Classic Motorcycle Wiring Tutorial

The electrical systems and associated wiring on classic motorcycles are relatively simple. Advances over the years have changed the basic configurations to utilize solid state systems in the ignition systems, for example, but in general, the wiring and systems have remained consistent.

As motorcycles get older, the electrical systems often need repairs, or at times, complete replacement. Although electrical systems are generally reliable, age will have its effect on the wires themselves at places where there is consistent movement—the wiring harness as it passes from the frame to the headlight is being a typical example.

The wiring connections often develop oxidization over time which leads to poor connectivity and eventual failure. In addition, vibration can cause wires to break, especially where a wire feeds into a connector (this is due to a concentration of stress at that point). Replacing a single wire or connector may be sufficient to repair or fix a specific problem, but if this happens to a number of items it may be time to completely rewire the bike. Another obvious time to replace the entire wiring systems is during a restoration as access to the various components and wires is much easier.


To completely rewire a motorcycle, the owner or mechanic must have considerable previous experience, or at the very least, the ability to read a schematic wiring diagram. Alternatively, the mechanic could buy a replacement harness if they are available for a particular make/model.

To make a wiring harness, and completely rewire a bike, the owner will need some basic tools such as:

  • Wire cutters/strippers
  • Crimping tool (often part of the same hand tool as the cutter/stripper)
  • Solder
  • Soldering iron
  • Soldering flux
  • Multimeter


The majority of motorcycles use either 18 swg. (standard wire gauge) or 20 swg. copper wire insulated with plastic. These wire types are commonly available at auto stores.

The plastic insulation is available in multiple colors but the mechanic should try whenever possible to replicate the original colors and sizes. If the wire colors must be changed from those listed on a schematic, the mechanic should make a notation for future reference (print off a copy of the schematic and write any changes on it).

Electrical Connectors

All wires will have a connector at each end, except for the type of connection where a bare wire is pushed into a receptacle (this is rare). If the bike is being rewired, it is not essential to utilize the original style or type of connector except where the connector fits onto a specialized plug or switch. Therefore, for most rewiring jobs, generic connectors will be acceptable. The generic connectors typically have an insulated section and are of the crimp on variety; however, many mechanics prefer to remove the insulation, solder the wire into the connector, then cover both the connector and wire for a short distance with heat shrink.

Harness Wrapping and Sheaving

With multiple wires traveling from one end of the motorcycle to the other, manufacturers would typically have wrapped the wires into a bundle and then taped them together with insulation tape (cloth or plastic).

This was done to give the wires an additional degree of insulation and also to protect them from wear and tear. Some manufacturers used plastic sheaving for the same purposes. However, modern alternatives are available such as a split plastic flexi tube which readily available from auto or electrical supply stores.


As previously mentioned, the ignition systems on motorcycles have been redesigned the most on motorcycles, going from a basic mechanically operated set of contact points to fully electronic capacitor discharge. However, the generating and rectification systems have also undergone considerable improvement over the years.

The older designs called for a Zener Diode to regulate the voltage produced by an alternator and a rectifier to convert alternating current to direct current (as stored and used from the battery).

More modern designs, as introduced for mass-produced motorcycles in the 70s and 80s by the Japanese, utilized voltage regulators that use a rotor with an internal field coil and an internal rectifier. The main advantage of this design is that when the regulator senses the battery is low, it allows maximum current to flow through the field coils maximizing the charge within a predefined range.

If the mechanic is replacing the wiring entirely, he or she should consider updating the electrical system to include: capacitor discharge ignition, solid state regulator rectifiers, high output alternators and converting to 12 volts from 6 volts where applicable.