Thermal & Vacuum Switches

The feedback system does not operate quickly enough to provide good driveability when the engine is cold or under full load. The thermal and vacuum switches are used to limit closed loop operation under these conditions. When engine coolant is below 140°F (60° C) and manifold vacuum is less than 8 in. Hg (Man. Trans.) or 5 in. Hg (Auto. Trans.), the thermal switch and vacuum switch II signal the electronic control unit, which switches the solenoids off. Closed loop is also prevented by vacuum switch 0.1, which is operated when the engine is warm and vacuum falls below 3.4 in. Hg.