Electrical aspect of Vespa

With this article I will try to explain the function of the electrical circuits and possible failures in classic Vespa's

Experience from several years tell me that many owners of classical Vespa's have problems with the electrical circuits necessary in every vehicle. A weak head- or stop light, or a tail light that almost diminishes during braking are some of the most common problems. Even experienced people may fail to find the cause due to constantly changed designs between models.

Someone once said, that the design of the Vespa is God worthy, but unfortunately he left the design of the wiring to someone else! In any case, it seems that the Piaggio engineers never became satisfied and therefore constantly tried a different approach. Or did they replace the engineers for every model? A lack of systematics seems to be the rule. But for those of you who restore or repair your own scooter, I will try to help you understand the mysterious ways of electrical currents.
I will focus on the most common classical models from 1955 to 1979. P-models are more complicated due to the electronic voltage regulator.

Lets start with basics, which is common to all models.
In the year 1820 the danish scientist ěrsted found that a current through a coiling wire creates a magnetic field. Fortunately the reverse is also valid, because this is how the generator in our Vespa functions. But the magnetic force needs to be varying and the generated voltage will vary accordingly. On the stator plate are mounted three or four coils wound on cores made from thin iron foils (to prevent magnetic losses). Around these coils 6 permanent magnets rotate thereby creating alternating currents in the coil windings.

One coil supply the spark plug with a high voltage pulse for each crank revolution, controlled by the contact breaker and the crank position. On all models (except GS150) this coil is connected in parallel with the braker point and the external high tension transformer. The braker point shorts the coil for about half the revolution, but opens at the same time the magnetic field is on its maximum. This creates a short voltage spike of a few hundred volts which again is transformed to thousands of volts in the high tension transformer. On the oldest models the high tension transformer is integrated inside the stator.

The other two coils supply your scooter with lights but they may be connected in several mysterious ways. A few models have a separate coil for the brake light.

So, the alternator generates an alternating voltage with a frequency (and amplitude) depending on the engine revolutions. Hence, the lights will be more or less weak at idle speed. On models without a battery all bulbs are fed with alternating voltage - usually 6V. Most Vespa models were designed with a battery as well as versions without battery. This is due to some markets demanding that parking lights could be lit when the scooter was left on a dark road. I cannot really imagine this to happend in practice - the battery would not last very long anyway.

On battery equipped models there are allways a mixture of AC and DC voltages. Several ways to connect these circuits are seen, but usually the headlight runs on AC directly from the alternator and the battery voltage is used for parking light, brake light and indicators, if fitted. Charging current to the battery comes from rectified alternator voltage.
So much about the generator in general terms. Details for each Vespa models may be seen in the wiring diagrams - more about that later.
As mentioned the battery models are equipped with a rectifier for charging. As we are talking about vehicles from the 1950's it is most likely a selenium rectifier. These have the habit of becomming poor over time. Internal resistance increases causing the charging current to fail and internal leakage current will eventually
discharge the battery through the stator coil. If this is a problem it should be replaced by a silicon diode, or in GS150, with a bridge rectifier.

Common to all models is that the magnets inside the flywheel must be sufficiently strong, otherwise the generator will not be able to deliver the needed power to the headlight (which is the heavyist load). In fact, poor magnets is the most common cause to a weak headlight, missing charging ability or poor ignition.
Permanent magnets loose power if they not for some time are magnetically shorted. If a flywheel is lying around for years without shorted poles it will eventually loose its magnetisme. To avoid this leave the stator (or pieces of iron) inside the wheel.

But how do one know if the magnets are OK? A rule of thumb says that each magnet should be able to hold a heavy wrench or similar weight. Any precise methode to check the magnet power is not known, but if in doubt (poor headlight maybe?) they should be remagnetized. This is done by applying a very strong magnetic pulse from a magnet charging piece of equipment. Unfortunately not many can do this. An alternative is to replace the flywheel with a new one, it really helps!


Troubleshooting the coils

In general it is difficult to trace a coil failure. The following feasable failures may be found:

The coil can be disrupted, i.e. the copper wire is broken somewhere. This can be tested using a multimeter or a homemade wire tester consisting of a battery and a lamp. DC resistance should be very low - a few ohms.

The coil may be shorted to chassis. Most coils allready have one wire connected to chassis. To check such a failure you must first disconnect all wires to the coil, then there should be no lakage to chassis.

The coil may be shorted internally. This situation is almost impossible to check without special measuring apparatus so try with another known coil.

Across the points is a capacitor (or condenser). If this fails it may lead to a weak or unstable spark in the plug. If you have ignition problems and decide to remove the flywheel anyway it is recommended to replace this capacitor. For those who own a multimeter capable of measuring capacitance I can say that the capacity is around 0.3ÁF. However, it may be OK when measuring and then fail when a high voltage is applied!

A word about bulbs. Most classic Vespa's use a headlamp bulb with a BA20d base. It has two filaments, one in front of the other and also has a screen to produce the city light. They used to be hard to find, particularly in 6V, but they are now produced in China. They also make halogen-versions but watch out! The ones I have tested have the two filaments placed side by side which means it will not focus the way it should. A pity because halogen bulbs are more resistant to vibrations.

Below is a short description of the individual wiring diagrams. I have divided them into 7 groups, 4 with battery and three without.


