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.
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.
start with basics, which is common to all models.
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.
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.
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.
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.
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.
no. 1 VB1, VGL1, VL1-3
no. 2 VSB1, VS5
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.
no. 3 VBA1, VBB1, VGLA1, VGLB
no. 5 VNB1-2
no. 6 VNA1
no. 8 VBA1, VBB1, VGLA1, VGLB1
no. 9 VNB2-5
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.