So I got a new (used) MIM fender jazz the other day, and was looking for some basic mods to get some better sound out of it. The first thing I ran across that didn't involve replacing pickups, bridge, etc, was the S1 or S-1 mod, to enable the pickups to work in series as well as parallel, essentially emulating a single humbucker. (no hatemail for that statement please. =P )

I hunted around, and found it very difficult to locate a good "howto" for doing this. There were a couple of poor diagrams floating around, and one good one which involved replacing a knob with a push/pull knob, which I wasn't interested in. I'm happy just sticking in a switch and calling it a day. So I resolved to do an S1 mod and write up a good howto when I was done. Thus, this page.
*edit* I have since been pointed to this page: http://acapella.harmony-central.com/forums/showthread.php?s=&threadid=250926

My goal was to do a basic mod that wouldn't require replacing expensive parts, or large purchases. That said, you do need one new part for this, as well as some wire. You'll need a DPDT (Dual Pole, Dual Throw) switch, which you can pick up at your nearest Radio Shack (or similar store) for under $5. But for $5 and a little time, this is a good (and easy) mod to get more from your Jazz style bass.

Why do this mod?
Good question. What this will do for you is extend the sound range of your Jazz-style bass. It gives you a tone which is closer to that of a P-bass, aka. deeper, warmer, and thumpier. It also leaves your stock Jazz sound intact so you're not losing anything, just gaining a more flexible sound.
*One thing to note, when you throw the switch for serial mode, your bridge knob won't do anything at all, and your neck knob will become a volume knob. The tone knob will function as normal.

Parts:

• 1 DPDT switch (I liked the 6A model, but a 3A would be fine too.)
• wire (small enough gauge to fit your DPDT switch terminals. I think I used 24 guage.)

• Soldering Iron
• Solder (i use silver-bearing really thin solder. melts easier.)
• A clampy thingy of some sort
• Philips-head screwdriver
• Wire cutters
• (Possibly needed: tiny allen wrench (hex wrench) for removing knobs. Depends on where you mount the switch.)

Now you're going to need to mount that switch somewhere. This is where it gets just slightly trickly. Since all basses vary, you'll need to figure out where you have space on your control board or pickguard to mount the switch. Personally my MIM fender Jazz had just enough space between the tone knob and the cable jack to mount the switch, but I had to route out a bit more of the control cavity to fit everything in. Consider switch placement carefully going into this. I didn't and had a heckuva time as you'll see.

To mount the switch, you'll need a drill with a metal-cutting bit, or just a cheapo bit if you're mounting on the pickguard. Bit size needed may vary based on the exact switch you use; mine was 1/4".

So what are we going to do exactly? Glad you asked.

We're going to wire in the switch as per the above diagram. When the switch is one way you pickups will run in parallel (aka. stock setup). When you throw the switch the other way, your pickups will run in series, giving you a deeper, warmer tone.

(For you newbies to this stuff, the electro-mechanical parts of the knobs are called Potentiometers, here called simply "pots". Also, the hot wire is the wire that's not the ground. Ground wires are usually black, hot wires vary. Fender MIMs currently use white as seen in the below photos.)

The DPDT switch:
We're going to name the terminals on the switch, for convenience.
Really easy:

E F C D A B

And no, it doesn't matter which way you hold it.

Step 1: Prep work Plug in your soldering iron to heat up. Remove the pickguard and the control plate from your bass. You should end up with something roughly like the picture to the right. Identify which wires are going to your neck pickup, and which are going to your bridge pickup, and which pot they're associated with. Label them if you're lacking in the shortterm-memory department.	Click on any image on this page for a larger version.
Step 2: Desolder Desolder the neck pickup ground wire from the pot. Desolder the bridge pickup hot wire from the pot.
Step 3: Solder the switch Solder a small piece of wire bridging E and F. Solder the neck pickup ground to C. Solder the bridge pickup hot to D. Solder a new wire from A to the ground. I soldered directly to where we removed the neck pickup ground earlier. (Use 3-6" of wire, depending on where you're mounting the switch. Be sure to leave yourself enough slack.) Solder a new wire from B to the bridge pot terminal that we desoldered earlier. (Ditto on wire length.) REFER TO THE EARLIER DIAGRAM OFTEN! When you're done this step, you should have a free-hanging switch, as in the photo to the right.
Step 4: Mount the switch Where you mount it is up to you. As I said before, I mounted mine between the output jack and the tone knob. Be aware that you may have to remove some wood to place your switch. I sure did. Photo at right shows mounted switch, compare to previous photo. As you can see, I had to rotate the tone pot to fit in the switch. The cavity was routed so close to the pots, that I had to 'stretch' the cavity on one side. I made short work of it with my favorite dremel and a sanding drum bit. You can see the cutout if you look closely at the large version of the photo.
Step 5: Test it Plug it in and test it out! You should notice a distinct difference when you throw that switch. If you don't, check your wiring and see where you went wrong.
Step 6: Reassemble If you did any cutting/sanding, clean out the control cavity thoroughly. Put the pickguard back on. Remount the control panel. Clean up! You're done!

Here are some images of my finished bass with the switch installed: