Modern Vespa

I've just finished a rebuild on a V91A and put in a new wiring harness. No yellow wire at engine junction but have one at switch and tail light as shown by wiring diagram. I could probably figure this out if I knew how the circuits work. Seems to me that switches BREAK the ground circuit.... How does that work? Should I be seeing 6v coming out of the engine? Guys that sold the harness said bridge yellow to blue (they come from the same coil in engine) Blue is for brake light and the switch disconnects blue from ground.... I'm confused[/img]

When the brake switch is open (brake pedal not pressed) it breaks a circuit and the power goes to ground. The brake switch works when the switch closes (brake pedal is pressed) it makes a circuit and power is sent to the brake light. Horn switch works the same way. Yes or somewhat in that range. Can't see the image you're trying to post. A picture of your stator will help. The stator has wires for a spark, wires for lights/horn and a ground wire. The original stator had 3 power wires but only used two power coils. If you have less then 3 power wires you "prob" can bridge a stator's power wire to another component wire. Really need to see what type of stator you're using.
Its sometimes better just to get a 6v regulator and run all power in one regulator slot and power out to the needed wires at the junction box.

My brake light switch breaks a connection from blue wire to black wire when depressed. (Opens circuit as far as I can tell) I can send photo of that later. I am going to check continuity in yellow wires and see if I can figure out what they might be connected to inside the harness also because it lights the taillight despite the fact that no yellow wire comes out of harness at engine junction


Old stator wires -working but deteriorated

Stator with new wires I soldered in

New wiring harness and new wires from stator. No yellow, but have a yellow at switch and tail light as indicated on diagram. Light blue wire replaces green.

The finished bike (almost)

V9A1 Wiring.pdf
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Filename:	V9A1 Wiring.pdf
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Old Vespa AC electric are crazy. You turn the lights OFF by shorting the circuit! The AC will find the path of least resistance which would be through the short, bypassing the brake light so it stays off. When you break that circuit, AC flows through the brake light bulb and lights up!

Red is ignition coil and kill switch
Green is headlight and horn
yellow is tail lamp, pilot bulb in headlight (if you have it)
Blue is for brake
black is ground

Not sure if your new harness bridged the blue and yellow circuits internally somewhere. I have seen some harnesses do that but usually tie in a regulator also.
Trace your yellow and you will find out.
Here is an explanation on the old Vespa AC stuff. Aimed at Large frames but the principal applies. Go to the bottom, talking about diagram 11. Then look at the diagram. It shows the floating-ground blue and yellow wire coil that powers the brake and tail lights.

https://www.vespa-klub.dk/EL_english.HTM

BTW I believe your wiring is closer to the early Primavera with an external coil. Very similar to the V9A diagram, very slight differences. I think your bike is external coil looking at the sheathed red wire coming out of your junction box.

Hope this helps.

I’m with Poi in assuming maybe that harness is meant to have a regulator in there somewhere. Do you have a branch off on the left side where you see blue and a yellow?

Light blue was never a V90 wire color. Where did you buy your harness? Did you make it? Are you handy with a multi meter?

I got the harness from "Piston Ported". There was no yellow at engine junction. Yellow ran directly from tail light to handlebar switch. I cut my old harness apart. It has yellow from switch to engine junction box yellow (coil) to taillight. I have spliced in a wire to the yellow to run to the engine junction yellow. That should make the new harness same as the old one(everything worked). I think bridging the yellow and blue as suggested by Piston Ported burned up the coil,(no juice now). New one coming.... The light blue wire replaces the green. Thanks for the input and the link, I'lllet you know what happens when the new coil gets here....

There was no regulator on the "balanced" AC systems. It made them cheap to manufacture, but once you lost one bulb, it was a failure cascade as a result.

^+1

I've seen some aftermarket harnesses that 'incorporated' a regulator and modified the original wiring layout, without implicitly saying that you needed a regulator, when you actually did. Just something to be aware of.