Why would transmission fluid leak?
First, we should acknowledge that the transmission, whether automatic or manual, is actually designed to hold the fluid without leaking. However, there are many factors which can cause a transmission to leak.
Drying or hardening seals can cause leaks. Failing pressure lines in an automatic transmission system can crack, dry rot, or be damaged by something hitting them while driving. Automatic transmission systems use a cooler unit usually included in the radiator and these can fail due to vibration or corrosion of the housing over time. If an external or additional cooler is used, it is often in front of the radiator and condenser and subject to damage from debris on the highway. Areas known for leaks include faulty components, misaligned or faulty gaskets, and loose drain plugs or lines. Temperature changes and the difference in metal and gasket types can lead to leaks over time. Modern electronically-controlled transmissions have electrical connections that go into the transmission case or through the transmission pan. I have seen porous casting in the past causing internal or external leaks as well. In older vehicles that used vacuum modulator valves to control shift points, the diaphragm would sometimes rupture, causing the fluid to be leaked into the engine through the vacuum line that was used to signal or control the valves’ operation.
As with any system, a regular check of fluid level and condition will help to avoid costly repairs. If you suspect a leak, have the vehicle inspected as soon as possible. Transmission fluid serves several purposes. It acts as the pressure agent to actuate clutch packs to control the different gear ranges. It also lubricates and cools the transmission. Loss of fluid causes aeration of the fluid, known as cavitation. Like all hydraulic systems, the transmission system depends on the ability of the fluids to not compress if the level get low. If they get air bound, parts can slip and damage will ensue. Keeping the level full and the fluid cool will extend the life of the transmission.