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I do not have the links for the other threads handy ATM, but yes, the conclusion recently reached by several of us was that [unlike most of the Mustangs which only have the small split pickup] the American Performer Mustang has the two halves of the small split pickup externally wired IN PHASE with each other and with the bridge pickup [no voodoo, really], AND, like most other small pickup Mustangs, the magnets are pointing in the same direction in both halves, and the coils of both halves are wound also in the same direction.
So in essence, electrically speaking, the American Performer Mustang bass has a single coil neck pickup like a standard passive wired Jazz bass, but constructed in two pieces to maintain the look, which ironically yields a cheap performance from Fender's most expensive currently produced Mustang bass.
As a consequence, same as with a standard passive wired Jazz, an American Performer Mustang also can suffer from hum when each pickup is soloed, and can cancel the hum if both pickups are set to a similar volume.
One noteworthy exception to things apparently are the small split pickup Mustangs which were made in Japan, which have a small split pickup wired like a typical Precision bass, where the winding direction of one half is reversed, and the magnet pole direction of one half is also reversed, thus cancelling hum while keeping the signal of all strings in phase with one another, and as a result having no issues if a non-split bridge pickup gets added, externally wired in correct phase with the split pickup, same as on a typical PJ bass.
If you managed to snag a small split pickup from a Japanese Mustang, your experiment might be successful, but bear in mind that your current Jazz type bridge pickup could still have hum, just like on most PJ basses.
Someone might have posted, but I forget without searching, what internal pickup construction is in play with any Squier Mustang CV or VM basses which only have a small split Mustang style pickup.
I hope that the above partial summary helps.
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Oh my god, I've been looking for a message from the universe like this!
I've always thought that the neck pickup on my American Performer wasn't up to snuff, output-wise or sound-wise, and was really worried that it had grounding issues with the (seemingly) split coil pickup humming. That's super annoying to learn, now the hypothetical pickup upgrade I've been considering has turned into a very real thing I should probably do. Thanks for the info!