Driving force distribution control device

A driving force distribution control device mounted on a four-wheel drive vehicle, configured to automatically switch a state between a two-wheel driving state and a four-wheel driving state, comprising:

• a pair of left and right main driving wheels to which a driving force of a drive source is transmitted both in the two-wheel driving state and the four-wheel driving state;
• a pair of left and right auxiliary driving wheels to which the driving force is transmitted only in the four-wheel driving state;
• a drive shaft configured to transmit the driving force to the auxiliary driving wheels;
• a coupling mechanism configured to switch the state between the two-wheel driving state and the four-wheel driving state by disconnecting and connecting the drive shaft from and to the auxiliary driving wheels, respectively, and change a distribution of the driving force to the main driving wheels and the auxiliary driving wheels by adjusting the fastening force while the drive shaft is connected to the auxiliary driving wheels;
• a coupling mechanism controller including a processor, configured to control operation of the coupling mechanism;
• a vehicle speed sensor configured to detect a vehicle speed of the four-wheel drive vehicle;
• an accelerator opening sensor configured to detect an accelerator opening of the four-wheel drive vehicle;
• and
• a slip detector configured to detect a slip of the main driving wheels,
• wherein the coupling mechanism controller connects the drive shaft with the auxiliary driving wheel and sets the fastening force to a first fastening force, when a rate of increase in the accelerator opening detected by the accelerator opening sensor becomes more than a given value while the drive shaft is disconnected from the auxiliary driving wheels and the vehicle speed detected by the vehicle speed sensor is below a given first speed, and changes the fastening force from the first fastening force to a second fastening force, when a slip of at least one of the main driving wheels is detected by the slip detector after the fastening force is set to the first fastening force, and before a given period of time has lapsed from the setting of the fastening force, or before the vehicle speed detected by the vehicle speed sensor becomes faster than a given second speed faster than the given first speed, and
• wherein the second fastening force at least immediately after the fastening force is changed from the first fastening force to the second fastening force is a value larger than the first fastening force.