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Automatic Transmission Hydraulic System
The Hydraulic system is a complex maze of passages and tubes that sends transmission fluid under pressure to all parts of the transmission and torque converter. The diagram below is a simple one from a 3-speed automatic from the '60s. The newer systems are much more complex and are combined with computerized electrical components. Transmission fluid serves a number of purposes including: shift control, general lubrication and transmission cooling. Unlike the engine, which uses oil primarily for lubrication, every aspect of a transmission's functions are dependant on a constant supply of fluid under pressure. This is not unlike the human circulatory system (the fluid is even red) where even a few minutes of operation when there is a lack of pressure can be harmful or even fatal to the life of the transmission. In order to keep the transmission at normal operating temperature, a portion of the fluid is sent through one of two steel tubes to a special chamber that is submerged in anti-freeze in the radiator. Fluid passing through this chamber is cooled and then returned to the transmission through the other steel tube. A typical transmission has an average of ten quarts of fluid between the transmission, torque converter, and cooler tank. In fact, most of the components of a transmission are constantly submerged in fluid including the clutch packs and bands. The friction surfaces on these parts are designed to operate properly only when they are submerged in oil.
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