Moped Electrical 101

The most common thing that most young or inexperienced moped mechanics struggle with is the machines electrical and ignition system.  Most systems are actually very simple and crude by todays technology standards. Often times people tend to over-complicate and over-think things when trying to diagnose their machine’s inability to run or function properly.  As a rule, 99.9% of  mopeds that don’t fire/get spark are not due to a faulty secondary HT coil (the one with the spark plug wire attached to it).  Also on that same note, electrical components usually break-down from either corrosion or heat or physical damage…in other words, usually, they don’t just break for no reason. The ignition parts that will most likely be the root of your inability to get a spark will either be in the contact points or the ignition switch/wiring.

Lighting system issues are often times caused by simple issues like a broke terminal, loose connections, blown bulbs, etc but the coils inside the magneto system that produces the electricity are much more prone to malfunction due to heat than in the ignition system coils. The lighting coils are not well vented and also produce more heat. Heat is the number one cause of coil break-down.

5 Basic terms you should know for sure

  • Volt (V) is the pressure of the current
  • Amp (I) is the amount of current
  • Ohm (R) is the resistance of that current
  • Watt (W) is a measure of power that is released every second (Watts=Volts x Amps)
  • Ohm’s Law is a physics formula that is used to calculate the interaction and effects of volts, amps and ohms. (Volts=Amps*Ohms  or  Amps=Volts/Ohms)

Using the water pipe analogy:

Often times these terms can best be visualized by thinking how water moves through a pipe or garden hose.

  • Volts=The pressure of water moving through a pipe.
  • Amps=The amount of water moving through a pipe
  • Ohm=The restriction of water flow due to pipe size or a restriction.
  • Electron=A droplet of water

Visualize the following:

You live in a house that has a garden out back. You water that garden with a tank of pressurized water that is up on a stand next to the garden. You use an air compressor to keep the tank pressurized. The tank has a 1/2” faucet valve and a 1/2″ diameter hose attached that is 25’ long with a 1/2″ triggered spray nozzle at the end.

Okay, you want to water the garden so you open the faucet all the way on the pressurized tank (volts) and water (electrons) starts flowing out the hose (wire) once you squeeze the spray nozzle trigger (switch). You can’t quite reach the arugula so you increase the tank pressure (volts) a bit and the stream gets stronger and the amount (amps) of water (electrons) coming out of the hose also increases and is soon flooding out the garden. You quickly turn the faucet (ohms/resistance) half way off and both the voltage (pressure) and amps (water amount) are reduced to a nice controllable flow. Makes a little more sense now, huh?

Okay, here is another one. The spinach on the other end of the garden looks awful because you keep trying to water it with high-pressure water but the pressure and amount of water combined (watts) is too much for the plant. All that force is destroying it. You need to water it with a bucket.  You reduced the tank pressure (volts) back to its normal pressure and open the valve (ohms) all the way.  You squeeze the spray nozzle trigger (switch) a little bit  and water starts flowing into the bucket you are filling.  The more you squeeze the trigger, the more water that squirts into the bucket. The water rate (amps) coming out of the hose is being regulated by the spray nozzle because the restriction (ohms) in the nozzle is adjustable. Your water nozzle is a variable resistor.

Now let’s talk about Watts while continuing to use the water and hose analogy. A watt is a unit of measure that is used to explain total electric power generated or in this case, total power that is generated from the flowing water. You get that measure from multiplying the volts x amps.

In the middle of the garden, you have a windmill thing that when it spins, makes the little character on top of it look like he is cutting wood or something. When you are out watering the carrots, you like to spray the windmill to make the figure move. The force of the water hitting the wheel determines the total power or watts. To make the wheel turn faster or harder, you can increase the water pressure (volts) or increase the amount of water (amps) hitting the wheel. If you spray that windmill with too much force and too much water, it will break and the little guy will no longer be able to cut wood. The amount of energy that the windmill can absorb would be it’s wattage rating just like a light bulb has.

Now move beyond the whole water and pipe analogy and back to regular basic electrical circuit definition and let’s get closer to understanding how your mopeds electrical system works.

Remember that :

  • Volt (V) is the pressure of the current
  • Amp (I) is the amount of current
  • Ohm (R) is the resistance of that current

Now that you understand the three basic components that determine how strong and how much electricity something generates, you will soon be able to use the following calculation wheel to help find answers to your electrical issues. As long as you know the voltage of your power supply, you can calculate the current through a bulb or some other device based on it’s wattage. If you know what your voltage is and how much amp draw something has, you can use the wheel to figure out what your capacity for additional bulbs are. These are just a couple of examples but once you master the theory, you will be able to diagnose or design or build many different electrical applications. Don’t worry if you still don’t quite comprehend everything, we got a long ways to go here but it will soon all make sense.


In most applications the voltage is constant at the supply. Your house outlets are always 110 volts (USA) and that doesn’t change. A car’s battery puts out a little over 12 volts and that stays constant. If you want to adjust the amount of energy that flows through a circuit, you normally do that with resistance or some kind of transformer component.

Your vintage moped is kind of that same way but not so efficient and it doesn’t have a real good constant voltage. The reason is that most old mopeds used a magneto system. A Magneto is simply a generator used to power the mopeds ignition and other electrical devices like lights and horn because your bike doesn’t use a battery (in most cases).


Basic Magneto Understanding

A magneto creates A.C. current which is how most moped electrical systems are designed. We will get into the difference between A.C. and D.C. current later but at this point, it’s not relevent.  The current is generated from the magneto by a magnetic field passing over a coil of special wire called magnet wire or resistor wire. The magnets are built into the flywheel and pass over the coils of wire by rotating over them. The rotation takes place because the flywheel is attached to the crankshaft. As the engine spins, so does the flywheel. As the magnets pass over the coils of wire and stop , developing electrons are pushed through the coil. The electrons moving through the wire is electricity. The faster the magneto spins, the greater pressure (Volts) of moving electrons.  This is why the faster your bike engine runs, the higher the voltage becomes.  The amount of current (Amps) flowing out is determined by how much pressure (Voltage) is being produced and how much resistance (ohms) there is in the coil of wire. The thicker the wire, the more current (Amps) capacity it has because the resistance (Ohms) is less. This is also true of the length of the wire. The longer the wire, the more resistance there is. Ohms determine and affect both volts and amps. The total amount of energy that the magneto coil circuit can produce is the Wattage Rating. If you are reading about your mopeds electrical system specifications or are trying to make sense of a wiring diagram, you will usually see something like the following: Bosch 2 coil 25w/25w magneto. This refers that the magneto has two coils and each one produces enough electricity for 2 seperate 25 watt circuits. Usually, one coil is dedicated for the ignition system and other (or up to 3) are dedicated to the mopeds lighting and horn circuits.

The main things that determine the Volts and Amps that are being produced are:

  • Wire length/number of windings
  • The number of times per second a magnetic field passes over a coil
  • The diameter of the wire
  • The length of the magnetic pulse generated.

Okay, lets fire up that moped and get the magneto spinning.