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 . As a first step in service, eliminate obvious items such as loose wiring connections, loose vacuum connections or damaged vacuum lines.