The servo orifice control valve acts as a timing device to prevent engine flare at wide throttle openings on a 2-3 upshift and conversely to prevent the transmission becoming "tied-up" or being momentarily in two gears during a 3-2 downshift.
It is a shuttle valve which can be positioned at rest by a spring to allow unrestricted fluid flow, or can be moved by governor pressure to insert a restricting orifice in the front band servo release circuit.
At low vehicle speeds, governor pressure is not high enough to move the valve and when the 2-3 upshift occurs, fluid flows unrestricted to the release side of the front servo.
Since this type of shift will occur at light throttle openings, the rapid clutch and band actuation does not cause engine flare.
With wider throttle openings, vehicle speeds will be higher before an upshift occurs and governor pressure will have built up sufficiently to move the valve, so that the fluid must now pass through the orifice.
This slows up the release of the front band and thus ensures the rear clutch is fully applied and able to transmit torque as the band comes off.
This prevents engine flare from occurring.
Similarly on a downshift at high vehicle speeds, the orifice slows up the release of fluid from the release side of the front servo and therefore the re-application of the front band.
This ensures the rear clutch comes off as the band is applied and prevents a lock-up condition in the transmission.
