Principles of Operation

Four Wheel Drive (4WD) Systems - Electronic Shift

Principles of Operation

Electronic Shift-On-The-Fly (ESOF) System

The Electronic Shift-On-The-Fly (ESOF) system is an electronic shift system that allows the operator to choose between 2 different Four-Wheel Drive (4WD) modes as well as 2-Wheel Drive (2WD). The operator can switch between 2H (4X2) and 4H (4X4 HIGH) mode at any forward speed. To engage or disengage 4L (4X4 LOW), the Transfer Case Control Module (TCCM) requires that the vehicle speed be less than 5 km/h (3 mph) and the transmission in NEUTRAL. This ESOF system includes a Constant Vacuum Hublock (CVH) system at the front wheel ends. The selector on the CVH, when turned fully counterclockwise to the AUTO position, engages/disengages the hub depending on the driver-selected 4WD mode of operation. The CVH also contains a manual override. When the selector is rotated fully clockwise to the LOCK position, the 4WD vacuum and electronic system used for coupling the front axle to the wheels is bypassed and the front wheels and front driveline remains coupled at all times.

The transfer case is equipped with a dual-cone synchronous clutch. This clutch is used to synchronize the front driveline to the rear driveline. All other internals operate in the same manner as the Manual Shift-On-Stop (MSOS) system. When the Mode Select Switch (MSS) on the instrument panel is turned, the TCCM powers the transfer case shift motor (part of the gearmotor encoder assembly), which activates a shift fork and engages the clutch. When the gearmotor encoder assembly reaches the desired position, as determined by the encoder position inputs to the TCCM, power to the gearmotor encoder assembly is removed. When the transfer case front and rear output shafts are synchronized, the lockup collar mechanically engages the mainshaft hub to the drive sprocket. At this point, the front axle CVH system is engaged. Instrument Panel Cluster (IPC) 4WD system indicator operation is controlled via the IPC which interprets outputs from the TCCM via the vehicle High Speed Controller Area Network (HS-CAN).

Feature inputs

- MSS
- Gearmotor encoder assembly position inputs A, B, C or D
- Individual wheel speeds (from the ABS module via HS-CAN)
- Ambient air temperature (from the PCM via HS-CAN)
- Accelerator Pedal Position (APP) (from the PCM via HS-CAN)

Feature outputs
- Transfer case gearmotor encoder assembly outputs
- 4WD system (from the TCCM via HS-CAN)
- Includes 4WD indicator commands (from the TCCM to the IPC via HS-CAN)

Electronic Shift Transfer Case

The transfer case is a Magna Powertrain 2-piece aluminum case transfer case design. Vehicles equipped with the 5R110 transmission include an aluminum adapter that fits between the transmission and transfer case assembly. The transfer case transfers engine power from the transmission to the front and rear axles. Under normal driving conditions, the unit is in 2H (4X2), but when desired, the operator may shift into 4H (4X4 HIGH) or 4L (4X4 LOW). 4H and 4L modes are intended for use in slippery conditions, such as ice and snow, or in off road situations only. 4H and 4L are not to be used on dry pavement. The ESOF transfer case is shifted electronically. The unit is lubricated by a positive displacement fluid pump that channels fluid flow through holes in the mainshaft. The encoder assembly used for mode indication is of a Hall effect type. The system uses a total of 4 independent transistors for mode recognition. Each transistor is associated with a specific range of motor movement. When the circuit to each transistor is closed, a ground path to any of the -A-, B-, C- or D- encoder pins is accomplished. When the circuit to each transistor is open, the connection to any of the -A-, B-, C- or D- encoder pins is pulled to voltage. Mode indication is "decoded" by the TCCM based on the varying combinations of signals coming from the transistors.

For a shift attempt from 2H to 4H or 4L, the following sequence occurs:

- MSS is switched from 2H to 4H or 4L.
- For shift attempts into 4L:
- transmission must be in NEUTRAL.
- vehicle speed must be below 5 km/h (3 mph).
- if the above 2 conditions are not met, the ESOF system does not attempt a shift and the IPC message center directs the driver to meet the appropriate shift conditions.

- gearmotor encoder assembly moves range fork in transfer case, which engages the dual-cone synchronizers.
- If a blocked shift condition occurs, the gearmotor encoder assembly performs 3 shift attempts (2H to 4H or 4H to 4L) with an approximate 1-second delay between each motor movement attempt.
- If 2 complete shift sequences are unsuccessful (6 total shift attempts) during an ignition cycle and the MSS has not changed state, either DTC C1728:00 or C1729:00 is logged. These DTCs are only logged if the vehicle is moving above 5 km/h (3 mph) for at least 1 of the unsuccessful shift sequences.

