Four-wheel drive vehicle control system and methods

1. A method comprising:

responsive to a desired shift from a four-wheel drive mode to a two-wheel drive mode:

decreasing a transfer case torque output to a secondary driveline to a lower first level and disengaging a disconnect device of the secondary driveline;

increasing the transfer case torque output from the lower first level to a higher second level; and

reducing the transfer case torque output from the higher second level to decouple the secondary driveline.

2. The method of claim 1 , wherein the desired shift from the four-wheel drive mode to the two-wheel drive mode is commanded by a vehicle operator.

3. The method of claim 1 , wherein the desired shift from the four-wheel drive mode to the two-wheel drive mode is commanded by a vehicle controller based on vehicle operating conditions, the operating conditions including one or more of: throttle position, accelerator pedal position, wheel speed, ambient temperature, steering angle, vehicle yaw, windshield wiper state, driver selectable mode controls such as sport, economy, winter, etc., brake pressure, anti-lock brake control status, altitude, tire pressure, fog light state, driver and occupant presence, driving habits, weather forecast, navigation routing road conditions, terrain, and traffic conditions.

4. The method of claim 1 , wherein the increasing the transfer case torque output comprises ramping up the transfer case torque output from the lower first level to the higher second level over a duration, and where after the duration, the transfer case torque is reduced to zero, to stop torque transmission to the secondary driveline.

5. The method of claim 1 , wherein the secondary driveline comprises one or more driveline components not driven by an engine during the two-wheel drive mode.

6. The method of claim 1 , wherein the increasing the transfer case torque comprises increasing one or more of a current and voltage to an actuator of a transfer case disconnect clutch to shift a clutch collar of the transfer case disconnect clutch to an engaged position where the secondary driveline is coupled to a transfer case and receives torque output from the engine.

7. The method of claim 1 , wherein the decreasing and reducing the transfer case torque comprises decreasing one or more of the current and voltage to an actuator of a transfer case disconnect clutch to shift a clutch collar of the transfer case disconnect clutch to a disengaged position where the secondary driveline is not coupled to the transfer case and does not receive torque output from the engine.

8. The method of claim 1 , wherein the decreasing the transfer case torque comprises stepping down the transfer case torque to the lower first level.

9. The method of claim 1 , wherein the increasing the transfer case output torque is initiated in response to the transfer case output torque reaching the lower first level.

10. The method of claim 1 , wherein the increasing the transfer case output torque is initiated after waiting a duration once the transfer case output torque reaches the lower first level.

11. The method of claim 1 , wherein the increasing the transfer case output torque is initiated in response to a determination that the disconnect device remains in the engaged position.

12. The method of claim 1 , wherein the increasing the transfer case output torque when shifting to the two-wheel drive mode occurs only when the disconnect device does not disengage after the transfer case output torque is decreased to the lower first level, and where, when the disconnect device disengages after decreasing the transfer case torque output to the lower first level, the transfer case output torque is maintained at the lower first level.

13. A system comprising:

a vehicle powertrain comprising:

an engine;

a transfer case;

a primary driveline including a first pair of wheels, the primary driveline drivingly coupled to the engine;

a secondary driveline including a second pair of wheels, the secondary driveline selectively coupled to the engine via the transfer case;

a secondary driveline disconnect positioned in the secondary driveline; and

a controller with computer readable instructions stored in non-transitory memory for:

engaging the engine and secondary driveline when a shift from a two-wheel drive mode to a four-wheel drive mode is desired; and

disengaging, and then re-engaging for a duration, before again disengaging the secondary driveline and engine when a shift from the four-wheel drive mode to the two-wheel drive mode is desired.

14. The system of claim 13 , wherein the controller further includes computer readable instructions stored in non-transitory memory for:

engaging the driveline disconnect when a shift from the two-wheel drive mode to the four-wheel drive mode is desired; and

disengaging the driveline disconnect when a shift from the four-wheel drive mode to the two-wheel drive mode is desired.

15. The system of claim 13 , wherein the transfer case further comprises a transfer case disconnect, where the transfer case disconnect is shifted to an engaged position by increasing an amount of electrical power supplied to the transfer case disconnect when engaging the engine and secondary driveline, where an amount of torque transmitted to the secondary driveline increases as the transfer case disconnect is adjusted towards the engaged position, and where the transfer case disconnect is shifted to a disengaged position by decreasing the amount of electrical power supplied to the transfer case disconnect when disengaging the secondary driveline and engine, where the amount of torque transmitted to the secondary driveline decreases as the transfer case disconnect is adjusted towards the disengaged position.

16. A method comprising:

increasing a transfer case coil voltage from a lower first level to a higher second level when shifting from a two-wheel drive mode to a four-wheel drive mode and then adjusting the coil voltage based on vehicle operating conditions in the four-wheel drive mode; and

stepping down the coil voltage to the lower first level, then increasing the coil voltage from the lower first level to the higher second level, and then stepping down the coil voltage from the higher second level to the lower first level when shifting from a four-wheel drive mode to a two-wheel drive mode, and then maintaining the coil voltage at the lower first level in the two-wheel drive mode.

17. The method of claim 16 , wherein the increasing the coil voltage from the lower first level to the higher second level when shifting from the four-wheel drive mode to the two-wheel drive mode comprises ramping up the coil voltage from the lower first level to the higher second level over a duration.

18. The method of claim 16 further comprising, indicating to a vehicle operator that a shift error has occurred when a secondary driveline disconnect device remains in an engaged position after a shift to the two-wheel drive mode.

19. The of claim 16 further comprising, stopping increasing the transfer case torque before reaching the higher second level when increasing the coil voltage from the lower first level when shifting from the four-wheel drive mode to the two-wheel drive mode, in response to a secondary driveline disconnect device disengaging.

20. The method of claim 16 , wherein the adjusting the coil voltage based on vehicle operating conditions in the four-wheel drive mode comprises adjusting an amount of torque transmitted to a secondary driveline based on one or more of a throttle position, a difference in speeds between left and right wheels of an axle, a difference in speeds between rear and front wheels, and a vehicle speed.