Basic Motorcycle Wiring
Electrical issues 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. To check an old electrical system for shorts, start with a fully-charged battery on hand. You'll be using it to test the electrical system and wiring.
Most automotive primary wire has a copper core, which has a low electrical resistance. Use multi-strand wire only - solid-strand wire is not recommended for motorcycle use. Primary wire is sold in 15-foot rolls and available from most auto parts stores. Colors available include black, red, blue, green, white, yellow, brown, purple, orange, and pink.
Automotive-type wire is measured in AWG (American Wire Gauge) and works backwards in sizing. Example: 20 gauge wire is smaller than 18 gauge, 10 gauge is bigger than 8 gauge, etc.
For a 20 or 30 amp circuits, such as to and from the battery and starter, use a 12 or 14 gauge wire. The key switch and regulator requires a 14 or 16 gauge wire. For most others, 16 and 18 gauge is good. 18 to 20 gauge wire works well with lights and switches.
A test-light is invaluable for finding electrical shorts. Be sure to use an automotive test-light which works with 12 volt systems. They sell for under $10. The leads of the test light are reversible. One end goes to a positive and the other end goes to a ground.
Always start by testing the light on the bike's battery. Depending on what you testing for, you may have to turn the ignition switch on. Electric 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.
If you were looking at someone else's bike, what would impress you more, wire ends that are soldered are plastic butt connectors?
Repairing A Broken Wire
Once a broken electrical wire on a motorcycle is discovered, the two pieces of wire should be soldered and shrink-tubed. Butt-connectors are easier to use, but soldering is far more vibration-proof. plus the solder itself is a good conductor of electricity.
A quality repair requires a soldering iron, some solder, and a bit of patience. Only use solders labeled "rosin-core" or are marked for electrical work.
A standard electric soldering iron or gun with a 100-watt heat rating is all that's needed. Heat until the solder melts (about 400° F) and flows through the open wire strands.
Procedure To Solder A Broken Wire
- 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
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.
Most old motorcycles use either 18g or 20g copper wire insulated with plastic. This is standard automotive-type wire commonly found in auto part stores. If the wire colors cannot be duplicated from original, make a note for future reference.
To make a wiring harness, or completely rewire a bike, you will need:
- Wire cutters/strippers
- Soldering gun and soldering wire
- Multi-meter or Test-Light
I find that drawing a wiring diagram out in a notebook (no matter how simple) helps visualize what needs to go where. Draw out 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.
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.