Thermal Vacuum Valve: Description and Operation

Fig. 18 Ported vacuum switches--PVS:

These switches, Fig. 18, control vacuum to emission components during engine warm-up. The 2-port and 4-port types open when coolant temperature reaches a predetermined value. The 3-port type switches vacuum from the center to the top or bottom ports. Electrical switches can be opened or closed until the PVS is fully cycled. All PVS units are temperature-activated and have a specific opening and closing temperature.