Vacuum Differential Valve (VDV)

Fig. 78 Vacuum Differential Valve (VDV):

On some vehicles with the Thermactor system and catalytic converters, a VDV, Fig. 78, is used to control the operation of the air bypass valve. Under normal operation, vacuum applied through the VDV holds the valve upward, blocking the vent port and allowing Thermactor air flow. During acceleration or deceleration or in case of system failure, the VDV momentarily cuts off vacuum flow to the bypass valve, diverting the Thermactor air flow to atmosphere. In case of excessive pressure or system restriction, the excess pressure will unseat the valve in the lower part of the bypass valve, allowing a partial flow of air to atmosphere. At the same time, the valve in the upper part of the valve remains unseated allowing a partial flow of air to the exhaust manifold.