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What we will do on this page is get into very basic wiring. What we will not do is get over-technical. If you need to get that technical, you probably don't need this help in the first place. This page is a basic how-to for the average do-it-yourself backyard mechanic. If you are above that level of expertise and still require wiring assistance, feel free to Contact Me for any help I might be able to provide you.
Here is a basic motorcycle wiring harness with accessory and ignition:
This harness shows an automotive ignition switch with accessory, ignition and starter:
If your ignition switch does not have an accessory or "Lights" position, but you have an extra wire for that purpose, move that wire to the ignition lug of the switch.
Here is a diagram of a Harley Davidson ignition switch, usually found on big bikes with a central dash cover.
Speaking of lights, there are many questions about aftermarket lights, lighting wire colors and more. Here, I will try to make it easier. First of all, there is no wire color standard for aftermarket lighting fixtures. A black wire, which is normally ground on most factory wiring harnesses, is sometimes a power wire on aftermarket light housings. Never assume any wire to be something, as the wrong choice can fry your new light, or worse, your battery. Some lights come with wiring diagrams, but most do not. It is much better to test it first, rather than find out later that you hooked it up wrong.
For headlights, if your headlight uses a stock 3 prong plug, like this one:
The top pin is usually ground. The other 2 can be reversed on some bulbs (kind of odd, but true). Testing is the only way to be sure. Use a 5 amp fuse in a test lead and ground the top pin. Test each of the side pins, one at a time to see which is the high beam and which is the low beam. Connect your wires accordingly, with the proper size fuse for your headlight.
As for taillights, the wiring colors are far too numerous to figure out, so testing is once again your best option. Ground the housing (most aftermarket lights do not include a ground wire, even if they have a black wire in the harness. Be sure to check) and test each wire, again with a 5 amp fuse, to see which does what. Wire accordingly. Make sure your housing is properly grounded when mounted on the vehicle.
For specific wiring issues regarding a particular vehicle make and model, get a factory manual if you can or at least get a Haynes or Clymer (or similar) manual for your specific make and model. These have wiring diagrams on the back pages. If you are attempting to make parts from one vehicle work in another vehicle, get both manuals, strictly for figuring out what wire serves what function and so you know what to connect to where and what to eliminate. That said, we'll go a little deeper....
Remember to always add a fuse to every individual circuit you create or add to the existing harness. Do not tap into existing fused circuits to add accessories as this will overload the original circuit. Fuses should also be mounted as close to the battery or ignition switch as possible. The main reason for this is if you have a circuit failure or pinch a wire at any point and it causes a dead short, the fuse will save your harness and/or your vehicle from self destructing. You may have one melted wire to fix or a burned out accessory to replace, but it beats replacing a harness or a whole vehicle. Keeping the fuses close to the power source eliminates long stretches of unprotected circuitry. A general rule of thumb on fuse sizes is 10% more than the load limit of the accessory being fused. For example, if your accessory is rated at a maximum 20 amps, your lowest safe fuse limit would be 22 amps, so a 25 amp fuse would be your choice.
A relay is nothing more than an electronic switch. Completing a circuit through a coil in the relay causes a reaction that in turn causes a connection between two or more terminals in the relay. In some relays, breaking the connection to the coil causes a connection between two or more different terminals on the same relay. Technical stuff aside, the following will describe how to use a standard Bosch 4 or 5 pin 12 volt relay to accomplish some things you couldn't do (or couldn't do as easily) without a relay.
Here is a basic "Bosch" relay and circuit diagram. These relays are about 1" square, are generally black plastic and sometimes have a mounting tab on them. They are all numbered identically with the exception of pin 87A. Some do not have pin 87A. These relays cost no extra to have that pin so don't worry about that part. If you don't need that pin, you simply do not use it. See the schematics below to determine if your circuit will require that pin for your application.
These relays are made by several companies and they are by far not the only available relays, but they are the most commonly used and most readily available relays out there. They are rated at 30 amps, but they are really good up to 40 amps. That's for Bosch only, the other companies that supply these may differ.
Pin connections are as follows:
Relays serve several purposes. We will explain a few of those purposes, but these are nowhere near all of the reasons to use a relay.
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