Two-speed transfer case with electronic torque modulation

In a vehicle having a first set of wheels and a second set of wheels adapted to be driven from a source of power, a power transfer system comprising:

• a transfer mechanism having an input member rotatably driven by the power source, a first output member interconnecting said input member to the first set of wheels for transmitting drive torque from the power source to the first set of wheels for defining a two-wheel drive mode, a second output member interconnected to the second set of wheels, a speed reduction mechanism driven by said input member for establishing a reduced-ratio low-range drive, a shift member movable between a first position for directly coupling said first output member to said input member for establishing a high-range drive and a second position for coupling said first output member to said speed reduction mechanism for establishing said low-range drive, a clutch assembly operable in an actuated state for selectively transmitting drive torque from said first output member to said second output member for establishing a four-wheel drive mode, coupling means for mechanically coupling said first output member to said second output member in response to movement of said shift member to said second position, and biasing means for urging said coupling means to a position de-coupled from said second output member upon movement of said shift member to said position;
• first drive means interconnecting said first output member of said transfer mechanism to the first set of wheels;
• second drive means interconnecting said second output member of said transfer mechanism to the second set of wheels;
• first sensor means for sensing the rotational speed of said first drive means and generating a first input signal indicative thereof;
• second sensor means for sensing the rotational speed of said second drive means and generating a second input signal indicative thereof;
• mode select means for enabling a vehicle operator to intentionally select one of a two-wheel high-range drive mode, a four-wheel high-range drive mode, a four-wheel low-range drive mode and an on-demand four-wheel high-range drive mode, said mode select means further operable for generating a mode signal indicative of the particular drive mode selected;
• coordinating means for coordinating movement of said shift member and said actuated state of said clutch assembly;
• actuator means for selectively moving said coordinating means;
• and
• controller means for receiving said first and second input signals and generating a speed differential signal in response thereto that is indicative of a speed differential between said first and second drive means, and said controller means controlling actuation of said actuator means in response to said mode signal for causing said actuator means to move said coordinating means to a 2WH position to de-actuate said clutch assembly and move said shift member to said first position when said two-wheel high-range drive mode is selected, said controller means causing said actuator means to move said coordinating means to a 4WH position to fully actuate said clutch assembly and move said shift member to said first position when said four-wheel high-range drive mode is selected, said controller means causing said actuator means to move said coordinating means to a 4WL position to de-actuate said clutch assembly and move said shift member to said second position and when said four-wheel low-range drive mode is selected, and said controller means causing said actuator means to modulate the position of said coordinating means between said 2WH and 4WH positions to vary the degree of actuation of said clutch assembly between said de-actuated and fully actuated conditions as a function of said sensor input signals for modulating the drive torque transmitted to the second set of wheels while maintaining said shift member in said first position when said on-demand mode is selected.