Engine "Small Cooling Fan" control unit based on a signal provided by the respective sensors, the use of internal storage target coolant temperature characteristic curve and the cooling fan speed characteristic to calculate the optimal running speed of the cooling fan, and the cooling fan speed data into account cycle data, and then sent to the PWM signal representing the cooling fan control unit, "Small Cooling Fan" duty control of the control unit 2 through the fan at a constant speed operation according to the engine control unit receives the duty cycle signal. Under normal circumstances, the engine control unit to the "Small Cooling Fan" control unit sends a control signal the duty cycle is 10% to 90%, when the cooling fan control unit receiving this interval of the duty cycle signal, will be based on the duty than the size of the cooling fan speed control.