Vacuum Operated Thermostatic

Fig.3 - Bi-metal Switch:

A vacuum operated duct valve with a thermostatic bi-metal control, Fig. 3, is used on some installations. The valve in the duct assembly is in an open position when the engine is not operating. When the engine is operating at below normal operating temperature, manifold vacuum is routed through the bi-metal switch to the vacuum motor to close the duct valve allowing only heated air to enter the air cleaner.

When the engine reaches normal operating temperature the bi-metal switch opens an air-bleed which eliminates the vacuum and the duct valve opens allowing only cold air to enter the air cleaner. During periods of acceleration the duct valve will open regardless of temperature due to the loss of manifold vacuum.