Originally Posted by
Simon Barden
You can check the output polarity of each unconnected pickup with a multimeter. You really need test clips to attach the wires to the multimeter, or someone to help you, as you need at least one other hand free. Connect the pickup main signal ground to the -ve terminal and the signal wire to the +ve terminal of the multimeter.
Put the multimeter on ohms and have it display the DC resistance. Move something reasonably large made of steel (say a big screwdriver or a spanner) towards the pickup. The resistance reading will either go down or up slightly depending on the pickup's output polarity. If both pickups' readings go down or up, then they have the same output polarity and will be fine wired as normal. If one goes down and one goes up, then they have opposite polarities and one will need the connection wires swapped over.
Moving a steel object towards the pickup induces a current in the pickup the same way that a string does. The multimeter detects the resistance of the pickup coil by putting a known voltage through the coil and measuring the voltage drop (which the gets converted to an ohms reading). The extra coil/voltage induced by the steel object moving towards the pickup coil either adds or subtracts from the voltage provided by the multimeter, and so changes the resistance reading (it doesn't change the actual resistance).
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