Hydraulic control system for an automatic transmission

Claim 1

In an automatic transmission for a vehicle comprising a speed change gearing, at least a first frictional engagement means and a second frictional engagement means each associated with said speed change gearing, a first hydraulic servo means for actuating said first frictional engagement means, a second hydraulic servo means for actuating said second frictional means, wherein the improvement comprises a hydraulic control circuit system for supplying pressurized oil to said first servo means and releasing pressurized oil from said second servo means under high speed conditions of the automatic transmission and for releasing pressurized oil from said first servo means and for supplying pressurized oil to said second servo means under low speed conditions of the automatic transmission, said hydraulic control circuit system comprising oil passages connected to said first and second servo means and a speed-change valve means, a timing valve means, and a manual valve which are connected to at least certain of said oil passages, said speed-change valve means arranged to receive a signal established by the running speed of the vehicle and based on the signal to effect a switching of the pressurized oil in said first servo means between the supply and release condition, said timing valve comprising a valve housing forming at least a first oil chamber and a second oil chamber positioned adjoining said first oil chamber and another oil chamber in communication with said first servo means, said first oil chamber arranged in communication through one of said oil passages with said second servo means, a valve body displaceable within said valve housing between one position for affording communication between said first and second oil chambers when pressurized oil is released from said second servo means and another position for preventing communication between said first and second oil chambers for supplying pressurized oil through said first oil chamber to said second servo means when the pressure of the oil supplied to said first servo means drops below a predetermined value relative to a signal corresponding to the engine output of the vehicle, said manual valve arranged to select the desired speed condition of the vehicle and having a plurality of positions including a position for selecting low speed conditions corresponding to the low speed conditions under which pressurized oil is supplied to said second servo means, said second oil chamber arranged in communication with said manual valve through another one of said passages when said manual valve is positioned for low speed conditions, said valve body being displaceable into position for regulating communication between said first and second oil chambers and when under high speed conditions the pressurized oil supplied to said first servo means drops below a predetermined value corresponding to the engine output of the vehicle so that if said valve body becomes stuck when it is to be displaced and cannot prevent communication between said first and second oil chambers when low speed conditions are to be established then pressurized oil is supplied to said second oil chamber from said manual valve for passage into said first oil chamber and then into said second servo means for obtaining low speed conditions.