Cooling system for working fluid used in automatic transmission of automotive vehicle

A cooling system for working fluid used in an automatic transmission of an automotive vehicle, comprising:

• a water-cooled oil cooler and an air-cooled oil cooler connected in series to each other and both disposed in an oil cooler line for cooling the working fluid;
• an oil pump located in the automatic transmission and disposed in the oil cooler line for pressurizing and sending the working fluid into the oil cooler line;
• a hydraulic control circuit disposed downstream of the oil pump for regulating a line pressure produced by the oil pump;
• a lubrication circuit located in the automatic transmission for lubrication of moving transmission parts;
• a temperature sensor that detects a working-fluid temperature;
• a first bypass valve that permits the working fluid to circulate via the first bypass valve through the oil pump, the hydraulic control circuit and the lubrication circuit, bypassing the water-cooled oil cooler and the air-cooled oil cooler;
• a second bypass valve that permits the working fluid to circulate via the water-cooled oil cooler and the second bypass valve through the oil pump, the hydraulic control circuit and the lubrication circuit, bypassing only the air-cooled oil cooler;
• and
• a control unit configured to be connected electronically to the temperature sensor, and the first and second bypass valves, for automatically controlling opening and closing operations of each of the first and second bypass valves depending on the line pressure as well as the working-fluid temperature and for controlling the line pressure depending on a sensor signal from the temperature sensor;
• the control unit comprising:
• (a) a first bypass valve control section keeping the first bypass valve opened in a first control mode area that the working-fluid temperature is lower than or equal to a first predetermined temperature and the line pressure is higher than or equal to a predetermined pressure;
• and
• (b) a second bypass valve control section keeping the second bypass valve closed in a second control mode area that the working-fluid temperature is higher than a second predetermined temperature higher than the first predetermined temperature.