A fully hydraulically-controlled transmission controls the shift points by a hydraulic control unit or valve body.
Fluid for transmission operation and lubrication is supplied by the pump. Pressure-control valves determine the pressures in the system according to operating conditions.
These conditions are signalled to the control valves and two inputs are critical.
Engine Load, and Vehicle Speed
The Engine Load signal can come via a cable connected to the throttle linkage. The Speed Signal is provided by a governor valve, attached to the transmission output shaft.
In an electronically controlled transmission, the speed of the vehicle and the throttle opening are sensed by the vehicle speed sensor, and the throttle position sensor. This information is sent as electrical signals to an ECU.
The ECU combines this input data with information in its memory, then sends output signals to electrical solenoids in the hydraulic control unit. The solenoids control the movement of the shift valves, which in turn controls the shifting of the transmission.
