Four Wheel Drive System Operation - 2 Speed Torque On Demand

(2H) OPERATION 

When the ATCM is in 2WD (2H), power is delivered to the rear wheels only. This mode is appropriate for normal on-road driving on dry pavement and provides the best fuel economy. Torque is passed through the transfer case to the rear drive shaft at a 1:1 ratio. In 2WD (2H):

• The ATCM sends a 2WD (2H) mode status to the TCCM via GWM.

• The TCCM outputs a 0% duty cycle to the clutch field coil (4WD_CLTCH_OUT # = 0.00%).

• The shift motor is in the full CCW position. Refer to TRANSFER CASE SHIFT MOTOR STATUS in this service information for further information.

• 2WD (2H) will momentarily be displayed in the message center at key up and after a 4WD AUTO (4A) to 2WD (2H) shift.

AUTO (4A) and (4H) OPERATION 

When 4WD AUTO (4A) or 4WD (4H) is selected on the ATCM, the 4WD system provides electronically controlled four-wheel drive with power delivered to all four wheels, as required, for increased traction. 4WD AUTO (4A) is appropriate for all on-road driving conditions, such as dry road surfaces, wet pavement, light snow or gravel.

4WD (4H) provides electronically locked four-wheel drive power to both the front and rear wheels for use in off-road or winter conditions such as deep snow, sand or mud. This mode is not for use on dry pavement.

Shifts from 2WD (2H) to 4WD AUTO (4A)/(4H) can be made at any speed. When performing this shift, release the accelerator pedal prior to the shift and wait until the Shift in Progress message disappears in the IPC before accelerating, this improves the shift performance. In 4WD AUTO (4A):

• The ATCM sends an AUTO (4A) 4WD mode status to the TCCM via GWM.
• The TCCM outputs a 0% to 97% duty cycle to the clutch field coil (4WD_CLTCH_OUT # = x.xx%).
• In 4WD (4H), the TCCM outputs a constant 97% duty cycle to the clutch field coil.
• The shift motor is in the full CCW position. Refer to TRANSFER CASE SHIFT MOTOR STATUS in this service information for further information.
• 4A or 4H is displayed in the message center.

In 4WD AUTO (4A), the TCCM continuously monitors conditions and driver input to send torque automatically to the front driveline by controlling the transfer case clutch, providing 4WD capability. The TCCM sends a duty cycle command to the transfer case clutch coil as a torque request based on combination of preemptive and wheel slip response algorithm. Preemptive response is based on steering wheel angle, vehicle speed, throttle positions and available powertrain torque. Wheel slip response is based on monitoring the average front and rear wheel speeds.

When 4WD is no longer needed (during cruising or steady state driving) system defaults back to RWD (rear wheel drive) mode by setting the duty cycle output to 0.

TCCM increases the duty cycle to prevent or control slip under any of the following conditions:

• Slip is detected.
• Heavy acceleration (throttle position).
• Straight-ahead steering wheel angle.

TCCM decreases the duty cycle to prevent or reduce vehicle bind under any of the following conditions:

• Low speed.
• Low acceleration (throttle position).
• Tight turn steering wheel angle.

The TCCM has a thermal hardware protection strategy which monitors the amount of energy going through the clutch. This strategy has 2 levels of protections:

AUTOLOCK:  If the system is in 4WD 4H and TCCM detects driving conditions that require greater 4WD performance, the TCCM temporarily turns on 4WD (4H) from 4WD AUTO (4A) after detecting driving condition and informs the customer via the message center indicating 4X4 TEMPORARILY LOCKED. The 4WD system automatically returns to 4WD AUTO (4A) after the system no longer detects these driving conditions and informs the customer via the message center indicating 4X4 RESTORED.

POWERTRAIN TORQUE PROTECTION:  If the system is in 4WD AUTO (4A) and TCCM detects excessive stress or high energy going through the clutch (clutch is slipping excessively while the system is commanding max clutch torque), clutch output will be turned off and message center indicates 4x4 TEMPORARILY DISABLED. When the system had the ability to cool off, 4x4 operation will be automatically restored.

(4L) OPERATION 

4WD (4L) provides electronically locked four-wheel drive power to both the front and rear wheels for use on low traction surfaces, but does so with an additional 2.64 gear reduction for increased torque multiplication. Intended only for off-road applications such as deep sand, steep grades or pulling heavy objects. 4WD (4L) does not engage when your vehicle is moving above 3 mph (5 km/h); this is normal and should be no reason for concern.

Shifts to and from 4WD (4L) can only be made below 3 mph with the transmission in Neutral. When performing this shift, wait until the 4x4 Shift in Progress message disappears in the IPC before moving the selector lever back to Drive. If the vehicle speed or transmission range is not within parameters, the message center will indicate the necessary action needed to complete the shift.

In 4WD (4L):

• The ATCM sends a 4WD (4L) mode status to the TCCM via GWM.
• The TCCM outputs a 0% to 97% duty cycle to the clutch field coil (4WD_CLTCH_OUT # = x.xx%).
• The shift motor is in the full CW position. Refer to TRANSFER CASE SHIFT MOTOR STATUS in this service information for further information.
• (4L) is displayed in the message center.

POWERTRAIN TORQUE PROTECTION:  If the system is in 4WD 4L and TCCM detects excessive stress or high energy going through the clutch (clutch is slipping excessively while the system is commanding max clutch torque), clutch output will be turned off and message center indicates 4x4 TEMPORARILY DISABLED. When the system had the ability to cool off, 4x4 operation will be automatically restored.

2 Speed Torque On Demand Transfer Case Shift Motor 

Vehicles equipped with the 2-speed torque-on-demand system use a shift motor to enter and exit 4WD (4L) mode.

The electric shift motor is mounted externally to the transfer case. It drives a rotary cam which moves the range fork within the transfer case. The shift motor moves during shifts to and from 4WD (4L), and to Neutral. It does not move during the shift between 2WD (2H) and 4WD AUTO (4A) or 4H. TCCM directly controls the electric shift motor and can reverse motor polarity to reverse rotary cam/shift fork direction. The message center may display \"Shift in Progress\" while the shift motor is operating. The shift motor sense plate, an integral part of the shift motor assembly, informs the TCCM of the transfer case position. The sense plates are a set of 4 contacts that are opened and closed to represent each valid transfer case shift motor position.

2 Speed Torque On Demand Transfer Case Shift Motor Status 

Torque-On-Demand Clutch 

The transfer case is equipped with an electronically controlled clutch which is located inside the case. This clutch is used to transfer torque in 4WD AUTO (4A), 4H, and 4L modes.. The clutch consists of a transfer case field coil, ball and ramp or cam plate, and a multi-disc friction and steel clutch pack.