It control the idle speed. To test this valve, you have to remove it and apply vaccum to it and check that it moves to full position. Then, plug hose with applied vaccum and check that it remain open for more than 30 seconds after vaccum has been cut off. If it's not then be ready to pay about $150 for a new aac valve.
or about $10 from the junkyard
The AAC operates much like the EGR in that there is a diaphram plunger thingy that moves up and down to let more/less air flow by. You should be able to move the plunger up and down freely with your finger. The above test from Bluestreak will confirm its operability. If this checks out, you may want to put you finger over the vac line coming from the vac module which directly controls this unit and see what kind of signal you get from it at idle.