This is how the hub operation is described in the Ford WSM: 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 hublocks, and a lower vacuum level is used to disengage the hublocks, after which the vacuum is released and the hublock holds itself in the proper mode. The vacuum signals are supplied to the hublocks by system components, including the GEM, wring harness, solenoid, vacuum harness and vacuum seals. As the first step in service, eliminate such obvious items as loose wiring connectings, loose vacuum connections, or damaged vacuum lines.