Making your accessory wires 'switched'
The accessory wires on the Ninja 250 are a really good thing, except that they are always hot, which has its drawbacks. If you are only running one or two accessories (heated grips or charging a phone, for example) this writeup will show you how you can arrange it so the accessory wires go off when you turn off the key. It may save you from the dead battery you'll get if you forget to turn off your accessory when you walk away from the bike.
If you have several farkles on your bike, it would be better for you to mount an accessory fuse box.
What is going on in this article is you are cutting a trace, then adding a jumper wire. The trace, which is a part of the copper layer on a printed circuit board, is about .001-.002" thick. It's copper, laminated to the board, which is probably made of fiberglass.
Compared to installing a fuse box, this method is easier, costs much less, and is totally invisible, unless you open the back of your fuse box. This was written for a '96 Ninja EX250F; F series bikes built since 1995 should be the same, but take a good look at yours to make sure before you start modifying. This won't work on '94 and earlier bikes. They have a different junction box, which contains not only the fuses but also things like the diodes and relays which implement the headlight logic and other interesting things.
Start by locating your fuse box, or "Junction Box" as the manual calls it, behind the right side seat cowl. Remove the seat first, and then the plastics on the right side of the bike. You should see a box that says "Fuse" on it. You will need to remove this.
Before you remove this box, take a moment to look at the fuses inside. Remove the cover. It comes off really easily with hand pressure. Take a look at the inside of the cover. You should see something like the picture below. The fuse being pointed at is the ACC (accessory) fuse. This is always powered, even when the ignition is in the Off position. That fact is what this article is going to change.
The box itself slides out of its rubber mounts by gently pushing it forwards, towards the front of the bike. When you have it out of its mounting slot, take out the two power wires. These should come out with just finger pressure, but you may need to use a screwdriver to depress the release button as you use pliers to pull them out. Be careful: they are fairly soft and you don't want to wreck them.
Once you've removed this from the bike, go to your favorite work station. You will need to remove the back of the box, so bring all of the tools and materials you haven't used yet with you. Take all four screws out with your small Phillips; be careful not to strip them, as they are soft. Remove the rubber baffle on the back; this comes off fairly easily by hand.
Now that you have the back off, you should see an exposed circuit board. This is where all of the fuses are connected. From now on you want to keep all metal tools away. You don't want to scratch this board (until you need to). Locate where the fuses are connected, and observe that the diagram on the cover will be mirrored. You can draw one yourself, just to make life easier. This makes it readily apparent what goes where on the circuit board.
This is where you're going to remove power from your ACC fuse when the ignition is Off. The only other item that receives power when the ignition is in the Off position is the fan, and you can see here that they are connected by the circuit (refer to earlier diagram). The best way to do this is to sever this printed circuit. At this point you need to take your knife and make a cut in the board, as in the picture below.
Make many slow motions with minimal force; you don't want to go off target and hit a portion of the board that you don't want scratched. You need to keep going until you see a yellowish color instead of the silvery metal color of the circuit; this shows you are through the metal and the link is severed. Use your tester to ensure that these two points aren't connected anymore. Put one arm of the tester on each metal contact (not on the board itself; there is a protective coating that is non-conductive on it).
You will now need the small piece of wire, preferably between 18 and 12 gauge – no larger than 12 for clearance issues. 16 AWG usually works well for moto applications. Strip a few mm off of each end, but not too much; you want the coating to occupy most of the length to prevent any issue of touching another piece inside the fuse box. Refer to your own board for exact length; you can see how it will be used in this photo.
Crush both ends lightly with your pliers, just enough to flatten them for clearance issues. Draw some solder into each end of the wire with the iron, making sure it is well saturated before you solder it onto the board. Solder one end at a time to the applicable connectors. You are connecting the power (before the fuse) of the horn to the power (again before the fuse) side of the ACC. This way, if your ACC fuse blows, you won't also risk blowing the horn fuse (as it is at a lower amperage after your modifications).
If you aren't brave enough to learn to solder on this job, take it to your local electronics nerd and show him the final picture. Should be that easy.
You can now re-assemble everything and test the finished product. Use some dielectric grease on the fuse box connectors. No modifying is obvious, and what has been modified is reversible (you only need to solder a wire to make the connection you severed whole again).
When you test the modification, be sure to plug in all the gear you plan on running and blow the horn. As you are drawing power from the horn, this could blow a fuse that would otherwise be safe without the addition of your accessories. Be sure to carry a spare fuse in the spare slot, just in case. You may also consider carrying a spare 15 amp fuse, but only use this in an emergency, for instance if you're 200 miles from home and need to keep moving.
If, for some reason, the horn is held on for more than 30 seconds, you may run the risk of heating up the wire (which should blow the fuse). While this is highly unlikely, it is possible. Just keep this in mind if you ever feel like playing Jingle Bells on your Ninja - try not to hold that one high note too long. Holding the horn for 10 seconds is no big deal at all. By no means should you be afraid of using the horn; just use some common sense.
The obvious limitations of this method are with something that needs more electricity, as this does not provide a relay as with a fuse box. But unless your particular farkle needs over 10 amps of power, this modification should do it for you. This method will only work on a bike with an "always-hot" horn. Other models will need to find another power source.