Automatic Hydraulic Transmissions consist of a number of primary components:
The torque converter which is a type of fluid coupling that hydraulically connects the engine to the transmission. It replaces the mechanical clutch, allowing the transmission to stay in gear and the engine to remain running while the vehicle is stopped, without stalling out the engine. A torque converter is different from a simple fluid coupling, because it creates variable amounts of torque multiplication at low engine speeds, thereby increasing breakaway acceleration. This is done by a third member of the coupling assembly called a stator, and by altering the shapes of the vanes inside the coupling to curve the fluid's path and flow into the stator. The stator captures the kinetic force of the transmission fluid, using the leftover force to enhance torque multiplication.
A gear pump which is mounted between the torque converter and the planetary gearset. It pulls transmission fluid from the sump and pressurizes it to the level needed for transmission components to operate. The pump is connected to the torque converter housing, which is bolted to the engine's flywheel, so the pump generates pressure when the engine is operating.
A compound epicyclic "planetary gearset", the bands and clutches of which are actuated by hydraulic servos controlled by the valve body, providing multiple gear ratios.
Clutches and bands effect gear changes. These use one of two types of clutches or bands that are used to hold a specific member of the planetary gearset motionless, while permitting another gearset to rotate and transmit torque and producing gear reductions or overdrive ratios. These clutches are controlled by the valve body, and their sequence is determined by the transmission's internal programming. Usually, a device known as a sprag or roller clutch is used for routine upshifts/downshifts. It transmits torque only in one direction, free-wheeling in the other. The bands are involved for manually selected gears (low range or reverse), and operate on the planetary drum's circumference. Bands are not applied once drive/overdrive range is engaged, with the torque being transmitted by the sprag clutches instead.
The "valve body," a hydraulic control center that gets pressurized fluid from the main pump to run a network of spring-loaded valves, check balls, and servo pistons. The valves use the pump pressure and the pressure from a governor on the output end. They also use hydraulic signals sent from range selector valves and a modulator to determine exactly which gearset ratio is needed. When the vehicle and engine accelerate or slow down, the pressures changes, causing specific valves to open or shut. The pressure regulated by these valves controls the clutch and brake band actuators, which controls the planetary gearset which selects the proper gear ratio for the existing operating conditions. In many modern automatic transmissions, the valves are controlled by electro-mechanical servos which in turn are regulated by the electronic engine control unit (ECU) or a separate transmission control unit (TCU).
Golden State Transmission and Muffler 10792 Olson Dr., Rancho Cordova, California 95670 916.853.1788