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Recently I bought 9 pairs of these pickups for 10 guitars I’m working on. I’m ready to install the first set and I noticed that some of these pickups have 3 leads, while others have 5. I’m working with the ones that have 3 leads and I don’t understand how the coil-tapping wire (white) is meant to work. I assume the switch will send the signal from the first coil through the white lead to ground, and when the switch is in the ‘off’ position the signal goes nowhere so it goes to ground through the other coil. I’m confused as to how the signal is able to do this through only one wire. When the first coil is selected, what is preventing the signal from going to ground and also through the second coil at the same time?

Please advise. I don’t know a lot about electronics.

Hi, wm17959??
I'm assuming that you've studied the instructions at the StewMac site- there's lots of good info on pickup wiring there.
I'm not sure what you are saying, but I think you may be analyzing the situation incorrectly. The 'signal' is not 'going to ground' through the coil, the signal is a voltage (potential difference) generated in/by the coil, and that signal can be detected by connecting a test instrument/amplifier to the ends of the coil.
For the 3-conductor pickups, it looks like the end of one coil is connected to a bare conductor (aka 'ground' or 'common'). To get the signal from one coil, connect to the bare conductor and the white, to get the combined signal from both coils, connect to the bare conductor and the red. Another way of thinking of this is that the white conductor is connected to the point where the two coils are joined.

Hope this helps!

Cheers
John

On the three conductor pickup, the red wire will be the "hot" and the start of the first coil, the white wire will be attached to the point where the two coils link inside the pickup housing called the "coil link" or "tap" where the first coil ends and the second coil begins, and the bare wire will be the pickup ground, which is the finish of the second coil and the pickup chassis soldered as one wire.
Since electricity follows the path of least resistance to ground, the coil link white wire can be attached to the middle lug of a single pole double throw switch, one of the outer two lugs can be connected to ground, the other left bare. When the switch is connected to ground, the signal ignores the second coil of the pickup, only conducting through the first coil of the pickup, effectively "grounding out" the second coil. When the switch is switched off of ground, the coil link wire is not connected anywhere, and the signal continues on through the second coil of the pickup.
In the Stew Mac diagram, the middle lug is the "output" of the switch, and the signal connected to the output of the switch is either the red wire, resulting in the full output of the humbucker, or the white wire resulting in only the one coil being introduced to the circuit. When the selector is connected to the red wire, the coil link is connected nowhere, and the signal conducts through both coils. When the selector is onthe white wire, the second end of the second coil is attached nowhere, and it is simply not introduced to the signal.
You can do the same thing with the 5 conductor pickup. Instead ofthe coils being linked and a wire attached to them, you link the coils yourself and solder the two wires together to the side of the switch where the white wire is, solder the start of the one coil to the other side of the switch, and solder the finish wire and bare ground to the back of the pot ground.
The five conductor version lets you "float" the bare ground wire and reverse the phase of the pickup if necesary.

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Thank you both. That helps.

Wolf