Modern Vespa

Hi everyone,
I'm Giacomo, new to the forum and currently restoring a Vespa GT 200. I've run into a strange electrical issue and could use some guidance from the experts here.

When everything is connected, the headlight stays on even with the key off, and pressing the glovebox seat-actuator button makes the starter motor crank. The 15 A main fuse is good, and I've checked most connectors. I suspect a short or cross-feed between the orange (ignition feed) and brown (starter relay) lines, possibly near the fuse/relay block or actuator harness.

If anyone has seen this behavior before or can point me to common pinch points or grounding issues in the front harness, I'd really appreciate it.

Thanks,

Here's the circuit diagram along with the workshop manual etc: https://manuals.wotmeworry.org.uk/Vespa/GT125%20GT200%20Euro2/

I suspect you've got a bad ground somewhere - but together with the circuit diagram and a multimeter you should find it fairly easily!

If this GT200 is new to you, check the ignition switch is the correct one. The Vespa ignition switches all look the same, but some have different internal connections.

Thanks, I've made some progress. First, I realized I made a mistake—the actuator is working fine, and pressing the actuator switch works as expected. So the problem is not with the actuator. Here's what I've confirmed is working: brakes and brake switches, light switch, turn signals and switch, all relays (they're new), 15A fuse, all other fuses, 12V line to all fuses, kill switch, starter switch, battery (new), spark plug (new), oil (new), fuel in the tank. I also cleaned and tested all dashboard connectors, and they're fine.
What I've found:

No code light when the key is off or on—no code light at all.
With the key on, the fuel light stays on even though there's fuel in the tank.
When I turn the key on and press start while holding the brake, 12V goes to the starter motor, and it attempts to crank, but there's no spark or ignition.
With the CDI disconnected, I have continuity between the purple cable and black, so I suspect the purple cable is grounded somewhere.
With the CDI connected, I have correct voltages at the red/black and white/green cables, so the CDI is receiving power.
However, when connected, if I measure continuity from the black cable to ground (battery negative or chassis), I get 125–130 Ω resistance instead of full continuity.

Questions:

Do you think the CDI is faulty?
Could there be a short to the purple cable?
Is the purple cable supposed to go to the HT coil correct, and not be grounded ?

This is the black wire onto pin 8 of the CDI? Yes, that should be solidly grounded - 0V between that and the battery -ve.

I think you've found the problem... The HT coil's primary (that the purple wire goes to) is a very low resistance - so it'll read as if grounded unless you're using a very low resistance range. The spec is 0.4 to 0.5 ohms.

See page 76 of the workshop manual...

I've checked everything, including the specs, HT coil, stator rectifier, and CDI receive voltages as per the manual. There's no code light on, off, or on. The only thing I need to understand is this: at the kill switch, I always have continuity between the orange and blue cables, whether the switch is in the on or off position. The switch itself works fine. Is that normal? Also single blue or orange cable testing continuity to the ground both let multimeter sound .

Did you replace any electrical component??

Do NOT rely on the multimeter sounder giving useable results - use voltage and resistance readings instead, unless you are very sure of the design of the circuitry under test. The sounder may go off with quite a sizeable resistance between the probes, and if voltage is present on either of them all bets are off.

Follow the circuit diagram to nail this!