Wiring a Relay for Accessories


This document describes how to wire a relay into a motorcycle's 12 volt electrical system in order to power add-on accessories such as radar detectors, GPS'es, satellite radio, heated clothing, aux lights, etc. There's also some photos of an install on the author's R1200GS.

Why use a relay?

There are several reasons you should use a relay instead of just wiring your accessories directly to the battery. The most important reason is so that your accessories won't be ON all the time. Sure, you can put a switch in the circuit, but what's to keep a little kid (motorcycles attract kids, you know) from flipping the switch on your parked bike? Or forgetting to flip the switch off yourself? If either of those things happen, you might find your bike's battery dead the next time you try to use it.

What is a relay?

In simplest terms, it's a little switch that throws a bigger switch. The little switch gets "thrown" by your motorcycle's electrical system when you turn the ignition on. The little switch triggers a solenoid (or solid-state equivalent) which flips the big switch ON. The big switch side of the relay is the "power" side that will be used to run accessories you're adding to your bike. The way we'll wire it, the relay will be ON when the bike's ignition is on, and off otherwise. Thus your accessories will also be on only when the ignition is on.

Here's a photo of a typical relay. In this case, a Bosch micro-relay, part number 0 332 011 007. The US quarter is shown for scale. Relays like this are available from auto-parts stores, truck stops, electronics supply places, and so on. They typically retail from $5 to $20US. I got this one from WayTek Wire.


Relays generally have 4 connectors (if they have 5, you'll ignore one of them). Refer to the schematic that came with your relay, or that may be printed on the side of the relay. There's an excellent primer on relays written by Joe 'Cuda' posted on the Airheads Website you might want to read if you're interested in learning more about the subject.


Note that this relay has slightly larger connectors for the main power side (numbered 3 & 5), and smaller connectors for the side that taps into switched power (numbered 1 & 2). Here's a typical schematic:


In the schematic above, the #3 connector would be connected to the battery's positive (+12v) terminal, and the #5 connector would power the accessories. These two wires should be 12-14ga, and the wire leading to the battery should have a fuse in it. I generally use a 20amp fuse (good for about 240w of power) but you can use a smaller fuse if you wish.

The big rectangle with the diagonal line represents a solenoid which is what turns the power "ON". Connector #1 would tap into an existing wire in the bike's wiring harness that has +12v when the ignition is on, and no voltage when the ignition is off. Connector #2 would go to ground.

The wires attached to terminals #1 and #2 can be much smaller wire. 22-24 gauge is fine, as the maximum current draw on it will be 100 milliamps or less (about one watt). I believe the rectangle shown above the solenoid in the above schematic represents a resistor, and means terminals #1 and #2 are interchangable -- i.e. you can hook either to switched power, and the other to ground.

Some relays have a diode in place of the resistor, and that means that only one of the "little" terminals (depending upon the orientation of the diode) should be hooked to switched power. If you don't know about diodes, ask a friend who does to help you decide which terminal goes to switched power. If you're lucky, the schematic will show you.

Wiring in the relay



The schematic above shows the simplest way to wire the relay, and the photo shows an actual example. Here's some step-by-step instructions for doing the job yourself. I'll use the specific relay above -- your relay may differ a bit in appearance and terminal numbers. Just make the approriate substitions.

  1. Turn the ignition off.
  2. Find a place for the relay. Near the battery is good, but it's also good to have it where it's accessable. Under the rider's seat is a reasonable choice for many bikes.
  3. Hook some 12-14ga wire with a ring-connector on one end to the +12v (Positive) battery terminal. Hook the other end of the wire with a spade connector to terminal #3 on the relay. There should be a fuse in this wire, as shown above. Use red wire if you can. The neat waterproof fuse in the photo above is available from WayTek Wire.
  4. Put another spade terminal on the POWER wire coming from your accessory (may be marked +12v). Later, you'll hook it to relay terminal #5 — BUT NOT NOW.
  5. Take some small gauge black wire (16-24ga) and put a small spade terminal on one end. Hook that to relay terminal #2.
  6. Put a ring-connector on the other end of the small black wire and hook it to the battery's NEGATIVE terminal, or to some good electrical ground.
  7. Take another longer piece of small gauge wire (use some color other than red or black if you can) and put a spade connector on one end and hook it to relay terminal #1.
  8. The only tricky part is finding a good source of "switched power" to tap into for the other end of the wire you just hooked to terminal #1. Here's where you might want to refer to your bike's wiring diagram, ask your friends for help, or use a mailing list or online forum. On many BMW models a green wire with blue stripe is a good source. Or you can just tap into one of the headlight wires. You're only going to draw 100 milliamps or so, so it isn't going to affect the circuit you're tapping into.

