There should be two vac valves connected togeather with their associated delay valves. These are routed to the back of the air cleaner and then into the vacume system. I think the cold door stays open if their is no vac but not sure. When the system is working, they regulate between and keep the air a consistant temp.
There is a good write up on this in the standard Haynes manual. The shop manual has better vac diagrams.
There are some variations in the setup. I'm referencing my 89 info.
89 Grandwagoneer, built 360, COMP cam set, port matched Edelbrock SP2P, 475 cfm Holley ORA, Thorleys, MSD6a, extra cooling and oiling.
76 Cherokee S Chief - rebuilt stock, Holley, Performer, MSD Dizzy.
89 GW parts waggy