Heater Lockout Switch: Description and Operation

Fig. 5 Thermal blower lockout switch:

The TBL, Fig. 2, is a combination vacuum valve and single-pole single-throw switch powered by a thermal element. The TBL is located in the heater supply hose in the engine compartment with the thermal element of the switch in contact with engine coolant. With engine cold and function lever in FLOOR position, blower is locked out and outside-recirc door is in recirc position.
When engine coolant warms to approximately 120°F, the electrical switch contacts close, permitting blower to operate and outside-recirc door to move to outside position.