Most likely, you have the “hot” and “ground” wires backwards.
I have a Gibson 57 Classic Humbucker, which has a really high output of 9K!! and it’s NOT an classic+, which I know. I installed it by itself in the bridge position, with the selector on treble, and it sounds thin, very quiet, and is humming slightly. What could be wrong?
Hi Sven. It sounds as though you have a wiring problem. Most likely, you have the “hot” and “ground” wires backwards. Triple-check the wiring diagram that came with the pickup. If you can confirm 100% that the pickup is wired correctly, then try touching the hot / ground wires directly to a guitar jack, and tap the pickup. There should be a loud and healthy “thud” that comes out of the amp when you do this. If not, then the pickup is damaged.