What valve is this...???

[vw_lee]

I have a 2001 1.8t passat AWT engine...Having boost problems with lack of it..There is a slight whistle & the slightest increase in speed but not much (like others ive driven).
Basically what im doing is working my way through the vacuum system, checking & testing as i go.
I have a few questions for anyone that may know..!!
what is the small black 3 way valve that connects to the turbo actuator diaphram & sits on the side of the main air pipe from airbox to turbo intake..??
Also how can i test this valve..??
What are the two vacuum units under the inlet manifold & how can i test them..??
I dont have use of VAG COM..
Some one please help..!!

[Ross 1.8T]

The valve you speak of is the N75 boost pressure control solenoid valve. It pressurises the wastegate actuator when commanded by the ECU to open the wastegate thus regulating turbo output.

To test without VAG-COM, disconnect it and blow down one of the ports. You should get air coming out of one of the two remaining ports but not both. This is a crude method of testing. Vag-com is the only reliable way i'm afraid.