Transmission fluid is an automotive lubricant pumped through the engine and transmission system to reduce the friction generated by turning internal components. Without this fluid, the transmission would overheat, seize up and destroy internal parts. While some car owners overfill their transmission fluid tanks so that the gauges read a higher level than "full," this can create problems further down the road.
Aeration is created by air bubbles entering the transmission lines and mixing with the fluid. If there is too much transmission fluid, then air can leak into the system when the car runs and develop air bubbles in the solution. These bubbles travel throughout the transmission system and interfere with the effectiveness of the fluid, giving the transmission components enough air to cause friction and create damage.
As the air bubbles gather in the transmission fluid, the transmission cannot transfer heat as easily, and the heat from its moving parts is localized in areas where friction occurs. This, combined with the oxygen from the air, can burn the transmission fluid. Owners can smell and see the brown color of the burnt transmission fluid, which will require replacing all the transmission fluid in the automobile.
Pressure problems occur when transmission lines overfilled and exert too much pressure against the seals that are meant to contain the fluid. These seals are designed to contain a full tank of fluid, not an overfull tank. Too much fluid and the seals will start to rupture, causing fluid leaks which many owners respond to by filling the tank again and making the problem worse.
Vent leaking occurs when the tank is too full of transmission fluid, and the fluid begins to leak out of the vents designed to allow air to flow through the system. This blocks necessary air and may show up as a leaking problem when the tank is actually overflowing.