Control system of a downshift by an automatic transmission gear and method for the same

A control system of a downshift by an automatic transmission gear comprising:

• a throttle valve sensor for sensing an opening of a throttle valve wherein variations of said opening of the throttle valve, according to an operation of an accelerator pedal, changes an electrical signal to be outputted;
• an input rpm sensor for sensing revolutions per minute (rpm) of an input shaft wherein variations in the rotational speed of said input shaft changes an electrical signal to be outputted;
• an output shaft rpm sensor for sensing rpm of an output shaft wherein variations in the rotational speed of the output shaft changes an electrical signal to be outputted;
• a transmission control unit connected to said throttle valve sensor, said input shaft rpm sensor and said output shaft rpm sensor and for determining that a vehicle is in a state of a downshift if the running state of the vehicle satisfies predetermined conditions, thereby outputting a control signal for a shift to a first speed after reducing hydraulic pressure fed to friction members to a predetermined state using a duty pattern when the vehicle is in a state of a second-to-first speed downshift, or a control signal for a second-to-first speed shift after a third-to-second speed shift is completed when the vehicle is in a state of a third-to-first speed shift;
• a first and a second transmission control solenoid valve wherein said control signal output from said transmission control unit changes their operation so that a a transmission ratio corresponding to the downshift state of the vehicle can be obtained;
• and
• a hydraulic pressure control solenoid valve wherein said control signal outputted from the transmission control unit changes its operation so that the operational state of pressure fed to every friction member may be changed;
• wherein said transmission control unit detects variable values according to said opening of the throttle valve determined by said output signal from said throttle valve sensor to set the duty pattern, outputs said control signal for controlling the operational state of said hydraulic pressure control solenoid valve according to the duty pattern and outputs a control signal for changing the operation of said first and second transmission control solenoid valves to be in the first speed after a predetermined period passes from the completion of a predetermined state of the transmission if the vehicle is in the second-to-first speed downshift state.