So on my guitar that i'm building i've got three pickups. Which are the Fernandes Sustainer (neck position), Seymour Duncan SH-2n (middle position), and Seymour Duncan SH-4 (bridge position). After getting it all wired up and having all of the pickups work i've ran into a few problems.
-The sustainer is very weak sounding with distortion turned on. I've replaced the battery, pickups are as close as they can be and the VBC is all the way up. What should I do?
-There is no significant difference between the harmonic, mix and standard mode. It could be a problem with the wiring. What should i do there?
-The two pieces for the FGC that allow you to turn it (white circular piece with part for screwdriver/silver cylindrical piece) fell off and I can no longer adjust it. How do I get them back on or can I turn it without them being on?
I doubt many here have a Fernandes Sustainer (i personally always wanted to dabble with them em; but eh, I have too many things I want to do)...

I know Vai uses them; and if I recall correctly, someone at the following forum decided to try them out and uses them, or at least has some experience. They may be able to offer you a little more help, but I really don't remember anymore:

Sorry I can't personally help; good luck!