To identify a fault in a vehicle's electrical system (a degraded cable, a oxidized connector, etc.), you can measure the voltage drop along the positive or negative line.

In an ideal situation (perfect cables, connectors, switches, etc.), the voltage drop along the line should be near zero. In the example diagram, when the load is activated (the light turns on), the voltage difference between the positive pole of the load (the light bulb) and any point on the positive line should be near zero, since the components should offer near zero resistance. If the multimeter indicates a voltage difference, it is proportional to the resistance offered by the part of the circuit between the multimeter leads.

An empirical reference value is that you may have a degraded cable or degraded component if you measure a difference of 0.3 volts or more.

WARNING: If you use a load greater than the one expected from the electrical system, you may measure a voltage drop greater than optimal. This is because the voltage difference is proportional to the circuit's resistance multiplied by the current flowing (amperes).

• Battery
• Starter relay
• 30A Fuse
• Main harness red cable (1.82 mm²)
• Bullet connector (inside headlight)
• Ignition switch
• 3P white connector (inside headlight)
• Fuses plate
• 9P mini white connector (inside headlight)
• 9P mini white connector (below tank, right)
• Turn signal/horn switch
• 9P mini white connector (below tank, right)
• Horn

The Ohm's law states the following:

current I (ampere) = voltage V (volt) / resistance R (ohm)

The voltage of the vehicle (eg. a modern motorcycle) is normally 12 volts, so knowing the internal resistance of, for example, a light bulb should be enough to calculate the amperes that will flow through the circuit to make it work. This value is crucial to use a fuse with the proper rating on the circuit and to use the proper wire gauge. However the devices of an electrical systems (light bulbs, motors, etc.) are generally rated in watt (W) instead of ohm (Ω), so you have to use the electric power formula instead of the Ohm's law:

power P (watt) = voltage V (volt) · current I (ampere)

For example an H4 halogen eadlight bulb with two tungsten filaments (low beam and high beam) is rated 55/60 W. Using the electric power formula you can calculate the ampere (A) from power (watt):

current I (ampere) = power P (watt) / voltage V (volt)

so the H4 bulb will absorb 4.8/5.0 A. The fuse for the headlight on this motorcycle wiring is rated 10 A, which make sense because this single fuse supports the low beam or the high beam, but also the two together when the flash overtake lever is triggered.

It is possible to use a multimeter to measure the ampere absorbed by an H4 bulb. Just connect the multimeter leads in series to the circuit: I measured 4.5 A for the low beam (e.g. 54 W). This means that the internal resistance of the low beam filament should be 2.7 Ω. But when I used the same multimeter to measure the filament resistance, I got a value of 0.4 Ω! What this means?

The fact is that the resistance of the filament varies greatly depending on the temperature of the filament itself. When it heats up and becomes incandescent, the resistance increases to the expected value.

It's not possible to measure the resistance of an electromechanical horn with a simple multimeter, because the horn is not a simple resistor. A typical motorcycle horn contains an electromagnetic coil, a moving armature, and a vibrating contact that opens and closes hundreds of times per second when the horn is powered.

With the multimeter in ohmmeter mode, with the horn disconnected from the system, you can only measure the coil's DC resistance.

A multimeter can measure the current drawn by the horn when it is activated. A digital multimeter generally takes numerous samples per second and calculates the average over time at a frequency of a few Hz, while the horn circuit opens and closes at a frequency of a few hundred Hz.