Wiring A Motorcycle From Scratch
Text and Pictures by Mark Trotta
Because I was upgrading from 6 volts to 12 volts and installing a non-stock front-end, I needed to wire my 1961 Harley trike from scratch. The following procedures should work for any classic motorcycle that is kick-start only and has a battery.
Before you start wiring a motorcycle from scratch, it's best to have all the electrical components on hand. This would include battery, generator, voltage regulator, ignition switch, headlight, taillights, and horn.
Most classic motorcycle wiring is 16-gauge or 18-gauge. Heavier 12-gauge or 14-gauge wire is required when you need something that can handle a bit more load, like starter and generator wires.
The wire rolls you buy in auto parts stores are usually copper wire insulated with plastic. These are SAE-rated, but there is also copper wire insulated with silicone, and higher quality AWG rated wire.
AWG sizing has more copper for a given gauge size than SAE-rated wire does, and more copper is better. This variation is about 10-20%.
Use quality connectors or solder joints to connect wires to terminals. If you choose crimp connectors, take off the colored plastic caps and throw them away. Seal the terminal butts with shrink tubing instead.
Read: Solder or Crimp Motorcycle Wires
Abbreviations for wire colors are usually "BK" for black and "BL" for blue, "R" for red, etc. If the wire colors cannot be duplicated from original colors, make a note for future reference.
Draw Out A Basic Diagram
Drawing out a diagram in a notebook lets you visualize what needs to go where. Start with the ignition switch in the middle of the page. One at a time, add each electrical component to the diagram. Use as many pages as you like.
Mark down where each wire is starting, where it will end up, and what color it will be. I usually reference the factory manual for this.
Best Place To Start Wiring
When you're wiring a motorcycle from scratch, you can start pretty much anywhere you like. For the Servi-car project, I started with the battery, which was being mounted inside the cargo box. I had no restrictions on physical size, so I looked for a semi-popular battery to keep the price down.
After doing a little homework, I purchased a 12N7-4A battery, which is rated at 7 amp-hours and 74 cold cranking amps. It's a fairly small battery, dimensions are 5-5/16" length x 3" width x 5-1/4" height, and has the vent tube on the positive side.
Important: The output of the generator will dictate how big a battery to use and what the amperage should be. Too high an output can cause damage. Not enough amperage will put extra work on the generator.
Circuit Breaker vs Fuse
Installing a 20 amp in-line fuse or circuit breaker from the battery will protect your electrical system in event of an electrical surge. I opted for a 20-amp circuit breaker.
Although wiring harnesses are available for most classic Harleys, if you're wiring a custom bike from scratch, you'll probably need to make a custom wiring harness or two. That 50-year old wiring is probably cracked and frayed anyway, and could use replacing.
There are several styles of wire loom you can use, including black plastic, black cloth, and original-style cloth with patterns. Although original-style cloth looms are most authentic, I bought black cloth looms in 3/16", 1/4", 5/16", and 3/8" rolls.
Shop: Black Cloth Wire Loom
Panheads, Shovelheads, and 1959-up Servi-cars have a large headlamp nacelle which housed the front wiring panel and wires. Since I was swapping a Wide Glide front end on my Servi, I had to find another way to wire up the headlight. This involved wiring the headlight switch inside the handlebars.
Taillights draw three amps or less so 16-gauge wire is fine.
To install a horn button like this you will need to thread-tap two holes. They are a machine-screw size 8-32 x 1/4".
Vintage Harley Dash
Since original 50 year old Harley electrical components are, well, 50 years old, I decided to buy a new aftermarket dash kit. This kit is from V-Twin and included a speedometer/tachometer, ignition switch, dash lamps, sockets, and hardware.
The two-light dash panel is correct for Knuckleheads, Panheads, and Servi-cars.
Five-Post Ignition Switch
Early Harley Big-Twins and Trikes had a five-post switch, which was replaced with a six-post switch sometime in the seventies. If you have the later six-post and don't need the extra terminal, two of them can be "tied" together with a small jumper wire.
Cycle Electric Generator
The single biggest expense of my electrical system (but well worth it) was the Cycle-Electric generator. The voltage regulator is built onto the generator, making it attractive and less cluttered.
Read: Install Cycle Electric Generator
With the built-in regulator, the Cycle Electric generator is a complete 12-volt charging system. It's a simple, two-wire hookup, and gives you higher output and longer service life. It's the last generator you will have to buy for your bike.
There are two wires that run from the Cycle-Electric generator, the larger wire goes to the ignition switch. Use a heavy grade wire here. The smaller wire from the A terminal goes to the generator signal light.
After the battery and switch are hooked up, the gen light comes on when you click the key switch to "on" but have not yet started the bike. The light should then go out when the bike starts up. If the light stays on after starting, it's telling you the generator is not charging.
Vintage Harley Speedometers
A 2:1 speedometer ratio is found on Harley models from 1936 to 1961, including 1947 to mid-1961 Servi-cars. From late-1961 to 1983, Harley Servi-car and Big-Twins had a 1:1 speedometer ratio. You can use either one for any year Harley, provided you use the correct speedometer gear along with it.
The speedometer/tachometer combo that came with the kit has a diameter of 4-11/16". The bottom nut for the speedo cable is 5/8" female thread.
Distributor and Ignition
I like points ignition for their simplicity and ease of diagnosing. Seems like a lot of modern mechanics don't like points because they don't understand them, and incorrectly assume they are unreliable. Once properly set, a points ignition will run fine for 5,000 miles or more, without needing to be "fiddled" with. So unless your classic bike is your daily driver, points ignition is fine.
6-Volt to 12-Volt Conversion
Harley-Davidson used 6-volt electrical systems on trikes until 1963 and motorcycles until 1964. If you're converting from 6v to 12v, make sure the new battery will fit. You may also need to replace the battery cables.
Ignition points will work with either 6-volts or 12-volts, but the condensers are different. If converting, buy a 12-volt ignition coil. They have more windings and will work much longer.
Make sure that both the battery and the motor are grounded to the frame.
Once everything is correct and working as it should, use plastic cable ties to secure wiring and harnesses.
Wiring a motorcycle from scratch is always a challenge. Don't rush, focus on one thing at a time, then move on to the next. If you break it down into smaller steps, it's easier to do.