    • Make sure you pick a wire that is ON all the time (you'll need the ignition on at this point). NOT one of the turn-signal wires.
    • A useful trick is to use a pin or needle to probe the wire first (with the ignition on, of course), in conjunction with a multi-meter, to find a suitable wire.
    • I suggest stripping a bit of insulation off, wrapping and soldering the terminal #1 wire to it, and finally wrapping the joint with good electrical tape (3M Scotch 33 or 88).


  9. If you want an ON/OFF switch for your accessories, put the switch in the wire that goes from terminal #1 to the source of switched power. The photo above shows two types of switches. The SPST toggle switch has a rubber cover to make it water-resistant, and came from a hardware store. The push ON/OFF switch with LED is from The Electrical Connection. I don't think it's waterproof, but it seems to hold up fine in the rain on my motorcycle.
  10. When you have everything hooked up but terminal #5 (the power to your accessories), turn on the bike's ignition and test terminal #5 with your multi-meter. It should read +12v (or thereabouts). You may also hear a little "click" when the relay goes on. If you've got +12v, then turn off the ignition, hook up the accessory power to terminal #5. Turn the ignition back on and check your work.
  11. Your accessories will also have a black wire that needs to go to ground before they will work, even if you have power to them. If you have more than one accessory, you might want to use a distribution panel for the power, and perhaps one for the grounds as well. Distribution blocks are available from Radio Shack, marine shops and electrical specialty shops.

Adding a relay to a BMW R1200GS

Here are some photos and specific information about how I hooked up a couple of relays to my R1200GS. I used two relays -- one to power my HID accessory driving lights, and one to power everything else (Passport 8500, Garmin 276C, heated clothing).


Finding a good source of switched power on the new GS is easy. At the rear, under the rider's seat, is a large black plug with twist-off cap. It pulls out of a clip and I believe is used to plug the diagnostic computer to the bike during servicing.

Find the green wire with blue stripe leading into the plastic gizmo. Strip off a bit of insulation, tap into it, and wrap it back up with tape. The red arrow points to the orange wire I used for this purpose. The green wire w/blue stripe has +12v power as long as the ignition is on, and is not interrupted by the engine start cycle -- it's the perfect wire to use.

Don't be tempted to tap into the red wire, it has +12v ALL THE TIME (even when the ignition is off).


I could have made this a bit neater I guess. :-) But my setup is a little complicated. See below. I screwed a white plastic "distribution strip" (sourced from Radio Shack) to the tool tray. I labelled the two relays and stuck them under the rubber strap holding the tire repair kit in place.

The orange wire from the 20A fuse (lower center of photo) runs from the battery to one of the relays, which in turn "powers" some of the distribution strip connectors. Some of them act as grounds.

The spiral sheathing covers the wires running to the front of the bike, and down to the switchbox shown in the next photo.


This is the tricky part. I wired my setup so that I can run the accessories off switched power (normally) or directly off the battery. The latter case is for when I'm competing in a motorcycle rally and want my GPS to remain on during the entire event (computing overall speed/time averages), even when the bike is turned off and I'm in a resturant.

The way I did this is to use a SPDT (single-pole, double-throw) switch in the wire hooked to the relay's terminal #1. I hooked up the switch as shown in the following diagram:


Likewise, I can run my HIDs in two modes: Normally they are "slaved" to the highbeams and controlled by the bike's high-beam switch. In the other switch position, they are controlled by a push on/off switch glued to the inside of the left brush guard. In both cases, they only operate when the bike is running.

For this I also used a SPDT switch. The difference from the above diagram for the accessories was that one wire tapped into the high-beam circuit, and the other went to the push ON/OFF switch mounted to my left handguard, and from there to switched power. The center switch terminal went to relay terminal #1 as above.

The power outlet on the switchbox in the above photo is for my heated Gerbings jacket liner. I intend to mount a HeatTroller to give me continuously variable power from this socket, but haven't done that yet. For now, it's just pure +12v power. The socket is from Powerlet (formerly JasTek). The black box is a small "electronic project" box I sourced from RadioShack and backed with a piece of aluminum stock which I painted black and cable-tied to the frame tubes. It's quite solidly mounted.

Note: Scot Marburger did a very nice job installing a relay and a bunch of accessories, using a neat trick so as not to modify the stock BMW wiring harness. See his Electrical Wizardry page...

Copyright © 2004-2007, by H. Marc Lewis. All rights reserved.