Vacuum 1 Way Valve

JME, I have just re written this post to make it clearer.
 
SW20 3SGE NA's run an ACIS, and not a TVIS.
To test: Firstly make sure that the butterflies are not rusted up and can move freely.
Reach under the activator and with your finger try and move the tab on the pivot. Can be tricky to do.
 
If you can move it OK, have someone start the motor and observe the ACIS activator.
If nothing happens check the rubber hoses.
Normally with motor stopped, butterflies are open.
Motor started, butterflies snap closed, meaning full length of the intake runners used for best torque.
If the revs go over 5k butterflies open, effectively shortening the intake runners for more power at high revs.
 
Here is a pic from the Gen3 motor manual (page EG266) that can be downloaded from the tech section on this website, and shows the vacuum switch valve (VSV) that controls it.
Although this is from the Gen 3 motor, the gen2 is similar.
The VSV can probably only be accessed from under the car. 
On my '90 model the ACIS VSV is green.(The blue one alongside it is for the fuel regulator).
The vacuum is taken from the botton tube (of the 3) down on the side of the intake manifold, and goes to bottom connection on the vacuum tank. 
From the center angled connection on the vacuum tank the hose goes to the center connection on the VSV.
From the angled connection on the VSV the rubber tube goes up to the ACIS activator.
Very easy to get them mixed up!!