Making a Motorcycle Wiring Harness

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The wiring on classic motorcycles is relatively simple. Making a new wiring harness, or rewiring a classic motorcycle, effectively starts with one wire. The mechanic should start to place the various wires on the bike, attaching labels to identify the location. For example, a wire from the battery to the ignition switch may be a good starting point. However, it is important to remember, at this point, that the battery must not be connected. For example, the battery ground lead should be left disconnected for safety reasons.

It is good practice to attach (permanently) a terminal at the start point of each wire--this will locate the wire in its final position and be the start point for establishing the total length. When the wire has been routed to the final location, and the length set, it can be cut to its final length and the other terminal fitting.

Affixing Terminals

The time-honored view that soldering is the best attaching method for terminals has been replaced in most industries by proprietary crimp-on terminals. Ultimately it is the choice of the owner—there are positives and negatives for each system of attachment. However, regardless of the attachment method, the mechanic should use heat shrink on every terminal (regardless of its polarity—positive or negative) to both insulate it and to give additional support at the point where the wire enters the terminal. Note: the interface between wire and terminal is the most common breaking point.

The entire rewiring job will simply be a multiple of the single wire manufacturing process.

Stress Points

Due to vibration, wires should be supported wherever possible. As previously stated, this is particularly important where a wire enters a terminal. Although heat-shrink at the joint will help considerably, adding a small pigtail just before the terminal then zip tying it, will take most of the load/weight off of the joint.

Electrical Noise

For the most part, electrical noise on motorcycles affects computer controlled systems only. Therefore, wires and control units associated with electronic ignition systems should be isolated from other electrical components. Of particular importance in shielding electronic ignition wires and units, is the need to protect them from components emitting strong magnetic fields or other wires carrying high currents.

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 is readily available from an auto or electrical supply store.

Routing

Manufacturers take a lot of care when routing cables and electrical wires on their motorcycles. They will carefully route a wire away from any heat source, for example. Of particular concern, when routing a single wire, or the complete harness, is the position at the headstock. Needless to say, it is imperative that wires not become trapped as the forks are turned from side to side. The mechanic must also ensure the wires do not get trapped when the forks are compressed.

Finally, wire routing must also allow for the bike’s movement through the air; wind speeds of more than 100 mph are easily attainable with performance machines and any loose wires could be blown back toward heat sources etc.

Zip Ties

In conjunction with routing, the mechanic must secure the individual wires and the harness wherever possible with zip ties. However, the ties must be of good quality or they can do more harm than good by cutting into the wires. Quality (stainless barb type) zip ties are expensive but very rarely break and will not cut into the plastic insulation of electrical wires as easily as their less expensive counterparts.

Tip: To save wasting the expensive ties, the motorcycle wiring can be initially located with inexpensive ties until the location is final, the mechanic can then replace all the cheap ties.

Up-Dates

If the mechanic is replacing the wiring entirely, he or she should consider updating the electrical system to include: