How to measure the polarity of the coils in a guitar pickup.
This simple procedure is very useful for identifying the wiring colour-code of an unmarked humbucker with 4-conductor wiring. It can also be a very handy technique for checking whether pickups are going to be "in phase" or "out of phase" with each other without having to test-fit them to the instrument.
It is not hard to work out the wiring if you have a test-meter with voltage and resistance ranges (very handy - every musician should have one!). All you need to do is to follow this process:
1 - with the meter set to a resistance range (0 - 20K or 0 - 200K), measure the resistance between any two wires. If you get a reading (could be anywhere between 2k to 10k), then you know that these are the start and finish of one coil, and obviously the other two are the wires for the other coil. If there is no reading, then you know that you have one wire from each coil.
2 - To confirm (and as a check that you do not have a faulty, open circuit coil), check the resistance between the other two wires.
3 - You can now draw a sketch, showing the two coils and marking the colours of the wires going to them.
4 - You now need to identify the polarity of the coils (so that you can connect them in phase). to do this, you need to connect the testmeter, set to a DC voltage range (0 - 2V or 0 - 20V), to the wires from one coil. Lay a piece of iron or steel (screwdriver?) onto the polepieces of that coil and watch the meter as you do it. If it is a digital meter you should see a momentary reading which will be either positive or negative, if it is an old-school analogue meter you will see the needle kick either forwards or backwards (if you get no reading, try the other coil!).
5 - If the reading was positive (digital meter), or the needle kicked forwards (analogue meter), then the wire connected to the red meter probe is the positive wire, if the reading was negative, or the needle kicked backwards, then the wire connected to the black meter probe is the positive wire. When you remove the screwdriver, you should see the meter give the opposite reading. Mark your sketch accordingly
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6 - Now repeat steps 4 and 5 for the other coil.
You have now successfully worked out the colour code for your pickup - for normal humbucker configuration, simply connect the negative wire of one coil to ground, and its positive wire to the negative wire of the other coil, Connect that coil's positive to your "hot" connection (Pickup Switch, Volume Pot etc), and you are done.
The same procedure can be used to check that two or more pickups are in phase with each other. This can save a lot of time when fitting new pickups to an instrument, as there is no "standard" polarity in the industry, and the only other way is by trial and error!