Transmission Shift Valves
Automatic transmissions use mainline, governor, and throttle pressure to control and lubricate the transmission. Some of the devices that control hydraulic transmissions have been replaced or work together with electronic controls.
Governor pressure increases with vehicle speed. Older transmissions had mechanical governors that consisted of springs, centrifugal weights, and a spool valve to control this pressure. Governor pressure causes a transmission to upshift, and throttle pressure causes it to downshift. Today’s transmissions use solenoids for the shift timing.
Throttle pressure indicates engine load. Some transmissions use a vacuum modulator, or throttle linkage to control the throttle valve. Late-model vehicles use electric solenoids to achieve the same results.
Shift Valves: Shift valves control hydraulic pressure flow to the reactionary devices (clutches and bands) that drive and hold members of the planetary gearset. Transmissions change gears by moving shift valves. Governor pressure works on one end of the valve, and a spring assisted by throttle pressure works on the other.
When a vehicle first accelerates from a stop, throttle pressure is higher than governor pressure, so the vehicle stays in first gear. As vehicle speed increases, the governor pressure (affected by vehicle speed) increases until it overcomes throttle pressure, moves the shift valve, and causes an upshift.
When throttle pressure overcomes governor pressure, the transmission downshifts. When a driver accelerates to pass another vehicle, there’s an increase in throttle pressure, causing a downshift. These two pressures control shift valve movement.