It may seem like a rather remote possibility, but engine coolant, or anti-freeze can get into automatic transmission fluid. The temperature of transmission fluid is regulated inside the engine 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 coolant to mix with, and contaminate transmission fluid. The engine coolant can also become polluted by the transmission fluid, as the pressures of either liquid vacillate. 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 anti-freeze is mostly water despite the additives involved. However, the transmission pump comes close to completely combining the two liquids. Automatic transmission pumps are comprised of gears that mesh to compress and impel the fluid, which is basically hydraulic oil. Because of the tight tolerances between the cogs, the gears are capable of mashing the diverse particles of coolant and oil into a frothy mess. Minor contamination may be noted by foam seen at the top of the fluid level displayed on the transmission dipstick. More severe cases are exhibited by a dipstick covered with a substance that resembles a strawberry milkshake.
The pump that pressurizes and circulates coolant uses vanes rather than gears to drive the anti-freeze. The vanes lack the tight tolerances of gear-style pumps, and the fluids are not mixed so well. Small droplets of transmission oil separate and rise to the surface of the coolant in the radiator. This inspection often involves the removal of the radiator cap, and this action should never be attempted until the radiator cools completely. The time required for the cool-down is more than sufficient for the droplets to form. An oily sheen, or oil droplets seen in the radiator or coolant recovery reservoir can indicate a ruptured transmission cooler tank. The underside of the radiator cap may exhibit a gummy residue as well.
The Damage Done
Automatic transmissions often succumb to seemingly slight deficiencies in fluid quality or quantity. Therefore, it is not surprising that grossly contaminated fluid can cripple the complex component. The fluid pressures required to operate and lubricate the transmission can not be achieved when the oil is diluted. The polluted fluid resists uniform compression necessary for hydraulic functions, and the transmission either slips in certain circumstances, or fails to engage at all. Lubrication failure can occur while underway, and the friction and resultant heat ruin vital transmission parts. Consequences can be less severe for the engine cooling system. Coolant function is compromised by the contamination, but overall effects are usually mild.
The transmission cooler tank in the radiator is not serviceable in modern applications, and the radiator and tank are replaced as one unit. The engine cooling system may be completely restored by a thorough flushing and the installation of new hoses and coolant along with the new radiator. In some instances, the transmission might recover after the internal filter is replaced, and the fluid flushed out and renewed under pressure by a professional. Should the transmission be in use for any appreciable time while the fluid is polluted, extensive repairs can be required. In some cases, a rebuilt or re-manufactured transmission costs less than restoring the original.
- Basic Car Care Illustrated 2nd Edition; William Flerx et al
- Photo Credit Mike Powell/Photodisc/Getty Images