Principles of Operation
Four Wheel Drive (4WD) Systems - Electronic Shift
Principles of Operation
The Electronic Shift-On-The-Fly (ESOF) 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. 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 be turned freely from the front axle.
The electric shift motor is mounted externally to the transfer case. It drives a rotary cam which moves the 2 shift forks in the transfer case to change between the 2H and 4H modes and between the 4H and 4L modes. For information on transfer case mechanical operation, refer to Transfer Case Transfer Case. The TCCM directly controls the electric shift motor and can reverse motor polarity to reverse rotary cam/shift fork direction. The shift motor sense plate, an integral part of the shift motor assembly, informs the TCCM of the transfer case position.
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 Mode Select Switch (MSS) is switched between 2H and 4H modes, the TCCM energizes 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
- MSS
- Wheel speed signal from ABS module
- Accelerator Pedal Position (APP) from PCM
- Transfer case shift motor contact plate position inputs A, B, C and D
Feature outputs:
- Transfer case electric shift motor outputs
- Pulse-Width Modulation (PWM) signal to transfer case clutch
- 4WD message center and indicators
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:
- MSS
- Wheel speed signal from ABS module
- TR position from TCM
- Transfer case shift motor contact plate position inputs A, B, C and D
Feature outputs:
- Transfer case electric shift motor outputs
- PWM signal to transfer case clutch
- 4WD message center and indicators
Electronic Locking Differential (ELD) Feature (optional)
The Electronic Locking Differential (ELD) system allows the operator to lock the rear differential side gears while in 2 different 4WD modes (and in 2H on SVT Raptor only). If the vehicle is driven above the disengage speed, the ELD indicator will blink for 10 seconds alerting the operator that the ELD is disengaged. Once the vehicle drops below the re-engage speed, the ELD lamp will illuminate solid again. The ELD cannot be engaged unless the MSS is in 4H or 4L mode (and in 2H on SVT Raptor only). When the operator pulls the MSS to activate the ELD, the ELD indicator will flash for 2 seconds then illuminate solid. The flashing indicates the ELD is attempting to lock. If there is a malfunction with the ELD system, a CHECK LOCKING DIFFERENTIAL message will be displayed in the Instrument Panel Cluster (IPC) and the ELD lamp will blink for 10 seconds, letting the operator know the ELD is not engaged.
The operator can engage the ELD per the following table:
a Raptor only with off-road mode off.
b The 4WD system engages/re-engages at or below the speed noted.
The differential locks when a PWM controlled magnetic field coil located inside the differential energizes. A collar within the differential moves toward a differential side gear and engages a dog clutch within the differential. This locks the differential side gears and creates the locked differential.
The TCCM activates the ELD. When the ELD is engaged, the TCCM will send PWM voltage to the ELD field coil located in the differential. This voltage is initially 98% duty cycle, and then tapers off to 40% duty cycle after 30 seconds. In order for the TCCM to activate the ELD, certain conditions must be met.
TCCM Input Conditions
- MSS to 4H or 4L (and in 2H on SVT Raptor only)
- MSS pulled to ELD position
- Average rear wheel speed versus average front wheel speed must be less than 15 km/h (9 mph) (via High Speed Controller Area Network (HS-CAN) communication from ABS module)
- Operating range (voltage between 9-16 volts)
- DTC setting range (voltage between 10-15 volts)
- Ignition in the ON position
- Throttle Position (TP) less than 50%
Off-Road Mode
The SVT Raptor comes equipped with a special off-road function. An off-road button is located on the floor console. A light on the button will illuminate when this feature is activated.
NOTE: Off-road mode is specifically calibrated for off-road driving conditions and should never be used on pavement.
Off-road mode affects 4 vehicle functions:
- It allows unique transmission shifting that optimizes off-road performance. Off-road mode shifting differs from standard on-road shifting in several ways:
- Transmission upshifts are delayed and occur less frequently. This ensures the best possible vehicle response and acceleration if the driver needs to briefly lift off the throttle to negotiate an obstacle or corner.
- Transmission downshifts occur more quickly improving vehicle response.
- Sixth gear (overdrive) will not engage. This prevents engine lugging (6.2L equipped vehicles only).
- Off-road mode provides optimized throttle response tailored for off-road performance driving.
- When off-road mode is engaged, the standard vehicle speed thresholds for the ELD are lifted. If the vehicle is in 4X4 mode (or in 2H on SVT Raptor only) and the ELD is engaged, the ELD will remain locked until the operator manually disengages it with the control knob. If the operator disengages off-road mode and exceeds the standard ELD speed thresholds, the ELD will disengage.
- When off-road mode is engaged, the AdvanceTrac settings are altered for optimized off-road performance.
To engage off-road mode, slow the vehicle to a speed of 5 km/h (3 mph) or less. Release the accelerator pedal then press and release the off-road mode button. The message center will display OFF ROAD MODE ENABLED once off-road mode has been enabled. If any of the above conditions are not met, off-road mode will not enable and the message center will display information guiding the driver through the proper off-road mode enabling procedures. To disable off-road mode, press and release the off-road mode button. The message center will display EXITING OFF ROAD MODE.
The operator can engage the ELD per the following table:
a The 4WD system engages/re-engages at or below the speed noted.