From Moped Wiki
Jump to: navigation, search


The electrical system on a moped is the most mysterious and crucial set of components on a bike. They control the primarily the lights, horn, and ignition since mopeds tend to not have other electrical subsystems. But it is because of the bare bones nature that they exhibit some of the illogical problems. Loss of spark due to a brake light failure and that sort of tom-foolery. The majority of mopeds fall into the category of fixed-magnet, ac generator with simple points and condenser ignition and a balanced load for lighting with a common ground through the frame. Bikes with blinkers installed in the system have blinker relays and voltage regulators primarily and may or may not have voltage rectifiers installed and a battery lighting system. (Tomos early models only have regulators and relays for signals while Vespa Grandes have battery system for instance) Some newer and much less moped-y bikes have electric starts and all the works that come along with that. (i.e. new Tomos mopeds) Since they are the exception and not the rule they will be addressed in the Specific Model section at the bottom of the page.


All electricity on a moped comes from an electrical generator called a magneto which is powered by the engine itself. The magneto consists of a flywheel with permanent magnets inside mounted to the crankshaft. The crank rotates the magnets around coils which in turn produce the electricity. This is different from the alternator on a car which has electromagnets to produce the electromotive force (EMF) and a regulator that can change the strength of the fields to suite the load on the system to account for turning lights on and such. Inside of the flywheel (or outside if it's a Jawa) there will be 2 or more coils. One coil is dedicated to powering the ignition system and the rest are for the lights and horn. These coils may be called the generator, low-tension or exciter coils although all the terms are a bit confusing when the word coil usually refers to the large thing that goes to the spark plug. So care will be taken to use specific terms when referring to coils here. Generator coils are grounded to the bike through themselves and a wire comes off that goes to whatever it is they power. Some generator coils have more than one winding and will have two wires coming off of them but both go to something and are not ground. Puchs are a good example of that style of generator coil. What is important to observe is that generators produce AC current, not DC. And that ground on a moped is not positive or negative it's just ground. And a DC voltmeter will not work on a moped.

The generator coil that powers the ignition system will then travel to either a set of points and THEN to the high tension coil (series) or will run to the points and ALSO the high tension coil (parallel(called a "energy transfer ignition")). If you disconnect the wire going from the generator to the HT coil and ground it through a continuity tester when you roll the motor over it will show an open circuit when the points open if its in series and will not change if its a parallel (meaning its a pain in the ass to set point timing). Always in parallel with the points is a condenser, which is just a fancy word for capacitor. The condenser's role is to hold off the back emf that comes from the high tension coil to the points until the gap is wide enough that they don't arc. Arcing across the point faces wear down the material quickly either leaving poor ignition firing, changing the timing, or it simply welds the points closed keeping the bike from running at all. The points are an electrical switch that is operated by the crankshaft and sets the timing of the spark. A wire leads from the points to the HT coil and will also branch off here to the kill switch. When the switch is set to run the switch is an open circuit to ground (does not conduct) and when the switch is set to kill it's closed (current can flow). While the switch is in kill mode the coil is still physically connected to the points the same way they were but what happens is the resistance between the points and ground through the kill switch is much lower than the resistance between the points and HT coil. Meaning all the current is diverted to ground instead of through the HT coil. It is effectively a short circuit in the ignition system that you control (unlike the ones you don't control that leave you stranded somewhere). The spark itself is created by the high tension coil which converts low voltage to high voltage when the current through it is changed (say when the points open and the current drops from whatever it is to zero instantly). The high voltage travels down the big wire to the plug cap then into the spark plug, across the spark gap, and back to ground through the head. It does this every time the cylinder fires, which is every revolution on a 2-stroke and every other on a 4-stroke. Which means that if you are going 9,000 RPMs then its sparking 9,000 times a minute, or every 6.7 milliseconds. Which is a lot considering that it must do it at the same point in every revolution with enough strength to spark strong enough to ignite the fuel.

