Control valves are simple "gate" type valves which either open or close off a circuit with no alteration in pressure.
The manual valve connected to the gear shift lever is a good example of this type of control valve, positioned by the driver to direct fluid to components when a drive range is selected.
Control valves can also react to changes in pressure.
This simple shift valve is held at the end of its bore by spring tension and blocks the fluid under pressure at port A.
The larger differential area at port C is subject to governor pressure and as vehicle road speed increases, the increasing pressure forces the valve to the left and allows fluid to flow across from port A to port B.
This can be directed to operate the next clutch or servo and change the transmission to a higher ratio.
As road speed falls and governor pressure diminishes, the spring moves the valve back to cut off the fluid supply and the transmission downshifts.