- Generally, the transfer case engages upon vehicle movement as the probability of gear teeth blockage is greatly reduced while rolling. The IPC message center prompts the driver to pull forward during a blocked shift attempt to alleviate the gear teeth blockage that typically causes blocked shift attempts.
- If no gear teeth blockage is present, the transfer case dual-cone synchronizers bring the front driveline to the same speed as the rear driveline.
- The shift fork completes movement and the hublock vacuum solenoid supplies vacuum to the CVH system which couples the wheels to the front wheel driveline.
- Once the vacuum is supplied to the CVH system and the 4WD shift is completed, the appropriate 4WD indicator is displayed in the IPC message center.
- The appropriate 4WD indicator is only displayed upon a successful 4WD system shift.

For a shift attempt from 4L or 4H to 2H, the following sequence occurs:

- MSS is switched from 4H or 4L or 2H.
- For shift attempts into 4L:
- transmission must be in NEUTRAL.
- vehicle speed must be below 5 km/h (3 mph).
- if the above 2 conditions are not met, the ESOF system does not attempt a shift and the IPC message center directs the driver to meet the appropriate shift conditions.

- Gearmotor encoder assembly moves range fork in transfer case, which disengages the dual-cone synchronizers.
- If a blocked shift condition occurs, the gearmotor encoder assembly performs 3 shift attempts (2H to 4H or 4H to 4L) with an approximate 1-second delay between each motor movement attempt.
- If 2 complete shift sequences are unsuccessful (6 total shift attempts) during an ignition cycle and the MSS has not changed state, either DTC C1728:00 or C1729:00 is logged. These DTCs are only logged if the vehicle is moving above 5 km/h (3 mph) for at least 1 of the unsuccessful shift sequences.

- Generally, the transfer case disengages upon vehicle movement as the probability of gear teeth blockage is greatly reduced while rolling. The IPC message center prompts the driver to pull forward during a blocked shift attempt to alleviate the gear teeth blockage that typically causes blocked shift attempts.
- The shift fork completes movement and the hublock vacuum solenoid removes vacuum from the CVH system which uncouples the wheels to the front wheel driveline.
- Once the vacuum is removed from the CVH system and the 4WD shift is completed, the appropriate 4WD indicator is displayed in the IPC message center.
- The appropriate 4WD indicator is only displayed upon a successful 4WD system shift.

Constant Vacuum Hublock (CVH) System

The ESOF system engages the front driveline to the front wheels via the CVH system. The Constant Vacuum Hublocks (CVHs) have 2 modes: AUTO and LOCK. Always set both CVH dials to the same position. When in the AUTO mode, the CVHs are disengaged (no vacuum) in 2H (4X2), engaged in 4H (4X4 high) and 4L (4X4 low), and automatically operate upon 4WD mode shifts. During a 4WD mode shift, the TCCM commands the CVH system vacuum solenoid to supply or remove vacuum to the CVHs which engages or disengages the front wheels to or from the front driveline. When in LOCK mode, the CVHs are engaged at all times and the vacuum system has no effect on the operation of the CVHs (refer to Manual Override for more information regarding the LOCK position). AUTO mode should be used for system operation convenience and increased fuel economy. A minimum vacuum level of 6 inches of Hg is required to correctly operate the CVH system. The vacuum signals are supplied to the CVHs by system components, including the TCCM, wiring harness, solenoid, vacuum harness and vacuum seals. On diesel engine vehicles, vacuum is supplied by a gear-driven pump that is mounted to the front of the engine behind the accessory drive cooling fan driver and is serviced as an assembly. For additional information, refer to Power Brake Actuation Testing and Inspection. As a first step in service, eliminate obvious items such as loose wiring connections, loose vacuum connections or damaged vacuum lines.

Manual Override

The CVHs have a manual override selector dial, which, when turned to the LOCK position, keeps the CVHs locked (via the dial) at all times, regardless of the position of the instrument panel MSS. (Always set both hublock dials to the same position. If the CVH dials are manually turned from the AUTO to the LOCK position, and vacuum is supplied to the CVHs (by switching the MSS from 2H (4X2) to 4H (4X4 high) or 4L (4X4 low), the CVHs are locked through the dial as well as through the internal spring mechanism. In this case, turning the CVH dials back to the AUTO position leaves the CVHs in (spring) locked mode until vacuum is removed from the CVHs (by switching the MSS from 4H or 4L to 2H). (This is not an issue if the CVH dials have been in the AUTO position and the MSS has been switched from 4H or 4L to 2H at least once.) The CVH should be left in the AUTO position for increased fuel economy.

Electronic Shift-On-The-Fly (ESOF) Constant Vacuum Hublock (CVH) Operation





Constant Vacuum Hublock (CVH) Replacement

NOTICE: Make sure to follow the Removal and Installation procedure exactly as instructed or damage to the hub can occur.

LH and RH side CVHs are not connected, other than by the common vacuum supply line. If a malfunction in either CVH is diagnosed, it should be installed as an individual unit; there is no need to "balance" an axle with new CVHs on both sides. It is highly unlikely that both CVHs are malfunctioning at the same time on a vehicle. If both sides appear to be malfunctioning, be sure to verify upstream integrity before installing new CVHs on both sides. Many system components are involved in the correct operation of the ESOF hubs. Before diagnosing the CVHs themselves as the cause of 4WD concerns, be sure to verify all related system components.