2. Controls the shifting pattern ofthetransmission. This is done by switching hydraulic pressure to programmed combinations of clutches and brake bands based on vehicle speed and engine load.

3. Circulates the transmission fluid through a remote cooler to remove excess heat that is generated in the transmission and torque converter.

4. Provides a constant fresh supply of oil to all critical wearing surfaces of the transmission.

The hydraulic system for an automatic transmission typically consists of the following.

Pump

The typical hydraulic pump (fig. 4-36) is an internal-external rotor or gear-type pump. Located in the front of the transmission case, it is keyed to the torque converter hub so that it is driven by the engine. As the torque converter spins the oil pump, transmission fluid is drawn into the pump from the transmission pan. The pump compresses the oil and forces it to the pressure regulator. The pump has several basic functions:

Produces pressure to operate the clutches, the bands, and the gearsets.

Lubricates the moving parts in the transmission.

Keeps the torque converter filled with transmission fluid for proper operation.

Circulates transmission fluid through the transmission and cooling tank (radiator) to transfer heat.

Operates hydraulic valves in the transmission.

Pressure Regulator

The pressure regulator limits the maximum amount of oil pressure developed by the oil pump. It is a spring-loaded valve that routes excess pump pressure out of the hydraulic system, assuring proper transmission operation.

Figure 4-36. - Typical transmission hydraulic pump.