Principles of Operation - Mechanical Shift-On-The-Fly

Four Wheel Drive (4WD) Systems - Mechanical Shift

Principles of Operation - Mechanical Shift-On-The-Fly

The Mechanical Shift-On-The-Fly (MSOF) system has 3 modes of operation: 2H (4X2), 4H (4X4 HIGH) and 4L (4X4 LOW). Integrated Wheel Ends (IWEs) are used at each front wheel to reduce rolling resistance when 2H is selected. When 4H is selected, the Transfer Case Control Module (TCCM) sends a duty cycle command to the synchronization coil which activates the clutch within the transfer case. Refer to Transfer Case Transfer Case.

Engagement and disengagement of the front wheels is done by the IWEs. The IWEs are normally engaged (no vacuum). Vacuum is supplied to the IWEs by actuation of the IWE solenoid when commanded by the TCCM. When actuated, the IWE solenoid allows manifold vacuum to pass through to each IWE. The IWEs disengage and allow the front wheel hubs to turn freely from the front axle.

The transfer case is equipped with an electromagnetic synchronization clutch which is located inside the case. The synchronization clutch is used to synchronize the front driveline when shifting between 2H and 4H modes. When the manual shift lever is moved, this exercises the Mode Indication Switch (MIS), triggering the TCCM to energize the synchronization clutch. When the transfer case front and rear output shafts are synchronized, the spring-loaded lockup collar mechanically engages (4H) or disengages (2H) the mainshaft hub to the drive sprocket. Finally, the front axle IWEs are engaged (4H) or disengaged (2H) and the synchronization clutch is deactivated.

Shifts from 2H mode to 4H mode can be made at any speed. The synchronization clutch is applied to equalize output shaft speeds before 4H mode is engaged. The following are the inputs and outputs needed by the TCCM to execute a change between these modes.

Feature inputs:

- Ambient air temperature from the PCM
- MIS
- Wheel speed signal from ABS module
- Accelerator Pedal Position (APP) from PCM
- Transfer case shift motor contact plate position inputs B and C

Feature outputs:

- Ignition switch position
- Pulse-Width Modulation (PWM) signal to transfer case synchronization clutch
- 4WD message center and indicators

When the transfer case is shifted into 4L, a message is sent to the ABS module. The ABS module disables the AdvanceTrac(R) system and the Instrument Panel Cluster (IPC) illuminates the sliding-car icon in the message center.

When shifting into or out of 4L mode, the TCCM requires the vehicle speed to be less than 5 km/h (3 mph) and the transmission in NEUTRAL. The following are the inputs and outputs needed by the TCCM to execute a change into or out of 4L mode.

Feature inputs:

- MIS
- Wheel speed signal from ABS module
- Transmission Range (TR) position from Transmission Control Module (TCM)
- Transfer case shift motor contact plate position inputs B and C

Feature outputs:

- PWM signal to transfer case clutch
- 4WD message center and indicators
- IWEs