Isolation Valve The isolation valve isolates the left and right sides of the bleed air duct during normal operations. The isolation valve is AC operated. With the isolation valve switch in AUTO, both engine bleed air switches ON, and both air conditioning pack switches AUTO or HIGH, the isolation valve is closed. The isolation valve opens if either engine bleed air switch or air conditioning pack switch is positioned OFF. (Any corner switch OFF) Isolation valve position is not affected by the APU bleed air switch.