Output jack wiring looks simple. Two connections: hot and ground. But the output jack is the most common source of signal failures on a finished build, and most failures come from the same two mistakes. Here is how to get it right.
The two output jack types you will encounter
Barrel (mono) output jacks are the most common on solid body guitar kits. They have a cylindrical body that threads into the side of the guitar. The tip contact carries the hot signal. The sleeve contact is ground.
Panel-mount (open cup) output jacks have a cup that mounts in a routed cavity, with a jack facing outward through the side. These use the same tip/sleeve logic.
Stereo jacks appear on some kits with active electronics or battery circuits. They have three contacts (tip, ring, sleeve). On a passive guitar, wire tip and sleeve only and leave ring disconnected. On active circuits with a stereo jack switch, the ring contact connects to the battery negative terminal (the cable insertion and removal turns the circuit on and off).
The most common mistake: mixing up tip and sleeve
On a barrel jack, the longer spring contact is the tip (hot). The shorter contact (or the threaded collar itself) is the sleeve (ground). When these are swapped, the guitar functions normally when plugged into the amp, because amp and guitar share the same ground. The problem appears when the guitar is plugged into a pedalboard or recording interface with a ground lift: the signal drops out or is reversed in polarity.
Wire the hot signal to the longer spring contact. Wire ground to the shorter contact or the sleeve lug.
The other common mistake: mechanical fatigue
Output jack wires in a side-mounted barrel jack are subject to mechanical stress every time the cable is plugged and unplugged. A wire soldered to a jack that is not anchored properly will eventually flex at the solder joint and break.
Two things prevent this: use wire long enough to have a gentle loop from the cavity to the jack (no tension at the solder joint), and use a jack wrench to tighten the barrel jack properly so it does not rotate with cable use. A jack that spins every time you plug in will eventually break the wire.
How to test before closing the cavity
Plug in with a cable before screwing the pickup ring or cavity plate closed. Tap the pickup poles with a screwdriver: you should hear a clear thump. Play a note: you should hear it cleanly. Roll the volume to zero: signal should cut completely. Tap the strings while touching the bridge: hum should stop or reduce (confirms ground is working).
If you get a constant loud hum
The most common cause is the hot and ground wires swapped at the output jack. Unplug from the amp first, then swap the two wires at the jack and retest.
If you get no signal at all
Check that the tip contact spring is under tension. On some jacks, the spring needs to be bent slightly toward the center so it makes firm contact with the tip of a cable. A spring that lies flat against the body of the jack will not connect.
Two minutes of testing before you close the cavity saves you 20 minutes of opening it back up later.