Transmission fluid serves three purposes. This slippery liquid lubricates the moving parts inside the transmission. In cars with automatic transmission, it also cools the inner workings and transmits power from the engine.
Overfilling the Transmission Fluid
Overfilling the transmission fluid will not cause permanent damage to your car. It can cause some changes you may notice. Frothing and leaks are the symptoms of an overfilled transmission.
When the fluid level is too high, it can come into contact with moving parts. The normal level of the fluid is below these parts. The moving parts can aerate the fluid, causing it to froth and foam. While this may look like cause for alarm, it is harmless.
Overfilling the transmission fluid will not cause seals to break, but they may leak. The transmission is vented so that pressure won't build in the unpressurized areas. Transmission fluid can leak out these vents if it is too high. The seals higher in the transmission may also leak, but these seals are not pressurised.