I have an Epi Les Paul 100, with GFS pickups. I wired EXACTLY to the diagram (and replaced all 4 pots), and still only the neck pickup works. I put a multimeter to the switch, and the readings (In Ohms) I'm getting are:

On Rhythm:
-the rhythm and middle prongs on the switch get a reading, while the treble side prong gets nothing (normal, I assume).

With the switch in the middle position:
-Nothing gets a reading, all 0

With the switch in the Treble position:
-The rhythm side prong gets a normal reading, while the middle and treble side prong are both at 0.

I'm assuming it has something to do with the switch or switch wires, maybe there is a ground where it shouldn't be? Something isn't grounded correctly? I did not mess with the switch wiring at all, just added the pickups.

Any and all help is greatly appreciated.
Was the switch working before you changed pickups? You're putting the multimeter contacts directly on the switch, not the wires right? When the switch is making good contact between two lugs it should read 0.0 ohms, this means there is no resistance, it is making a connection. If it is not making contact (that side is "off") it should read 0.L or whatever yours reads when it's not touching anything.

If it should be making a connection and reads a number other than 0 I'm pretty sure the switch is dirty (signal might still get through, but not sound right.). If it should not be making a connection and you get a value other than 0.L then it is partially making contact for some reason and is probably broken.

Of course, this is all assuming you're testing with the switch disconnected from everything. If you're testing with things actually connected to it you may get other readings.

I don't know what kind of switch is in those guitars, but if it's like this

It may have a piece of trash or something in it, since it's completely open. Those types of switches are very easy to take apart and clean, though you should be able to just spray some kind of contact cleaner and work the switch a little.
Last edited by The4thHorsemen at Aug 17, 2014,
I have been testing it wired up. I wasn't aware that the switch should be disconnected from everything. I will try it disconnected and put up the readings.
I cleaned it and nothing worked, never got around to testing it disconnected.

However, I wired my bridge pickup to the neck pots and the neck to the bridge pots, and now the opposite is working. The bridge works (in neck position) and the neck does not work (in bridge position).