Bronco L6-300 4.9L VIN Y 1-bbl (1983)
Thermal Vacuum Valve: Description and Operation
Temperature Vacuum Switch (TVS)
Fig. 27 Temperature vacuum switch--TVS
This switch, Fig. 27, used on some vehicles, has a bimetal disc which locates itself in one of two positions, depending on its temperature. One
position allows free air flow in the vacuum line; and the other blocks air flow by sealing itself against the O-ring. The switching temperature is
below the range of normal engine operating temperature.
The TVS may be used to control the vacuum signal to the thermactor dump valve, to reduce emissions.
The normally open TVS may block the purge vacuum signal to provide satisfactory cold driveability and reduce cold engine emissions. It may be
also used to hold off EGR operation during cold engine operation.
The normally closed TVS may allow cold spark advance to provide satisfactory cold engine driveability.
