Classic Motorcycle Build

Electrical Wiring For Motorcycles

Electrical problems are one of the biggest challenges when building a classic or custom bike. Unlike mechanical issues, what's not working may not be visible to the eye.

A factory service manual for your year and model motorcycle is helpful, but I've found discrepancies in manuals numerous times. Your common sense should override anything you read.

Most old motorcycles use 16g or 18g copper wire insulated with plastic. For starter and generator wires a heavier 14g is needed. You can use standard automotive-type wire found in auto part stores, but you should be aware that there is a difference between SAE-rated wire and AWG (American Wire Gauge) sizing.

AWG Rated Wire

Voltage drop is the wire's ability to carry current with a minimum amount of electrical resistance. AWG sizing always has more copper for a given gauge size, and more copper is better. The variation is about 10-20%.

Remember that battery cables are the most critical wires on your motorcycle.

Making A Wire Harness

Wires and their connections develop oxidization over time, which lead to poor connectivity and eventual failure. Replacing a single wire or connector may fix your electrical problem, but if this happens several times, consider rewiring the entire bike.

custom bike wiring

On several of my motorcycle projects, I started from scratch with nothing but a simple wiring diagram I drew out. I usually start by mounting the battery, then adding the other electrical components one at a time; headlight, taillight, ignition switch, coil, etc.

To make a wiring harness, or completely rewire a bike, you will need:

Drawing a wiring diagram out in a notebook (no matter how simple) helps visualize what needs to go where. It should show where each wire is starting, where it will end up, and what color it will be. Abbreviations for wire colors are usually "BK" for black and "BL" for blue, "R" for red, etc. While rewiring your motorcycle, take pictures of everything for future reference.

electrical wiring for motorcycles

Use quality connectors or solder joints (preferred) and heat shrink your connections. Make sure that the battery and motor are well grounded to the frame. If the wire colors cannot be duplicated from original colors, make a note for future reference.

I prefer to route any length of wire in either cloth or plastic wire looms. Some builders like wrapping wires in electrical tape, or only use heat shrink tubing on the ends. All these methods will work, what's important is that the wires are secure and are away from any sharp edges. They should also be kept away from heat (cylinders and exhaust).

Generator Charging Problems

Your classic bike's generator or alternator really doesn't charge the battery until engine rpm is considerably higher than idle. Trips that are less than 15-20 miles are typically not enough to recharge the battery's losses from starting. Consider upgrading to a trouble-free aftermarket generator like Cycle-Electric.

generator charging problems

buy Cycle Electric 12V Generator with Regulator

The Cycle Electric generator is a complete 12V charging system with a built-in regulator, simple two-wire hookup, higher output, and longer service life. It's the last generator you will have to buy for your bike!

Amp Gauge vs Volt Gauge

An amp gauge will tell you the amount of amps passing into or being drawn out of the battery. A voltmeter will tell you the "pressure" behind the amps.

read Remove and Install Harley Bendix Gear

Motorcycle Battery Care and Maintenance

Getting tired of buying a new motorcycle battery every couple years? Seems we neglect them until there's a problem, unnecessary damage is done, and we're spending money for a new one.

A motorcycle battery has a harder life than a car battery. Not only are the plates inside them more fragile, but they are subject to much greater vibration.

electrical wiring for motorcycles

read Motorcycle Maintenance

Handlebars are one of the first things riders change. If you're thinking of installing taller handlebars, you'll need to extend the switch wiring harness. Many wire extension kits are available that have the right colors and terminals for your year and model. Or you can be creative and make your own.

read How To Wire Through Handlebars

wiring handlebars internally
1974 Triumph Trident restoration

read Troubleshooting Motorcycle Electrics