Coolant Temperature Sensor / Switch HVAC: Description and Operation

Thermal Blower Lockout (TBL) Switch:

The Thermal Blower Lockout Switch (TBL) is a combination vacuum valve and a single-pole single-throw switch power 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 the engine coolant. In the FLOOR function lever position and with a cold engine, the blower is locked out and the outside-recirculate door is in the recirculate position.

When the engine coolant warms up to about 49° C (120° F), the electrical switch contacts close permitting the blower to operate and the outside-recirculate door to move to the outside position.