Switched Accessory Wiring

In this article, we’ll walk you through the necessary electrical theory for adding bulletproof accessory wiring to your motorcycle. I know folks find electrical wiring a little intimidating, but once the basics are solidly in your tool belt… you’ll find plenty of future opportunities to draw on the knowledge & experience gained here.

I say “accessory wiring” because the parts list and installation guide that follows can be used power all manner of electrical accessories on your motorcycle. Thnk cell phone chargers, GPS, Go Pro cameras, additional LED lighting, heated grips and whatever else you want to take along with you for the ride. (12 Volt blender for campsite margaritas? Nice.)

The following diagram shows the simplest and most bare bones schematic for accessory wiring.

This system totally works, but has one Achilles heel … it’s “un-switched” power; meaning it’s “always on”. If you forget to unplug your accessory when not parked, it will continue to drain your battery leaving you dead in the water the following morning (Ask me how I know). A 10 amp fuse is added to the (+) positive lead to guard against any possible shorts down the line.

Here’s a much better option:

What we’ve done here is add a relay to the (+) positive lead. A relay is an electrical switch that will only pass current when certain conditions are met. What conditions need to be met will be depend on how we wire up the relay. Do you feel empowered yet? You are about to make electricity your Bitch!

Let’s take a closer look at a typical 4 prong relay:

There’s usually a schematic printed on the side of the relay using numbers and symbols to reference the different prongs and internal functions of the relay. Hieroglyphics at first, but we can talk it out buddy.

A relay is a “conditional switch”. In this case, when the relay sees (+) power at prong 86, (+) electricity flows unhindered to ground through prong 85 (completing a circuit). Electrically (+) is always looking to flow to ground, so it’s kinda like setting a trap for (+). The flow of electricity through prongs 86 & 85 flips an internal switch within the relay that connects prong 30 (“always on” … direct from the battery) to prong 87 (and your accessory). You can actually hear an audible click when the relay is activated, there’s a physical switch in there.

Finding your switched power source

We’ll need to find a source of electricity (+) on the motorcycle somewhere AFTER the ignition switch to use as the “decision circuit” for the relay. Any circuit that is activated by the ignition switch will do. Go ahead, experiment with your bike’s ignition. What circuits turn on ONLY when the ignition switch is activated? Every bike is different, but usually the most convenient source of switched power is lighting of some kind. Does your headlight or turn signals have a running light function? Your motorcycles taillight is also controlled by your ignition switch. Both are perfect if they receive power only when the ignition switch is turned on… and power off when the ignition switch is in the OFF position. (if so, you’ve found your control circuit!)

I know what you’re thinking… Why are we bothering with a relay if we’ve already found a source for switched power? Well, there are two advantages to using a relay. Firstly, with a relay, your accessory is wired directly to the battery. Different accessories require different amount of current, so you can use the appropriate gauge wire appropriate for the accessory you are installing (For example: lighting draws much more power than a cell phone charger). To blindly assume that the wire we’ve found has enough capacity to handle your accessory is asking for trouble. Secondly, a relay allows you to add additional protection in the form of a dedicated fuse for your accessory. If anything goes wrong with your wiring, it takes down the accessory circuit only, not the rest of your bike!

OK, hardest part done (finding your switched power source). Let’s take a look at how we are going to make all these connections.

One of the easiest ways to make all the necessary connections will be with crimp connectors.

Crimp connectors are conveniently color coded as to wire size. Red being for thinner wires (22-18 gauge), blue for thicker (16-14gauge) wires. No soldering necessary, one simply crimps the connector on to the wire with a crimping tool like this:

Notice that the crimping tool has color coded jaws to apply the appropriate crimp to your color-coded connector. (I know, they have thought of everything!) The prongs of the relay are a male ¼” spade connectors, so we’ll be using ¼” FEMALE blade connectors to connect to the relay. Use the insulated style shown here:

As for our switched power source, we’ll need to find a way to tap into that wire somehow. One of the easiest ways to tap and splice into a wire is use of something like a Posi Tap as seen below:

This allows you to easily “T” off your control wire and connect to the relay through the female spade connector. (How awesome is that?) To finish the decision circuit, we need to connect Prong 85 of the relay to a good ground. Use a female spade (on the relay side) to a ring terminal to attach to a good bare metal ground on the frame.

Here’s a look at the relay all wired up:

This way, when the ignition switch is on, the relay sees (+) power at prong 86, triggering the relay. Once triggered, the relay will allow the flow of electricity from prong 30 to prong 87 (and to your accessory). With the ignition switch OFF, the relay switches off to disconnect the power to your accessory!

Well, well well. Nicely done. You made it through the whole article without your brain exploding! (Well, maybe you did have to read a few sections a 2nd & 3rd time… 🙂

Now it’s time to ponder the of your particular installation. You’ll need to locate a good location for the relay, and you’ll need to decide the best route to run your wires. Spend some time thinking of the most eloquent way to accomplish this, and your time will be rewarded! A clean installation is the most reliable. (please, no birds nest of extra-long wires anywhere, not sexy!)

Do it once and do it right!

Should you need to locate supplies or additional advice for your particular project, may I suggest contacting your local Community Motorcycle Garage? As opposed to a traditional motorcycle shop, CMG’s typically have parts for projects like this on hand and can provide valuable advice and assistance if necessary. We’re here to help!

Now go relay the crap out of something!