http://www.n54tech.com/forums/showthread.php?t=13114
Check out that link, the parts to test the solenoid were about $30 total from advanced auto parts.
Unless someone has any other info to add thinking it may not be turbo related are there any N54 turbo replacement DIY's posted anywhere? I have the Bentley manual but I'm a little unclear about lifting the front end of the engine up. I do believe that is part of the process?