Basics of bike's electrical system

When it comes to the electrics of your machine, the words “watts”, “volts” and “ohms” can make you practically dizzy. To be able to successfully repair electrical defects at the vehicle, the mechanic must be familiar with and understand these words and also a number of additional important fundamental physical principles of electrical systems. Here we'll explain watts, volts and so on for our DIY mechanics, and also allay the widespread fear of working on the electrical system of the motorcycle.

The intensity of the current and effect of a conductor can be explained by comparing it to the flow of water in a pipe. The water pressure at the end of the pipe depends on its diameter and – the higher up the water source is and the greater the diameter of the pipe, the greater the water pressure will also be. 

With current flow, the cross-section of the connected electrical cable corresponds to the diameter of the pipe. The thicker and shorter the cable is, the more effectively it conducts current, and therefore the less "resistance" the current encounters. The longer and thinner the cable is, the higher its "electrical resistance" will be. This depends on the material, cross-section, length and temperature. The resistance is measured in ohms (Ω) and uses the formula symbol R. The cable is made of metal and does not conduct the current completely smoothly. This friction produces heat. If the cable is too thin or corroded, it can become considerably hot and even cause the insulation to burn (cable fire). To avoid this kind of damage, electrical fuses are used. They work in a similar manner to a predetermined breaking point on a mechanical component, by interrupting the circuit, e.g. in the event of a short-circuit. They therefore prevent damage to the battery and electrical components and also cable fires.

In addition to ensuring the cables are thick enough for the relevant application, contacts, such as terminals and connectors should be kept free of corrosion and soiling to keep the contact resistance as low as possible. Use of terminal blocks and branch connectors on the vehicle should be avoided whenever possible as they increase the resistance in the cable unnecessarily and do not establish a safe and vibration-proof connection. The quality of negative/earth cables should be as good as positive cables.

The voltage is measured in volts (V) and has the formula symbol U. The voltage is the potential difference between the negative and positive terminal: In our water analogy, this is the difference between a full and an empty tank on the same level connected by a pipe. The water flows between tanks until the fill level is the same in both tanks. This condition corresponds to a discharged battery. In order for the water to flow again, it must be transferred/pumped from one tank to the other; in the case of the battery this is done by a charger or power generator, referred to as the "alternator" of the vehicle. This generates voltage through the rotational movement of an electrical coil in a magnetic field.

Another important factor in the circuit is the amperage. This is measured in amps (A) and has the formula symbol I. The amperage states how many electrons flow through a conductor cross-section/resistor (power cable) over a specific time. In our water analogy, the amperage corresponds to the quantity of water that flows through the pipe for a specific time, which depends on the pipe diameter and gradient.

Faults in two wheeler electrics normally arise due to trapped defective cables with damaged insulation or poor, and possibly also wet, plug-in connections. These lead every now and again to short-circuits or leakage currents. A leakage current (the "little brother" of the short-circuit) occurs if there is an unwanted connection somewhere in an electrical circuit due to moisture or corrosion, e.g. at an open contact such as the ignition switch (when switched off). Although very weak, it drains the battery over time when the vehicle is parked. This happens much faster with short-circuits. These are noticeable because the fuses pop out. The best way to pinpoint the cause of these kind of problems is to use a circuit tester. This detects and measures DC currents. Leakage currents in the electrical circuit can be detected with the adapters for mini and standard blade fuses when the ignition is switched off. All you have to do is pull one of the fuses out of the fuse box and plug it into the appropriate adapter. Then insert the adapter in the position from which you removed the fuse, switch on the device, and take a reading.

To find sources of faults and perform electrical checks on electrical components, a multimeter is useful for measuring the battery voltage, resistance of a cable or component, current flow and much more. You can obtain motorcycle-specific multimeters from Louis with instructions for use with application examples. This makes troubleshooting much easier.

If the aim is only to determine whether a component is receiving power or the supply line has been interrupted, a simple timing light is useful. It should be a part of every tool set. The cable of the LED multifunction tester is connected to the earth (e.g. a bare metal engine screw) or to a negative cable from the vehicle wiring harness (e.g. on the negative terminal of the battery) and the test probe is held at the positive input of the component being tested (e.g. the input cable of the ignition). If the lamp remains off, power is not being supplied to the component in question and the supply line must be checked for interruptions. If the lamp lights up, the power supply is ok and something may be wrong with the component internally.

Other malfunctions occur due to corroded cables for example, whose internal resistance is so high that they no longer conduct well and become hot. Older ignitions and switches frequently do not conduct the current sufficiently which leads to failures. If the alternator, or only its cable connection, is defective, this can cripple the power supply to the battery. Defective voltage regulators can cause overcharging of the battery. Worn carbon brushes can cripple a starter motor. For more information, see Testing the electrics and electronics in our DIY tips.