Hydraulic control system for automatic transmission

A hydraulic control system for an automatic transmission performing an up-shift to a predetermined gear ratio by engaging a first frictional engagement element and disengaging a second frictional engagement element comprising:

• an input shaft which receives power from an engine output shaft;
• an output shaft which is connected to vehicle wheels;
• a plurality of frictional engagement elements for selectively changing torque transfer from the input shaft to the output shaft between plural torque transfer paths and comprising the first frictional engagement element and the second frictional engagement element;
• hydraulic servos that engage and disengage the frictional engagement elements;
• a hydraulic operator system which controls at least hydraulic pressures applied to the hydraulic servos for the first frictional engagement element and the second frictional engagement element;
• and
• a control unit which receives input signals from sensors which monitor various vehicle operating parameters and which outputs hydraulic control signals to said hydraulic operator system, said control unit comprising an input torque calculator which calculates an input torque, an engagement side pressure controller which signals said hydraulic operator system to control the hydraulic pressure applied to the hydraulic servo for the first frictional engagement element, and a disengagement side pressure controller which determines if the calculated input torque is above or below a predetermined value and signals said hydraulic operator system to control the hydraulic pressure applied to the hydraulic servo for the second frictional engagement element in accordance with the calculated input torque and said determination, said disengagement side hydraulic controller operating in a maintain mode, when the calculated input torque is determined to be above said predetermined valve, to set the hydraulic pressure for the second frictional engagement element to a pressure which is higher than a base pressure, and operating in a slip mode, when the calculated input torque is determined to be below said predetermined value, to set the hydraulic pressure for the second frictional engagement element to a pressure which is lower than the base pressure, said base pressure varying as a function of the calculated input torque and providing the second frictional engagement element with a torque capacity corresponding to the calculated input torque.