Automatic transmission fluid is a special oil designed to allow for proper transmission operation. Transmissions are equipped with a fluid cooler that prevents the overheating of the fluid, which can result in damage to the transmission. The transmission's oil pump is the source of all pressures with the hydraulic system. It provides a constant supply of oil under pressure to operate, lubricate, and cool the transmission. Pressure-regulating valves change the oil's pressure to control the shift points and shift quality of the transmission. Flow-directing valves direct the pressurized oil to the appropriate apply device to cause a change in gear ratios. The hydraulic system also keeps the torque converter filled with fluid.

Electronic Controls

Electrical and electronic circuits are used to perform work or control the operation of a device. An electrical switch is a simple electrical control that turns on or off a circuit, and consequently turns on or off the electrical device connected to the circuit. Switches can be opened or closed by the driver, mechanical linkages, or a predetermined condition. The latter typically is a low or high hydraulic pressure.

Electrical sensors are also a type of control; however, rather than merely opening and closing a circuit, sensors vary the flow of electricity through the circuit. Sensors are typically potentiometers or variable resistors that respond to changes in conditions. The sensor's resistance reflects the condition it is monitoring.

Solenoids and motors are commonly used in automobiles. These devices convert electrical energy into mechanical energy. A motor provides for a rotating power, whereas a solenoid provides for a linear movement. Solenoids in automatic transmissions are used to control the direction and flow of hydraulic pressure.

Until recently, all automatic transmissions were controlled by hydraulic circuits. However, many transmissions now control the operation of the torque converter and transmission through a computer. Based on information received from various electronic sensors and switches, a computer can control the operation of the torque converter and transmission's shift points. Computer-controlled electrical solenoids are typically used to control shifting.