MotoMojo: Wiring Accessories the Right Way

Aug 19, 2017 View Comments by

More and more electrical accessories are becoming available for motorcycles, putting an ever-growing demand on charging systems and batteries. Before you begin cutting and splicing wires, take a few minutes to consider the best way to approach such a project, as doing the wrong thing could result in a dead battery, fried electrical system, or even a fire. Improperly wiring a motorcycle that’s under warranty could also result in a denial of coverage.

Determine how much current the device draws. Power draws are denoted several different ways; you need amps to determine wiring and fuse sizes. Some items, such as lights, are rated in watts. Watts/volts = amps. So, for example, a 60-watt auxiliary light draws about five amps (60/12 = 5). Some items such as electrical motors draw more power at first when starting and then the current requirement goes down as it reaches operating speed. Make sure you have enough capacity to handle that initial surge, which can be several times the running current.

Wire Sizes and Capacities
Always use flexible insulated multi-strand copper wire designed for automotive use. Wires come in various diameters, known as gauges. With the American Wire Gauge (AWG) system, the lower the number, the heavier the wire. (Metric wiring gauges show diameter in millimeters, and increase as wire diameters increase.) Using a wire that’s too small for the load is dangerous and must be avoided. However, using a wire that’s heavier than needed is harmless. It just adds unnecessary bulk and cost.

For short runs such as on a bike, use the following gauges (according to the National Electrical Code): 18 gauge < 6 amps, 16 ga. < 8 amps, 14 ga. < 15 amps, 12 ga. < 20 amps, 10 ga. < 30 amps. Choose wire insulation colors that are different than the originals found on the bike to differentiate them when tracing.

Fuse Sizing
Fuses come in a variety of styles and ratings. The most popular types for modern motorcycles are ATC and mini fuses; try to match the type you already have on your bike. Fuse holders are readily available in auto parts stores and motorcycle shops. Select the next highest fuse capacity than your accessory’s load. If you have two 35-watt lights, 70 watts/12 = 5.83 amps; use a 7.5- or 10-amp fuse.

Circuit Protection
All electrical circuits should be protected from short circuits by either fuses or circuit breakers. Fuses are compact and most commonly used. Circuit breakers are usually a bit bulkier, but are available as auto and manual reset, which means you don’t have to carry spares with you, as with fuses.

Switches for accessories should be designed for use on vehicles, rated for the load, and weatherproof. Switches that light up with a built-in LED are a good idea to indicate when the item is on. Often switches can be mounted near the instrument cluster, or with a handlebar mount or bracket. Bosch-type automotive relays should be used when current draws are relatively high, such as with auxiliary lamps.

Where to Connect?
For items like battery maintenance chargers, small air compressors to inflate tires, heated clothing (which you unplug when leaving the bike), and power outlets for USB charging (which you may want on while parked for a meal, or camping), you may want to connect directly to the battery. For other items such as auxiliary lamps, GPS, heated grips, and audio systems, connecting to a switched circuit will help prevent battery drain when the engine is off.

With conventional motorcycles, you have several choices for tapping into the wiring system. Connecting downstream of the ignition switch allows you to have power only when the switch is on, and it shuts off when the ignition is switched off. If you go directly to the battery, the power will be available all the time, but if you forget to turn something off, you may have a dead battery when you return to your bike. A direct connection is needed

Consult the shop manual for wiring diagrams of your motorcycle. You can also use a voltmeter or test light to determine which wires are turned on by the ignition switch. If your bike has an accessory fuse and circuit, use that. Otherwise, the best circuit to tap into is usually the lighting circuit, which has a small amount of excess capacity. You should be able to run a USB outlet or a GPS directly. Heavier current draws should only use the circuit as a trigger for a relay, with the main current coming from the battery. Avoid tapping into gauges, ignition or engine controls, ABS, or other delicate circuits. Wire splicing needs to be done carefully and properly. Most people use crimp-type connectors for the convenience, although I prefer soldering connections and covering them with heat shrink tubing for greater reliability. Carefully strip wires without cutting any strands. Connect where it’s easy to access the wiring, and route wires so there’s no strain when the steering or suspension moves, and nothing rubs or touches hot parts.

CAN bus
CAN bus wiring is being introduced into motorcycles and is found on bikes such as BMW’s K 1600 GT/GTL, liquid-cooled R 1200 RT, and liquid-cooled R 1200 GS models. CAN bus does away with a bundle of wires and allows various onboard computers and peripherals to link and communicate. Every electrical device is connected to the CAN bus system, which monitors current flow, voltage, etc., by computer. Tapping into the electrical system with a new electrical gadget may cause the CAN bus computer to “think” there’s a malfunction or short and indicate a trouble code or could even shut down the system.

The CANopener by Clearwater Lights is an aftermarket wiring kit and control module that plugs directly into the existing CAN bus connectors without cutting or splicing wiring. The stock switches on the left handlebar of BMW CAN bus models can then control the accessory lights.

Another method is to install a small auxiliary fuse block near the battery and use a wire with an inline fuse to connect the block’s main input terminal to the switched output of a constant-duty relay. Run power from the battery positive (+) terminal to the power input of the relay. Then, use the wire at the diagnostic connector, which gets voltage when the ignition is on, to activate the relay. This allows the load to be drawn from the battery, but it’s switched, and only a trickle of power is drawn from the diagnostic connector to trigger the relay. This is how many BMW dealers cope with adding accessories to CAN bus bikes.

Text and Photography: Ken Freund


Tours, tankbag maps, tips, and more: subscribe to RoadRUNNER today!

Tags: , , Categories: Chronicles