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Basic Wiring - Customs by Ripper


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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:

Wiring Diagram with Accessory and Ignition
Full Size Image
To save this image on your computer, click HERE to download the ZIP file (both diagrams are included).

This harness shows an automotive ignition switch with accessory, ignition and starter:

Wiring Diagram with Accessory, Ignition and Start
Full Size Image
To save this image on your computer, click HERE to download the ZIP file (both diagrams are included).

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.
Harley Ignition Switch Wiring

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:

Headlight Socket

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....

Turn Signals:

One Switch:
Signals with one switch

Two Switches:
Signals with two switches


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.

Standard Bosch 5 Pin Relay - Example Standard Bosch 5 Pin Relay - Diagram Standard Bosch 5 Pin Relay - Bottom View



Bottom View

Use a 1N4000 series diode (available at Radio Shack and other electronics stores) as a drain for static electricity that builds up from deactivating the coil. The band side always points toward the side of the coil that has positive power. This is not required, but it will save some of your components from possible electrical damage.

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:

  • Pins 85 and 86 are your control inputs. This is where the magic happens. Applying 12 volts to one side and ground to the other will cause the coil to energize and the relay to activate (actually, these relays will activate with as little as 8 volts, but that's another story for a much more detailed discussion). Polarity is not important here and you can put your switch either in the power side or the ground side, depending on what you're doing. Generally though, you usually apply power to pin 85 and ground to pin 86, but this is not really important (you can reverse the polarity on these 2 pins without any problems, just remember to point your diode band to whichever side has the positive lead going to it. The coil creates a magnetic field which turns back on itself when power is removed, creating a power surge of little current but very high voltage. This can damage electronic devices in the circuit if left without this added safety device).
  • Pin 30 is usually your high current input, either directly from the battery (fused, of course) or from an ignition or accessory circuit output from your ignition switch (also fused). In some cases, pin 30 is used for output instead of input, but we won't discuss that here since it doesn't apply to basic wiring.
  • Pin 87 is for 12 volt power to the device you want powered up when the relay is activated by the coil being energized (when you press the horn button, for example).
  • Pin 87A is only used if you need power to flow through the contacts when the relay is NOT powered up (such as a starter kill in an alarm harness) or if you are diverting power from one place to another by activating the coil (for example, if you are using the relay as a headlight dimmer switch). This is explained in the schematics below. Note that in the following diagrams, a fuse is not shown, but a fuse is always needed. The main input (usually going to pin 30) must be fused with the proper sized fuse for the accessory being connected. This is not an option. See the above section on fuses to determine the correct one for your application.

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.

  1. Relays can be used to invert a switched input. Basically what this means is if you need something to have power that has a positive input but your switch provides a negative output, you need a relay. A basic example of this is a horn in most cars and trucks. Most vehicles use a ground output to activate a horn, yet most horns are physically grounded to the body or chassis, making a ground switched input useless. This is where a relay comes in. Using this example and the diagram above, we will create a basic wiring schematic to power up your accessory device. For those not familiar with schematics, this symbol represents chassis ground:
    Schematic Ground Symbol
    In this diagram, our switch is connected on the "ground" or negative side. Grounding the switch will create a positive 12 Volt output at pin 87.
    Negative Switched Relay Schematic Diagram
  2. Relays are used to send full power to a particular device while allowing a switch for that device to run significantly smaller wiring. This works extremely well for motorcycles where small wires can be run inside handlebars or other tubing yet can still be used to control high power devices like headlights, for example. In this diagram, our switch is connected on the "hot" or positive side. The wire to the switch and from the switch to the relay can be much thinner than the power wires. *Note that you can connect the switch input and the power for the relay to the same point, after the proper size fuse, of course.
    Positive Switched Relay Schematic Diagram
  3. Relays can also be used to divert power from one device to another using the same input wire. These can use either of the above type of switched input. We will use a motorcycle headlight circuit as our example. Using your manual, determine which wires in the handlebar housings control the headlight dimmer switch (most states require headlights on a motorcycle to be on at all times so we will use that as a guide, if you do not want your headlight on at all times while the bike is running (or when the ignition key is on), you may alter this diagram for that purpose. If you need help, feel free to ask, a link is provided at the top and bottom of this page for that purpose). In this diagram, pin 87A is used for low beam power (on when the relay is not powered up by the dimmer switch). 87 is used for high beam. (on when the relay is powered up). Run a new 14 gauge wire (fused, of course. A 10 or 15 amp fuse should be fine, depending on the power requirements of your headlight.) from the headlight output of the ignition switch to the headlight bucket and connect that to pin 30 on the relay inside the bucket, making sure all exposed contacts are taped or insulated against shorting out (DO NOT CONNECT TO THE BATTERY or your headlight will stay on constantly, even when the key is off). If possible, mount the relay solidly (either rivet or screw it to the headlight bucket or use a wire tie (or tie strap) to connect it as solidly as possible). If not, insulate it against vibration and possible short circuits with foam rubber. Connect the dimmer switch input (the wire coming from the ignition switch to the handlebar switch) as if the switch were being used normally (if wiring from scratch, this can go to the same place as the wire going to pin 30, but can be much smaller than 14 gauge (18 or 20 gauge is fine for this purpose). The high beam output from the switch will go to pin 85 of the relay instead of to the headlight's high beam lug. The low beam output from the handlebar switch is unused in this case. Cut it back and tape it off to prevent a short circuit (it will still have power when the low beams are on. You can use this as an additional output for fog lights or other running lights that will shut off when the high beams are switched on, but you must use an additional relay for those).
    Power Diverting Relay Schematic Diagram
    One tip for all motorcycle headlights. Do not use the ground wire supplied that mounts to the inside of the headlight bucket. Always ground the headlight to the chassis of the bike (anywhere on the frame itself or go directly to the negative battery terminal). Grounding the headlight through the shell or bucket can at the least cause the headlight to flicker, not be at full strength, burn out faster and in extreme cases can cause your steering head bearings to seize (rare, but it can happen).

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