Electrical Wiring For Motorcycles
Text and Pictures by Mark Trotta
One of the biggest challenges when restoring a classic motorcycle are electrical problems. Unlike mechanical issues, what's not working is not visible to the eye.
Thirty Seconds Of Electrical Theory
The electricity in your motorcycle travels in a circle. Power leaves the battery from one terminal, passes through components (lights, horn, coil, etc.) and ends up back to the battery on the opposite terminal. Electricity doesn't care which way it goes, it just needs a path to come back. If something in that circle breaks, either a component stops working or the bike doesn't start.
Read: Basic Motorcycle Wiring
Motorcycle Electrical Systems
There are three major parts of a motorcycle electrical system - primary, secondary, and charging system. What hooks them all together is wiring, switches and relays. So let's break these down into smaller categories.
Primary Ignition (Low-Tension Side)
In a points-operated ignition system, the primary, or low-tension side of the ignition system starts with the battery, goes to the ignition switch, then to the primary windings of the coil. A key-operated ignition switch connects or disconnects the power.
- Coil (primary winding)
- Wiring, switches, and relays
Secondary Ignition (High-Tension Side)
An ignition coil is basically a step-up transformer. The voltage from the primary windings of the coil go into the secondary windings of the coil, where they come out at 20,000+ volts. The secondary, or high tension side of the ignition system goes to the contact breaker (points), which gives the necessary volts to the spark plugs.
- Coil (secondary winding)
- Points and Condenser
- Spark Plug Wires
- Spark Plugs
Motorcycle Charging System
To check your charging system voltage, with the engine off, check the voltage across your battery with a multimeter. Next, start the bike and check it again, with the lights off and engine running about 1500 rpm. With the engine running, the charging voltage should be higher than the basic battery voltage. A reading of 13.8 volts is ideal, but anything over 12.8 volts is good.
If your readings are lower than 12.8 volts, the battery is not getting completely charged. For a final test, turn the lights on, and check the voltage again. If it drops below 12 volts, your battery's going to be in a state of discharge when you're running with lights on.
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.
Read: Harley Generator Repair or Replace
Simple Generator Test
The first sign of a generator not charging properly is a dead battery. The simplest way to test a generator is, with everything hooked up properly, start the engine. Then see if the headlight gets brighter when you rev the motor from idle.
Another way to test generator output is to compare the difference between voltage at the generator and voltage at the battery. There should be higher voltage at the generator.
Wiring A Motorcycle From Scratch
On several of my motorcycle projects, I started with nothing more than a simple wiring diagram drawn out on a piece of paper. I usually start by mounting the battery, then adding the other electrical components one at a time; headlight, taillight, ignition switch, coil, etc.
Read: Custom-Wiring A Motorcycle
You can use standard automotive-type wire found in parts stores, but keep in mind there is a difference between SAE-rated wire and AWG (American Wire Gauge) sizing.
Shop: AWG Primary Wire
AWG sizing always has more copper for a given gauge size than SAE-rated wire does, and more copper is better. This variation is about 10-20%.
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.
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 wiring your motorcycle, take pictures of everything for future reference.
I prefer to route any length of wire longer than a couple inches 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).
6 Volt vs 12 Volt
Harley-Davidson used a 6-volt electrical system on motorcycles until 1964 and on Servi-Cars until 1963. If your 6-volt battery and electrical system are in good shape and the motor has not been extensively modified, you should not have any problems with the original 6-volt system.
6-Volt to 12-Volt Conversion
A 6 volt wiring system and switches are more than enough to handle 12 volts. This is because it's heavier because 6v has to carry more amperage than 12v. Less volts means more amperage. Changing to 12v cuts the amperage load approximately in half. So there's nothing wrong with using 6-volt wiring in a 12-volt system.
6v to 12v Gauges
If your gauges are mechanical, there's no problem converting 6v to 12v. If your gauges are electrical, they will need a voltage reducer to make them work. Headlight, tail-light, parking lights, dash lights; all 6 volt bulbs must be replaced with 12 volt bulbs. If you have warning lights on your dash, these bulbs should also be changed.
You can re-use your headlight switch, brake-light switch, and high-beam switch. They will work in either a 6v or 12v system.
Motorcycle Battery Care
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.
Read: Motorcycle Maintenance
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.
Battery cables are the most critical wires on your motorcycle.
Always make sure that the battery and motor are well grounded to the frame.
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.
Read: How To Wire Handlebars Internally
Wire extension kits are available that have all the right colors and terminals for most motorcycles. Or you can be creative and make your own.