ISSUE This TSB provides information on the 4X4 system, particularly the front locking hubs. ACTION Refer to the following text for information regarding the 4X4 system and the front wheel hub locks. SERVICE PROCEDURE 4X4 ESOF LOCKING HUB: DIAGNOSTIC AND SERVICE ISSUES Many system components are involved in proper operation of the ESOF (Electronic Shift-On-The-Fly) locking 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. 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 Generic Electronic Module (GEM), wiring harness, solenoid, vacuum harness, and vacuum seals. As a first step in service, eliminate such obvious items as loose wiring connections, loose vacuum connections, or damaged vacuum lines. “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, they 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 2 minutes depending on how the 4X4 mode 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 switch position. Verify that both dials are in “auto” before evaluating ESOF operation. MATCHING PAIRS NOT REQUIRED 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 replaced 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 system integrity before replacing both hub locks. HUB LOCK REMOVAL After removing the hub lock retaining ring, be sure not to use tools other than hands or “grip” gloves as damage may occur to either the paint or function of the hub lock. Pliers or channel locks should be only considered as a last resort as they will usually damage the hub lock, making replacement necessary.
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