Now here is an alternate wiring method for the Jazz Bass, or any other two pickup bass. This method has the pickups in series wiring. This wiring method only works for passive pickups. NOTE: You cannot do this with active pickups because of the built in pre-amplifier inside the pickups.
This method (passive series wiring) will give your bass that “fat sound”.
Note: that the pickups are wired this time in series, not parallel. This method has some benefits, and some draw backs too.
On the plus side, you will get more volume out of your bass, because the pickups are wired in series, and just like batteries, the voltage is additive. The sound will also be very punchy.
On the minus side, you will only have one volume knob, and your bass sound will be darker (not as much highs in the sound since you have twice the coil). You also will get a higher background noise from the pickups too, since they are in series.
Pickup Series Wiring
Notice that the coils are actually out of phase (from the stand point of external noise) here. It does not matter in this case that the coils are out-of-phase. If the coils are wired in-phase, then it becomes a single coil type of wiring, and you can expect to hear hum when your near electrical fields.
As of this writing, I believe that the pole pieces are reversed (front to back) so that the induced electrical current by the strings are in-phase, while the coils themselves are out-of-phase for electrical noise in the area.