Fix out of phase pickups to be in phase

[joakals]

I have got myself a J. Mascis telecaster which I want to change so that the middle-position is in phase.
I'm not that techincal so I want to make sure my plan is correct.
I'm thinking of swapping the black and white wire at the neck-pickup.
Next, I will also swap the grounding on the back of the pickup.
Can anyone assure me that this is a good solution to the problem?

It is maybe easier to change the wires at the control?
I think it looks complicated since several wires there soldered together there.
I don't want to untangle unnecessary wires.

Thank you very much for any help.

Guitar-scheme: SM_0140262326_J_MASCIS_TELECASTER.pdf

[joakals]

I tried this but it didn't work out. The sound of the neck pickup became low and scratchy. Anyone know what I'm doing wrong?

[timtam]

There are 3 things that needed to be done if you are permanently flipping electrical polarity ('phase') at the neck pickup. Only the minimal number of changes at the pickup end would normally be done, as there is a danger of the thin coil wires that also connect to the same terminals as the black/white wires on the bottom of the pickup being disturbed/damaged.
1. Cut the short bare-wire jumper that links the pickup cover and the coil ground terminal on the pickup.
2. Add a new wire from the 'cover' side of that cut jumper to ground just the pickup cover (eg a wire to the back of the volume pot)
3. Swap the coil hot (white) and coil ground (black) wires at the controls end (switch and back of pot).
(Alternatively, a similar process could have been done at the bridge pickup).

Since you appear to have made all the changes at the pickup end, it's a matter of whether or not they have been wired in a way that should achieve the same result. Changing polarity should have no effect on the sound of a pickup when selected on its own. On the Mascis tele, the stock, previously OOP middle position should be the only thing that changes - it should now be in phase.

The original common ground jumper linked the pickup's metal cover to the black (ground) wire. That cover still needs to be grounded. So if you have resoldered that cover ground connection somewhere else, does the cover still have electrical continuity (zero resistance on the multimeter) with the guitar's ground, eg back of the volume pot ? And does the side of the pickup that the jumper wire previously linked to now no longer have that continuity to ground ? Because it should now be the hot. So instead that side of the pickup should now have continuity with the top right terminal of the 3-way switch in the wiring diagram. And the other side of the pickup should now have continuity with the back of the volume pot (ground).

[Guppy]

I tried this but it didn't work out. The sound of the neck pickup became low and scratchy. Anyone know what I'm doing wrong?

You should definitely not solder ON the pickup. But I guess it's already to late. You should have just switched the wirte at the controls. Even if it's more complicated. You are not going to ruin anything. And you could have even learned a new skill if you where more patient.
If the neck pickup sounds weak and scratchy on it's own I think it's already damaged. If it's only in the middle position then you actually got it out of phase.