Engine vacuum, stored in the vacuum tank in the engine compartment, controls all of the valves except the temperature valve. Vacuum is routed by a vacuum switch in the control assembly to either of the spring-loaded, three-position vacuum actuators. One vacuum actuator is linked to each valve.

The mode knob controls the vacuum switch. In each mode knob position, the vacuum switch directs engine vacuum to specific hoses and vent the remaining hoses. When a hose is vented to normal air pressure, the vacuum actuator's internal spring expands and holds the associated valve in one position. When vacuum is routed to the hose, the force of the vacuum overcomes the internal spring pressure, and moves the valve to another position.