Hydraulic control system for automatic transmission

In a hydraulic control system for an automatic transmission having a gear shift mechanism with a first frictional engagement element adapted to be engaged to achieve a first specific speed stage and a second frictional engagement element adapted to be engaged to achieve a second specific speed stage faster than and closest to the first specific speed stage, wherein the hydraulic control system comprises a hydraulic control unit for controlling engagement and release of said first and second frictional engagement elements of said gear shift mechanism by feeding hydraulic pressure thereto or discharging hydraulic pressure therefrom, and an electronic control unit for controlling said hydraulic control unit by solenoid signals, said hydraulic control unit comprising a first directional control valve for switching feeding of hydraulic pressure to one of said first and second frictional engagement elements and discharge of hydraulic pressure from the other, responsive to a first solenoid signal outputted from said electronic control unit, and a second directional control valve adapted to be switched by a second solenoid signal output from said electronic control unit to achieve a third speed stage still faster than the second speed stage, the improvement comprising:

• a discharge line connecting said first frictional engagement element to a drain line via said second directional control valve;
• gear shift sensor means for sensing a gear shift;
• and
• tie-up determination means for determining tie-up of one or both the frictional engagement elements from a detection signal from gear shift sensor means, whereby upon determination of tie-up by said tie-up determination means, said second solenoid signal is output to said second directional control valve.