that is the air powered vacuum generator. it has to let air out to generate the vacuum. it doesnt take much to make the vacuum. mine was always on and was always pushing air through it.
the pressure regulator is in that unit so one line is the air source (the big one), one small one is a vacuum line and the other is a vacuum line that comes from the softball size tank behind the dash that goes to the regulator. when i replace mine with a 12v one i mounted in the front bay behind the drivers side headlight, i had to plug the one that goes to the regulator with a golf tee.
it uses vacuum for the vent switchover. it is turned on by the key and the push button for defrost and vent.
call me if you need me to talk you through it. i used a hand vacuum pump to test my system and to watch the louvers on the bottom of the plenum under the passenger side headlight to see if they moved. the round ones ont he bottom are for vent and if it is quiet you can hear the defrost ones move too.
tom
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