Modern Vespa

Hi all.

The headlight on my PX isn't working (either dipped or full beam) but the green and blue lights on my headset are on. The blue full beam light on headset is on when switches are in dipped mode.

Brake lights are fine and indicators are fine.

My first thought is short circuit or bad earth somewhere?? But thought I would check in on the knowledge here before I dig out the circuit diagram and multimeter.

What's the thoughts?

Edit: I should add that first thing I did was swap the bulb 😂

So making sure I understand the issue...
- Known Good headlight doesn't work at all
- Hi Beam indicator light works whether the switch is set to Low or High beams

Given the indicator lights are working, the power to the switch and the switch itself are working. Past that, it's a bit of a red herring.

Which leaves literally only three wires: The ground wire and the lo- and hi- connections from the switch to the bulb.

If they both failed at once, I agree with you it's most likely the ground.

However, given that the high beam indicator is always on, that makes me think that the wiring between the switch and the bulbs is also suspect, either rubbed wires or a broken wire.

Start with the ground and if that fixes it, decide if you care enough to fix the indicators.

Also, scooterhelp has awesome wiring diagrams, e.g. https://www.scooterhelp.com/electrics/wiring/VSX1T.px200e.nobatt.pdf

I've got the multimeter out.
Checked the black earth wire to the frame and it's a connection, so earth seems okay.
Checked the voltages on the grey wire going into the switch and at tickover I'm getting 9v ac and 1.5v dc. This is coming straight from the rectifier so what voltages should the rectifier be putting out?

You should be getting 13.2V on both AC & DC.

Did you confirm that you're getting those same craptastic voltages at the headlight?

If so, it's time for a new reg/rec.

Craptastic 😂👍

May well be an issue at the rectifier, I've a spare on the shelf I can try if necessary.

Update ..

Despite the multimeter telling me that the black earth wire was making connection with the frame (1.0ohms), when I ran a temp wire between headlamp and a clean screw head .. lights!

So it means that there is an issue on the black wire somewhere between the headlamp and the earthing point. It's telling me it's earthed, but obviously not a good earth when current running through it.

Too late in the day for me to start taking off horn covers, so just soldered a wire to it and earthed it at the nearest bolt.

All working okay, and I'll trace the damaged cable/loose connection the next free day I have 👍

If it's working, I wouldn't worry about doing anything further unless you're looking for projects to kill time. Grounding the headlight to the headset is normal practice on all the sixties bikes, as well as Lambrettas.