mess. Minor contamination can be noticed from the foam seen at the top of the liquid level shown on the transmission dipstick. The most serious cases are shown with a dipstick covered with a substance that resembles a strawberry milkshake.
The pump that pressurizes and circulates the coolant uses vanes instead of gears to drive the antifreeze. Vanes lack the tight tolerances of gear pumps and fluids don’t mix as well. Small droplets of transmission oil separate and rise to the surface of the coolant in the radiator. This inspection often involves removal of the radiator cap, and this action should never be attempted until the radiator is completely cool. The time required for cooling is more than enough for the droplets to form. An oily sheen or oil droplets seen on the radiator or coolant recovery tank may indicate a broken transmission cooler tank. The underside of the radiator cap may also have a gummy residue.
The damage done
Automatic transmissions often succumb to seemingly minor deficiencies in fluid quality or quantity. Therefore, it is not surprising that a highly contaminated fluid can paralyze the complex component. The fluid pressures necessary to operate and lubricate the transmission cannot be achieved when the oil is diluted. Contaminated fluid resists the uniform compression required for hydraulic functions, and the transmission slips under certain circumstances or does not engage at all. Lubrication can fail while running, and the friction and resulting heat ruin vital parts of the transmission. The consequences may be less severe for the engine cooling system. The function of the refrigerant is compromised by contamination, but the overall effects are usually mild.
The transmission cooler tank in the radiator cannot be repaired in modern applications, and the radiator and tank are replaced as a single unit. The engine cooling system can be fully restored by thoroughly flushing and installing new hoses and coolant along with the new radiator. In some cases, the transmission can recover after the internal filter is replaced and a professional flushes and renews the fluid under pressure. If the transmission is in use for a considerable time while the fluid is contaminated, extensive repairs may be required. In some cases, a rebuilt or remanufactured transmission costs less than restoring the original.