Basic Motorcycle Wiring
Article by Mark Trotta
Engine vibration, exposure to the elements, and poor quality wiring all contribute to old motorcycle electrical problems. Components such as spark plug wires and coils go bad with time. Regular inspection of your wiring and electrical components can keep you from breaking down on the roadside.
Troubleshooting A Motorcycle Electrical System
Broken or melted wires are the most common source of electrical failure. The first thing to do is visually inspect all wiring, front to back, side to side. If you're lucky the problem may be obvious.
To check an old electrical system for shorts, start with a fully-charged battery. You'll be needing it to test the electrical system and wiring. Having a workshop manual for your specific bike is very helpful. If you don't want to buy one, you may be able to download one online.
Electrical Test Light
A test-light is a must-have for finding electrical shorts. Be sure to use a 12 volt (automotive) test-light. They sell for under $10. The leads on the test-light are reversible. One end goes to a positive and the other end goes to a ground.
Shop: 12 Volt Test Light
When diagnosing an automotive electrical problem, always start by checking the test light on your bike's battery. Depending on what you testing for, you may have to turn the ignition switch on.
Most test lights have a sharp pointed end. Use this to pierce the plastic insulation on a wire. Now you can test the circuit without disconnecting anything.
A multimeter is an instrument designed to measure electrical values such as voltage (volts), current (amps), and resistance (ohms). There are two main types of multimeters, analog and digital.
A Digital Multimeter is known as a DMM, and has an easy-to-read numeric display. Digital is by far more common, and they are inexpensive.
Although you can run dozens of tests with a multimeter, I use mine for about two. I use it mostly to check if a certain wire is still getting power or if there's a problem with it.
Using A Multimeter To Test A Wire
Connect one test probe to one end of the wire, and the other test probe to the other end of the wire. With the multimeter on the low ohm setting, it should read .02 or less. Anything much higher than that tells you there is a blockage in the current. Replace the wire and/or the terminals.
Another use for a multimeter is measuring battery output. Vintage Harleys are not known to have great charging systems, so a quick check assures everything's working OK.
After re-wiring my Harley Servi-car, I double-checked that all the lights and switches were working correctly. I then proceeded to wire-tie all the wires, and tighten all the components down.
A few days later, I went out to the garage, turned the ignition switch on, and had no power.
My first thought was that the battery had gone dead, but here was the big clue. Every time I turned the ignition switch on, I heard a "click". Something was tripping the circuit breaker every time I turned the power on.
After a little head scratching and poking around, I found the problem. I had pinched a wire when tightening the gas tank to the frame, thus grounding out the system. Having that circuit breaker on the battery protected my bike from any damage.
Repairing A Broken Wire
Once a broken electrical wire on a motorcycle is discovered, the two pieces of wire need to be spliced and reconnected. This can be done by using either crimp connectors, or soldering the two wires together. Crimp connectors are much easier to use, but soldering is much more vibration-proof.
Procedure To Solder A Broken Wire
You'll need a soldering gun, some solder, and a bit of patience. Most soldering pencils don’t create enough heat, so a soldering gun is best.
- Strip both ends of wire with a pair of wire strippers
- Cut a piece of heat-shrink tubing
- Slip it over one end of the broken wire, sliding it up and away from the repair area
- Solder the wires back together using resin-core solder
- After the repair has cooled to the touch, slip the shrink tubing back down over the repair
- Once the repair is covered evenly, heat the tubing and allow it to shrink over the soldered wire
I've had this same Weller soldering gun for 30 years. If you bought a brand new one today, it'll look just like this one. Another example of "if something works, why change it?"
Shop: Weller Soldering Gun Kit
The Weller D550PK 260-Watt/200W soldering gun kit includes three tips, solder, and storage case.
Although soldering and shrink-tubing wires is the most permanent way to repair electrical wires, I don't always solder wires every time there's a repair needed. Time is often a factor, and when a repair needs to be done quickly, it just takes too long to solder. It's not that the actual soldering process is long, but the setting up process takes awhile.
If you choose crimp connectors, take off the plastic sleeve it comes with and throw it away. One or two inches of shrink tubing will look and work better.
After crimping the terminal, check tightness by pulling on it with your fingers.
Heat the shrink tubing with a lighter. The wire and terminal connection is now sealed.
Best Electrical Wire for Motorcycles
Most classic motorcycles use 16-gauge or 18-gauge copper wire insulated with plastic. For starter and generator wires, a heavier 14g or 12g is needed. Heavier 12-gauge wire is required when you need something that can handle a bit more load.
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%.
Once everything is correct and working as it should, use plastic cable ties to secure wiring and harnesses.