The generator coils that go to the lights and horn make up the rest of the electrical system. The wires go to the switches and then to the lights and then back to ground. Usually there is a headlight/taillight, a brake, and a horn. Sometimes there are turn signals and sometimes there are high and low beams for the headlight. The headlight is usually wired to the taillight so that both come on with the same switch. The brake light comes on with switches controlled by the brake levers or the coaster brakes if it is a Sachs original. The horn is also just run through with a button. The biggest thing with lighting is the way the switches work. A switch is either run in parallel with the light it controls or in series. This is a big distinction and is worth noting because many problems come from not understanding this. If it is wired is PARALLEL then the switch is OPEN (does not let electricity through) when the light is ON. If it is wired in SERIES then the switch is CLOSED (does let electricity through) when the light is ON. Headlights and taillights are run together and therefore running on the same circuit. The two are typically run in parallel with each other, which lends itself to some interesting effects. With a variable voltage and low current source like a moped magneto at low rpms, say at idle, lights are dim; at high RPMs they get brighter because the generator can supply more voltage. Having a battery tends to fix that BTW. But since there is only so much power to go around with such a meager generator the load must be balanced between the headlight and taillight so that both burn as bright as possible. If either blow then the current it was using goes to the other bulb and the light gets really bright as a result, and usually blows promptly afterward since its not used to dealing with all the wattage. So replacing bulbs quickly keeps you from having to replace two bulbs. It should be noted when replacing bulbs to be sure to get a bulb rated for the correct voltage. Bikes all run on either 6v or 12v and the bulb should be able to handle that. Using a 12v bulb on a 6v bike usually means a weaker light because the wattage on the bulb requires higher resistance for a higher voltage and the 6v will not give the same wattage for the bulb. Using a 6v bulb on a 12v bike usually causes them to blow. These are are rules of thumb though. It could just upset your current balance between the headlight and taillight and give you a weak headlight all of the sudden (which can also happen if you hook up the sides of a dual filament bulb backward). When buying, pay more attention to the wattage of a bulb because most will be for 12v since cars are all 12v and some quick math can get you the right resistance value for your bike. (resistance = voltage^2/wattage) So if your headlight and taillight are wired in parallel and you want a brighter headlight try a lower wattage taillight.

It should be noted that since most mopeds run an AC voltage system with no voltage level regulation that buying LED taillights does not work very well. LEDs allow current through in one direction and not the other usually, but they do have reverse bias breakdown when large voltages are applied to them in the wrong direction. Which means when the voltage gets high at high RPMs they breakdown every other time the voltage switches polarity. And since they aren't meant to be run like that they actually break. Some LEDs work better than others but in general they don't live very long. But you're more than welcome to give it a try.


Note: check all continuities with a ohmmeter to see if anything has a high resistance. A high resistance in a wire is a sign of either a poor connection or corrosion on the connections or along the wire and should be fixed.

No Spark

  • Check to see if kill switch is set to run, and if it is that is is not a defective switch
  • Check spark plug for severe fouling or object between gap
  • Check wiring to make sure nothing has come unhooked, wiggle wires around to make sure connections are not loose
  • Check the ground from the coil back to the motor
  • Check for continuity through components (make sure plug cap hasn't become corroded or broken and has the correct resistance value along with all other connections and wires)
  • Check to see if everything is wired up correctly when compared to manual diagrams
  • Check points to see if they open and are clean
  • Check to see if condenser has not failed (you should NEVER be able to detect continuity through a condenser, it can have a very high resistance but a low resistance is a sure sign of failure)
  • Check to see if the lights are working when you pedal (sounds dumb but it can tell you if the whole wiring is grounded out or if it is just the ignition, also some systems do not run if lights are blown)
  • Check HT coil to see if it has resistance values in the proper ranges.
  • Check to see if generator coil is outputting voltage ( this may take some clever rewiring around the points to work, and remember set meters to AC voltage)

If all of these things check out and there is still not spark, try switching out one component at a time until the faulty part is found, remember to put the original piece back if changing it does not fix the problem, and only change one thing at a time. Good things to start with are the HT coil and new plugs, if a generator coil is suspected you may be able to wire a different coil from under the magneto into the points and try it again, if it sparks then you know the coil for it is bad.

Weak Spark

  • Is the spark plug fouled? Is the gap correct?
  • Check connections for high resistance, clean ground connections
  • Are the points clean? Are they aligned and close properly? Is the point gap correct?
  • Is the condenser working properly? Low or no resistance across it? Is the casing cracked or broken on it?
  • Is the resistance across the HT coil within spec?
  • Is the generator coil putting out enough voltage?
  • If everything is working fine and seems to check out fine and it still is weak, or if it seemed ok for a bit and then it became weak again it is more than likely a failing coil. change out the HT coil first and second the generator coil for the ignition. they sometimes fail very slowly and it becomes a pain to find the source.

Light Failure

  • Is the bulb good?
  • Check the wiring going to the bulb for continuity, check the switch to make sure it works
  • Make sure the grounds are good and it is not grounding in the wrong spot
  • Check to see if generator is supplying voltage when running

Lights blink or do not stay on

If your lights blink or do not stay on:

  • Check for loose wiring, bad grounds, faulty switch
  • Check for constant voltage from generator

Lights emit strange blue light

If you sometimes see a strange blue light coming from your lights:

This is a weird one where the wire from the high tension side of the spark plug is grounding out (usually the kill switch wire) and the high voltage coming off the HT coil is going through the light, check for faulty wires involved with that, accidental grounds and the like

