Principles of Operation
Principles of OperationSpecial Tools:
Special Tool(s)
Transfer Case-Electronic Shift
The four-wheel drive, electronic shift-on-the-fly feature electrically shifts the vehicle transfer case between 2WD, 4X4 HIGH, and 4X4 LOW. The system mode is selected by the operator through the Mode Select Switch (MSS) on the instrument panel. The operator is informed which mode the system is in by two instrument cluster indicators, one for 4X4 HIGH which is 4X4, and one for 4X4 LOW which is LOW RANGE (in 4X4 LOW both the lamps are on). Shifts into 4X4 HIGH can be made at any speed. When shifting into 4X4 HIGH with the vehicle stationary, tooth blockage may occur, preventing shift completion. When the vehicle is driven above 8 km/h (5mph) the shift will complete. When shifting in or out of 4X4 LOW, the Generic Electronic Module (GEM) requires that the vehicle speed be less than 5 km/h (3 mph), the brake pedal be applied, and the transmission in NEUTRAL (automatic transmission) or the clutch pedal be depressed (manual transmission). (The digital Transmission Range [TR] sensor informs the GEM when the automatic transmission is in the NEUTRAL range position.)
The gear motor encoder assembly is mounted externally on the transfer case. It drives a rotary cam which moves the mode fork and the range fork within the transfer case between the 4X4 HIGH, 4X4 LOW, and 2WD range positions.
The GEM uses two relays to control the gear motor encoder assembly shift between 4X4 HIGH, 4X4 LOW, and 2WD modes. The GEM controls the pulse vacuum hub lock (PVH) solenoid which supplies engagement and disengagement vacuum to control the hub locks.
The GEM accomplishes shifts by interpreting inputs from:
^ Mode Select Switch (MSS)
^ vehicle speed signal (transmitted from the ABS system)
^ gear motor encoder plate position
^ brake pedal switch
^ digital Transmission Range (TR) sensor (automatic transmission)
^ Clutch Pedal Position (CPP) switch (manual transmission)
^ ignition switch
Based on these inputs, the GEM controls the shifts into 2WD, 4X4 HIGH, or 4X4 LOW with the following outputs:
^ low to high relay (clockwise)
^ high to low relay (counterclockwise)
^ Pulse Vacuum Hub lock (PVH) solenoid
^ gear motor encoder assembly
The Electronic Shift-On-The-Fly (ESOF) system has a feature which allows the driver to override the vacuum operated hub locks. When the front hub locks are manually turned to the LOCK position, the hub locks are locked at all times, overriding the vacuum operated system. If the front hub locks are manually turned to the AUTO position, the hub locks can only be locked by turning the MSS to 4X4 HIGH or 4X4 LOW position.
Electronic Shift Vacuum Schematic
Hub lock Operation
The Super Duty and Excursion 4X4 ESOF system uses timed vacuum sequences to lock and unlock the wheel ends. A high vacuum level is used to engage the hub locks, and a lower vacuum level is used to disengage the hub locks, after which the vacuum is released and the hub lock holds itself in the proper mode. The vacuum signals are supplied to the hub locks by system components, including the GEM, wring harness, solenoid, vacuum harness and vacuum seals. As We first step in service, eliminate such obvious items as loose wiring connectings, loose vacuum connections, or damaged vacuum lines.
Disengage Time
"Slow" release of the hub locks is not considered abnormal for this system. Anytime vacuum is applied to the hubs, whether for 4X4 or 4X2, the hub locks will initially engage. If 4X4 was requested, the hub locks will remain engaged, but if 4X2 was selected, the internal mechanism will release only after the GEM timers expire and vacuum is vented from the hub. This normally takes 15 seconds, but can take up to two minutes, depending on how the 4X4 mode select switch was operated. After the hub mechanism releases, internal springs must work the hub lock gears to the disengaged position. Road bumps, vehicle speed, acceleration cycles, or momentary reversal of direction can assist this process, varying the length of time the hub locks remain engaged in each situation.
Manual Override
The hub locks have manual override selector dials, which, when rotated to the "LOCK" position, will keep the mechanism locked regardless of the instrument panel 4X4 mode select switch position. Verify that both dials are in "AUTO" before evaluating ESOF operation.
Hub lock Replacement
Left and right side hub locks are not connected, other than by the common vacuum supply line. If a malfunction in either hub lock is diagnosed, it should be installed as an individual unit; there is no need to "balance" an axle with new hub locks on both sides. If both sides appear to be malfunctioning, be sure to verify upstream integrity before installing new hub locks on both sides.
Many system components are involved in the proper operation of the ESOF hubs on 1999 and newer Super Duty and Excursion 4X4 vehicles. Before diagnosing the hub locks themselves as the cause of 4X4 concerns, be sure to verify all related system components.
After removing the hub lock retaining ring be sure not to use tools other than the hands or "grip" gloves to remove the hub lock, as damage may occur to either the paint or the function of the hub lock. Pliers or locking Channel-Locks should be considered as a last resort, and will usually damage the hub lock, making it necessary to install a new hub lock.