Automatic transmissions use mainline, governor, and throttle pressure valves to control and lubricate the transmission. Some of these 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 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: Transmissions change gears by moving shift valves. Governor pressure works on one end of the valve and throttle pressure aided by a spring 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 and causes an upshift.
A downshift occurs when throttle pressure overcomes governor pressure. This is because of the increased engine load. 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. Shift valves control the reactionary devices (clutches and bands) that drive and hold members of the planetary gearset.