Brake light switch and brake light circuits

First thing to note is that most old bikes use A.C. or alternating current which does not have a typical positive or ground that you would find in a D.C. or direct current circuit. That being said on a typical A.C. electrical system on an old bike there is a "common ground" meaning one side of the coils on your stator either points or cdi are common to each other, the stator plate, the engine,frame, handlebars and any other metal part that is not isolated or insulated from the path of electricity. "common" or "ground" would be used to refer to this, the other side of the coils that usually have an insulated wire coming off them; and usually exiting the stator to power your accessories ( lights, brake, horn e.t.c ) or the ignition would be refered to as "hot" or "positive". there are a multitude of different types and functions of switches but for the sake of getting to the point I will explain the 2 most common types you will run into in this circuit. N/O and N/C which is normally open and normally closed, this refers to the continuity between the contacts when the switch is at the rest position. ( N/O electricity does not pass when at rest ). ( N/C electricity does pass when at rest) . this can lead to some confusion as at rest can be different when installed. The seller or manufacturer should state e.x. ( N/C at rest or N/C uninstalled ).

The brake switch and its circuit could be wired different for different makes and models of bikes. I will try my best to explain a couple of common types here, unfortunately i don't know the electrical circuits of all bikes so if its not here you must do your homework.

First off if you have 2 bulbs in your brake light assembly one is usually a taillight the other would be a brake light in this instance the brake light circuit could be triggered or turn on by "ground" at the switch, the "hot" to the brake light bulb would be paralleled off the taillight run through the brake light all the way to the switch and when the switch is closed the other end of the switch and/or the body of the switch would send the "ground" back through that "hot" line to the bulb, completing the circuit. In that circuit if the switch malfunctions or shorts to the body of the switch, the bulb would be on as long as the taillight is on also if that hot line is pinched or cut causing a short the brake light would be on as long as the taillight is on .

The brake light could also be triggered by the "hot", as the brake lights ground is paralleled off the taillights "ground" and the switch sends the "hot" to the bulb to complete the circuit. if the line to the switch is pinched,cut and/or shorted it will not cause the brake light to turn on but if you replace the switch with one that the body act as a third contact when the switch is closed the circuit will short to "ground" causing your light coil to short which will most likely kill the rest of your lights or any other accessory that is on that coils circuit.

Three wire sealed beam or three wire led lights same as below. On a side note led's dont typically work well or last very long in an A.C. circuit.

Single bulb dual filament taillights, one bulb 2 different amperage ratings or 2 different light intensities one is for taillight the other is for brake light. This circuit should only be able to work in one way as the two filaments share the same "ground" mechanically (can't be changed by wiring) in this circuit the switch sends a "hot" trigger to the bulb to illuminate the brake light, if the switch is replaced with one that the body acts as a third contact or there's continuity between the contacts and the switch body the "hot" will short to "ground before it has a chance to complete the brake light circuit also most likely shorting the light coil.

Why this happens or why it may not happen to you is a matter of the "common ground" of your bike going through the handlebars and in turn to the body of said switch. If you isolate your ground from your handlebars through powder-coating or thick paint or something else this may not affect you right away but be weary as time goes on if you did not purposely isolate the "ground" from this switch vibration, wear and or by removal and re-installation or a change of parts this may be a problem in the future.

In short be sure of your circuit and the workings of each component before changing or replacing anything. GET A MULTIMETER--Hawaii kai 21:11, 30 May 2011 (EDT)

Brake lights either don't come on or stay on

If your brake lights either don't come on or don't stay on:

  • Check for faulty bulb
  • Check for loose wires around taillight, and switches at handlebar
  • Check to see if switches operate correctly (change open or close circuit when switch is pushed)
  • Check to make sure voltage coming from generator

Brake light only works when one lever is pulled

If your brake light works when you pull one lever, but not when you pull the other lever:

  • Check the switch on that side for proper operation
  • f you are wiring up a new system or a new brake lever make sure that the switch is compatible with the way the brake light is wired up, don't use a switch that opens when you push it with one that closes when you push it, it will cause funny brake light condition (some switches are both and have three leads coming off or them and can be wired up either way, be careful when wiring them up)


  • Get a multimeter and learn how to use it
  • Electrical tape is awesome
  • Heat shrink is even better
  • When crimping connections make sure you use the good crimper and the right gauge connectors
  • Solder is a good electrical connection, but only a poor mechanical and makes the wire connection brittle, which can fail under stress and vibration unless supported in some way
  • A can of electrical connection cleaning spray is godlike with a corroded electrical system
  • Dielectric grease should become your new friend. It prevents water and dirt from entering into connections and thus corrosion, its non conductive so you can use it on connections that have multiple wires in them without shorting
  • Use rosin-core solder, never acid-core or solid (unless you use flux paste too), and if soldering to a flat surface prime surface with a wipe on flux paste, otherwise the solder will just bead up and not stick to the surface
  • When cleaning your bike high pressure seems like a good idea, but it gets water into everything electrical, like points if you're not careful, or can just straight knock wires off from the force
  • This has a lot of manuals with the wiring diagram and electrical component information, use it

Specific Model Details

People with good specific bike electrical system knowledge should fill out the models in more detail if they can


Individual Component In Depth Information