Technical how to test overboost valve?

[daddygez]

As the title says. I get fault code P0236 overboost voltage low.jtd110. I've taken it off and can suck through it with or without a voltage going through it but not sure if it needs a vacuum applied. the variable vanes on turbo move on mouth suction, about a quarter of an inch at the piston rod. there is very little suction on the pipe going into the overboost valve, they all dive around the back of the engine. any help folks would be appreciated. thanks Gerry