Hydraulic control system of an automatic power transmission

Claim 1

A hydraulic control system of an automatic power transmission for selectively producing a multiplicity of forward-speed gear ratios and a reverse-speed gear ratio, comprising a source of line pressure, a throttle valve responsive to load on an engine for producing from the line pressure a throttle pressure related to the engine load, a governor valve responsive to vehicle speed for producing from the line pressure a governor pressure related to the vehicle speed, a hysteresis valve for producing from the line pressure a substantially constant hysteresis pressure which is higher than said governor pressure, a kick-down valve communicating with said throttle valve and operative to pass therethrough the hysteresis pressure when the throttle pressure is increased to a level substantially representative of kick-down condition of the engine, and a plurality of shift valves each having a downshift position and an upshift position, at least one of said shift valves having first differential pressure-acting areas and second differential pressure-acting areas and formed with a first fluid port to communicate with said source of line pressure during automatic drive range condition of the transmission, the line pressure in the first fluid port acting upon the first differential pressure-acting areas for urging the shift valve toward the downshift position when the shift valve is in the downshift position, the line pressure in the first fluid port being isolated from the first differential pressure-acting areas when the shift valve is in the upshift position, a second fluid port which is in constant communication with said throttle valve, the throttle pressure in the second fluid port urging the shift valve toward the downshift position, a third fluid port communicating with said governor valve, the governor pressure in the third fluid port urging the shift valve toward the upshift position, a fourth fluid port in constant communication with said hysteresis valve and located to be closed when the shift valve is in the upshift position, the hysteresis pressure in the fourth fluid port acting upon said second differential pressure-acting areas for urging the shift valve toward the downshift position when the shift valve is in the downshift position, and a fifth fluid port communicating with said kick-down valve and located to be closed when the shift valve is in the downshift position, the hysteresis pressure passed through the kick-down valve to the fifth fluid port acting upon the second differential pressureacting areas for urging the shift valve toward the downshift position when the shift valve is in the upshift position.