Transmission Pressure Control
Automatic transmissions use mainline, governor, and throttle pressure valves to both control and lubricate the transmission. Some of these have been replaced or work together with electronic controls. It’s important to know the principles behind these devices.
The mainline pressure is controlled by the pressure regulator valve. Mainline pressure is the source pressure for the torque converter, valve body, clutches, and bands in an automatic transmission. As pressure builds from the transmission pump, the regulator exhausts excess pressure preventing any damage that may occur to internal components.
Governor pressure increases with vehicle speed. Older transmissions had mechanical governors that consist of springs, centrifugal weights, and a spool valve to control the pressure. Today’s transmissions typically use a solenoid to control governor pressure. Governor pressure controls upshifting while throttle pressure controls downshifting.
Throttle pressure is a more of a signal pressure indicating 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.
Transmissions actually change gears by the movement of shift valves. Governor pressure works on one end of the valve while throttle pressure aided by a spring works on the other. When a vehicle first accelerates from a stop throttle pressure is higher than governor so the vehicle stays in first gear. As speed increases the governor pressure (affected by vehicle speed) becomes higher until it overcomes throttle pressure and causes an upshift.
A downshift occurs when throttle pressure overcomes governor pressure do to increased engine load. If the driver were to accelerate to pass another vehicle, the sudden throttle opening or drop in manifold pressure would open the throttle valve causing an increase in throttle pressure. These two pressures working at opposite ends of the shift valve are what control the appropriate reactionary devices (clutches and bands) involved in automatic transmission shifting.
