MotoMojo: Basic Electrical Troubleshooting

Jun 18, 2014 View Comments by

MotoMojo: Basic Electrical Trouble ShootingMany motorcycle owners are intimidated by electrical systems and fear working on them. Electrical issues can be mastered with some simple step-by-step diagnostics. Let’s start with a few terms. Voltage is electrical potential measured in volts (V for short). Amperes (shortened to amps or A) is a measure of electrical current (represented by the letter I). Watts (W) is a measure of electrical power. When voltage and current are present, power is consumed and is the product of voltage and current (P (watts) = V (volts) x A (amps). The ohm (denoted by the Greek letter Ω) is a unit of resistance to electrical flow. The higher the number, the more resistance (designated by the symbol R).

Motorcycle electrical problems typically occur when something doesn’t work either because it’s not getting voltage (or enough voltage or adequate current), or because a part has failed. Your job is to find out whether the part is faulty or if it’s not getting the power it needs. Once you know that, you can either repair the faulty wiring or replace the component.

Electricity must be able to flow uninterrupted (per the original electrical system design) from the battery through the entire circuit and back to the battery. Circuit faults fall into two major categories; open (or partially open) circuits and short circuits. Examples of circuit problems are corroded connections, cut wires, or failed components such as the switch or a device that doesn’t work. If something doesn’t work, but the fuse isn’t blown, suspect an open or degraded circuit.

An example of a short circuit is a wire with frayed or cut insulation such that the bare wire touches a grounded location. Fuses and circuit breakers prevent electrical overloads by limiting excessive current flow, and a blown fuse or a tripped breaker usually indicates a short circuit (or possibly a failed component).

Wiring diagrams (found in most shop manuals) are helpful in tracing circuits and diagnosing problems. Follow the problem circuit from the battery to any switches and eventually the component, then to its ground connection. Note the wire color coding; it is essential for locating actual wires on the bike.

If the inoperative part (light, horn, fan, starter, etc.) is getting full voltage and has good connections and ground return, it may have failed. These cases can often be tested with a test light, jumper wires, and an inexpensive multimeter (volt-ohm-ammeter).

If a light is out, check the appropriate fuse, then the bulb. If the bulb filament appears good, check for voltage at the connector. If there is power at the positive terminal in the socket, then temporarily connect a jumper wire between the socket shell to the negative battery terminal. Reinstall the bulb and switch it on. If the light works now, the ground connection is faulty.

To use a test light, connect the alligator clip to a good, clean metal ground connection or the battery negative terminal. Then touch the tip of the test light to the point where you want to check for positive (+) voltage. If it lights, voltage is present, but for an accurate measurement you need a multimeter.

The battery stores power that is generated by the charging system. The most common charging problem is rectifier/voltage regulator failure, which can result in either over- or under-charging. When the engine’s running, a properly performing charging system should provide all of the motorcycle’s electrical power needs, plus some excess to charge the battery. Charging is controlled by a voltage regulator. Voltage at the battery terminals with the engine off and a fully charged battery should be about 12.6 volts. Soon after engine start, voltage should rise. With the gearbox in neutral and engine at fast idle, voltage should be in the 13.4 to 14.8-volt range. Above that indicates a faulty voltage regulator; values below that may be caused by a bad regulator, wiring, or alternator.

Fuel injection systems consume a lot of power to run the electric fuel pump, injectors, and engine management computer. If voltage is too high or too low it may cause poor running or complete shutdown. Shop manuals have instructions for testing electronic ignition, ignition coils, and spark plug wires. Owners can also check fuses and make sure the control units have sufficient voltage and good grounds.

The starter motor may not work properly if there’s a problem with the charging system, wiring, connections, solenoid, switches, or battery. For a quick check connect a DC voltmeter to the large terminal on the starter. When the starter is activated, there should be at least 9.6 volts present during cranking. Most starter problems are actually due to a weak or discharged battery or loose or corroded battery terminals. Check these first before replacing other parts.

For additional information refer to Motorcycle Electrical Systems by Tracy Martin; available from Motorbooks; (800) 458-0454;


Text and photography by Ken Freund

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