With Battery:

Diagram no. 1    VB1, VGL1, VL1-3
These are the widebody models from the late 1950's. Of the three coils in the stator one is always used to generate high tension for the spark plug. The other two are connected in parallel in order to double its current capacity. As on all other battery-models here is a mixture of direct current (DC) and alternating current (AC). In one switch position the battery drives the parking, tail and speedo light. In the other position all bulbs (except parking) are connected directly to the two stator coils and hence they are AC sourced. In this situation the battery is not used at all but is always held charged from the same coils through another coil and a rectifier. This last coil is supposed to keep the charging current constant irrespective of the motor revolutions. AC is always used for the horn.

Diagram no. 2    VSB1, VS5
These are the sport models. VSB1 GS160 rarely has a battery, so this diagram is first and all typical for VS5 GS150 (I disregard VS1-4 as they are rare). This model is the only one that ALLWAYS is equipped with a battery, and it differ from all other models in that ignition is battery-supplied! It is a mystery why Piaggio suddenly (and only on this model) chose this form for ignition and then discarded the idea in all subsequent models. It works like in cars: The external ignition coil is under influence by a constant direct current as long as the points are closed - which is about half a crank revolution. When points open the magnet field suddenly stops, thereby greating a high voltage pulse to the spark plug. Remember this, dear GS owners, that when ignition key is in ON position and before motor is started, there is a 50% probability that the coil draws 4 amps from the battery !!
Two stator coils are parallelled to yield twice the AC current for the head light.

The ignition switch is complicated but works like this: In center position everything is disconnected. If key is turned left parking light and tail light is ON. In this position it is possible to remove the key. In first position to the right (clockwise) the ignition is ON and motor can be started. Next key position couples everything except headlight to the battery, but in the extreme right position head and tail lights run on AC directly from the stator coils while ignition, brake light and horn runs on the battery. The tail light runs on AC or DC depending on the key position. The AC from the last coil is rectified and used for battery charging.
GS150 is the only model that require a battery for starting.

Diagram no. 3    VBA1, VBB1, VGLA1, VGLB
Diagram no. 4    VBB1, VLA1
These circuits has minor differences especially in the minus-wire, but the overall function is the same.
The ignition is supplied from a separate coil and is not dependant on the light switch. That is, the motor can always be started, and it is stopped by a short push on the kill switch. The two stator coils are wired separately to the switch but are nevertheless connected together in the switch outer positions. If switch is set to "pilot light" all bulbs are supplied from the battery, but in "head light" position bulbs are fed with AC directly from coil 1 and 2. Only the brake light is allways fed from battery. The models are randomly supplied with either a AC or a DC horn.
The battery is charged through a single rectifier which on diagram 4 is coupled to coil 2 and on diagram 3 on coil 1 via an extra winding on coil 1. I guess this extra coil winding has a stabilising effect on the charging current.

Diagram no. 5   VNB1-2
VNB has an integrated high tension coil inside the stator plate. Again coil 1 and 2 are directly paralleled in the junction box, and again the lamps are switched between DC (without headlight) and AC (with headlight). The pilot lamp has a somewhat special connection between a tap on coil 2 and the switch. The battery is charged through a diode from an extra coil winding on coil 1 and is here used for brake light only.


Without battery

Diagram no. 6   VNA1
Diagram no. 7   VNB1
VNA/B has internal high tension coil.
Being without battery all voltages are AC, so here an extra coil (3) has been provided for - just to drive the brake light - if excists. Otherwise the design is more or less standard - all lamps are driven by coil 1 and 2 in parallel, and the horn is of course an AC horn.

Diagram no. 8    VBA1, VBB1, VGLA1, VGLB1
Traditional ignition with external high tension coi and kill switch on the handlebar. The stator coils 1 and 2 are connected together when needed and feed head- and rear light. Except in position "pilot light" where the voltage comes from an extra winding on coil 1. A third coil (3) feeds brake light only.

Diagram no. 9      VNB2-5
Diagram no. 10    VBB2, VLA1, VSB1, VSC1
Diagram no. 11    VNB6, VNC1, VNL2-3, VBC1, VLB1, VSB1, VSD1
All these models have a very different way in which to distribute the electrical power. Coil no. 2 drives horn and headlight in series. One has to realize that the coil supply is a fairly constant alternating current rather than a constant voltage. The generator has a certain internal resistance which makes the voltage dependant of the load (the bulbs). If these are of the correct power, the load (and therefore the voltage) will be constant. But in this case the load is variable because head lamp and horn are coupled in series. Note, that to shut the head lamp off it gets shorted! If neither light nor horn is needed the stator coil is directly shorted, and the induced current is running around in the wires. To use the horn the switch now interrupt the short it normally makes. It has a slight disadvantage in that the headlight will be lower when using the horn. But the generator will partly compensate by supplying a larger voltage.

Coil 1 is even more curious by not beeing related to chassis, it is "floating". On one side of the coil the current can go through the brake light - if the switch has "interrupted". On its other side are the bulbs for rear light, pilot light and speedo light in parallel. If all lights are out both ends of the coil will be shorted to chassis.

It is important to note, that this type of connection demands that horn and all bulbs are of the correct wattage (load) in relation to eachother. Just one wrong bulb may ruin the balance, which in worst case may cause all the other bulbs to blow! Note that also the brake light switch is "reversed" - it interrupts when using the brake.

The diagrams
Click on the links above to download the diagrams in PDF-format (your PC must be able to show this format).