'79 vespa grande electrical

My headlight works but my tail light (bulb is blown) and all blinkers (bulbs are good) do not work.

I did some testing with a test light. when I clip the ground onto the negative battery terminal and touch the positive battery terminal i get a weak current. When I clip the ground onto the nearest grounded nut (the ground between numbers 18 and 19 on the picture) i get a strong current.

two questions:

Would the burnt brake light bulb cause the blinkers not to work?

what would cause the battery to act so strange?


Re: '79 vespa grande electrical

also the speedometer and odometer don't work (I don't think that's relevant) BUT if you look at the schematic you will see 2 wires inside the headlight assembly that are violet (D) they are disconnected and shrink wrapped together.

Re: '79 vespa grande electrical

# 20 fuse?

the violet, they bypassed the ignition switch #2 kill function

speedo / odo is purely mechanical.

instead of a test light, try to get a voltage off the battery +/- with the ped running.

with the ped running, ground the test light to between 18/19 then touch the light to the grey wire on 19, test light should illuminate / flash

then yellow /black, also brown off #11 with the turn signal switched to turn

test light should flash

i'm betting bad connectors or blown fuse 20, also poor grounds at any/all of the turn bulbs.

Re: '79 vespa grande electrical

I checked #20 fuze. Its good. I checked all the grounds i could locate. All seemed clean of rust or corrosion. I 'fixed' that. But there is another thing i noticed. When i clip the test wire ground to ANY ground other than the #18 ground the test light wont light up. Is it possible the frame isnt grounding properly?

Re: '79 vespa grande electrical

flasher unit could be out, but your violet wires are off your ignition. They are the main power to your coil. if you ground them it should kill your spark. If it runs with the tailight burnt out, someone has most likely been into the wiring before.

if you don't want to hard wire it, what i'd do is re-map that wiring diagram only on the circuit your needing (the turn signal circuit). it's easier to see things when there isn't so much going on. go through it with your test light and check every connection in order. if you have no current going through you'll know that part or wire is defective.

Re: '79 vespa grande electrical

If i choose to hardwire it will i need anything more than wires and connectors? Will there be resistors and capacitors to worry about? If its literally just put this wire here and that wire there i am ready to do it.

Re: '79 vespa grande electrical

Nope. Mine is hardwired and it only has 4 wires: ignition, ground, lights, and spark plug. No resistors or fuses.

Re: '79 vespa grande electrical

Re: '79 vespa grande electrical

I think i need to find my volt meter. I wonder if it has 6v output for testing. What are the lines in the center of the magneto? A capacitor? Is that preinstalled or would it need to be replaced? Also if there is no switch between the magneto and the coil how do you turn it off?

According to my 'current' schematic if i rip out all the wires EXCePT the ones between the magneto and the coil/plug will it still run?

Would it be as simple as keeping the current mag-coil-plugs wire

Then rip out the rest: discard the blinkers attach a wire from the mag-headlamp-tail light-ground. Would that provide me with a running bike, constant headlight and running tail light?

Then maybe i could run a wire from the mag or headlight - a brake switch - a brake light - a ground.


Re: '79 vespa grande electrical

I don't know anything about the stock wiring, but the only capacitor on my bike is the condenser in the ignition circuit.

Re: '79 vespa grande electrical

Here is a thought. Since i cant find a 6v bulb anywhere and i can get 12v bulbs forfree i was thinking about trying to convert. But im told i need a 12v magneto to do that.

If you look on the schematic it shows 3 coils. If i keep 1 for the spark and hook the other two up in series would it effectively create a 12v current? Or would it need to be parrallel? Or would that only double the amps/watts?

Re: '79 vespa grande electrical

If you have points ignition you have two coils inside the flywheel: one for ignition, one for lights. I think the CDI has 3 coils, but one is a trigger.

You could use both coils to charge a 12 volt battery through a voltage regulator/rectifier, then run both the ignition and lights off of the same battery, but that would probably be more trouble than it's worth.

Re: '79 vespa grande electrical

Ok so lets try to explain how i tried to simple it up.

On the schematic. There is a wire coming from the magneto (mag) that connects to 2 purple wires. I reconnected them but kept them the same. I trimmed them. One purple wire goes to the coil/plug set up. The other goes to the suplimental engine stop switch. The other end of the switch grounds out.

There is a blue wire from the mag that i attached to a blue wire. That wire goes to the tail light and grounds BUT it continues onto a SINGle green wire that also grounds out. There is also a RED wire coming from the mag that i attached to the yellow wire for the headlight which then grounds out.

There are two whitish wires with no charge that comes from the mag that i grounded out.

It WAS running. So i put all the screws back in. It no longer idles . Seems to have insufficiant spark.

What should i change?

Re: '79 vespa grande electrical

It doesnt run. It tries real hard.

Re: '79 vespa grande electrical

Make sure your grounds are good, and don't assume that any metal part of the engine/frame is grounded. I've run ground wires from all my circuits to a single point. Also make sure there's no frayed wires or broken insulation that could be grounding out.

Re: '79 vespa grande electrical

i would replace your condensor. with a bad condensor you'll have intermittent spark. it's cheap, simple and could cause a ton of problems if it is bad. it's only like $7 at any orielys or auto zone. just get a condensor for points ignition and splice it in. mine is grounded on the top of the cylinder where your plastic shroud screws on. i think i'm running one for an el camino on one of my bravo's and one for a Nova on my other.

your internal firing coil has one wire that goes to the points and another that grounds. attach your condensor to your points. then run one more wire from your points to your ignition coil. three wires coming from your points. you can stop it by slowly rubbing your foot on the variator, or run a kill switch that breaks the connection between your points and ign. coil.

Re: '79 vespa grande electrical

I think by monday i should have a chance. I think i will just put all new wires in. All that electrical shit i took out (horn, blinkers, etc) plus my pedals and chain weighed 8.8 poundslol.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account