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| Transmission Pressure Control |
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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
while late model vehicles electric solenoids.
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.
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
while late model vehicles electric solenoids.
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.