Modern Vespa

[UPDATE 10/24 @ 2100]
Hello everyone,
So the battery voltage on my piaggio mp3 250 is 12.4 without any load. Then i start the engine and it falls immediately to 12.1 then continues to steadily drop. Revving the engine has no effect it just keeps dropping.
So i unplug the connector between stator and regulator and measure the unloaded voltage of all 3 phases while revving the engine. They all measure 0.12v @ idle and ramp up to 0.95v @ full throttle.
So i pull the stator (it shows no sign of damage or overheating) and measure resistance across all 3 polarities = .4 ohms.
So i check for shorts between output leads and the laminate core. There aren't any.
i can't measure the flux of the magnets in the rotor, but i did an informal test with a steel knife. They all grip the blade well and evenly.
Anyone ever seen anything like this?

The voltage should be a lot higher than that with open circuit output around 30 or more volts. You have the stator out so measure the individual coils if possible. Never had mine apart so don't know if its possible.

Mean't to add that voltage is measured between phases

My meter's not sensitive enough to do that. But it shouldn't matter if the phases are all .4 ohm. It doesn't explain why the phase voltages are between 3 and 5 volts.

where are you taking the initial measurement at?

Resistance between the yellow wires should be 0.2 - 1 Ω. Then check individual insulation between each yellow to ground…

Just one shorted turn in the stator could lead to hardly any voltage output.

i unplug the harness that goes from the stator to the regulator and stick the probes into the connector and run the engine. Three combinations, all show the same voltage/rpm curve. 0.12 @ idle and 0.95 @ full throttle. Then i shut off the engine and measure .4 ohms across the phases, which means .2 ohms per coil/phase.
i pulled the stator and got a steel knife and the blade stuck well and uniformly to the magnets in the rotor.

well i measured resistance between the 3 output leads, all 3 combinations were .4 ohms. then checked for shorts between each lead and the laminate core. There weren't any.

well these are all 3 showing the same voltage/rpm curve. And the stator is only 6 months old. Never had no trouble til now.

When measuring the voltage between the phase wires, did you have your multi-meter on the 'AC' setting ?

Beat me to asking this question! Now we wait….

Yeah I made that mistake already, but this time i double triple checked and made sure it was the right reading.
Then I reconnected the regulator and started her up. The voltmeter on the dash reads 12.2 and falling steadily. I use a trickle charger over night.

Since the magnets and coils are fine it is almost impossible not to get a voltage out. Your multimeter might have a blown fuse or is broken. What I would do is get 2 12V bulbs, put them in series and connect between 2 of the 3 stator wires. I recommend at least 2 bulbs as the stator might generate more than 30V (my old Harley makes like 36 V). Try for all 3 combinations of stator wires.

That's a damn good idea:-)
except i just reconnected everything, fired er up n now the voltmeter on the dash is reading 14.4 which is exactly what it's supposed to read.
So the problem just vanished, except it's done that before, and an intermittent failure is still a failure.
So I'll wire up the lite bulbs with alligator clips n just wait till it fails again, which i know won't be long.

Intermittent ground on the regulator?

And it sure wasn't long and now i have another clue because twice now i seen it. i start er up for the 1st time in the morning n the voltage is right what i it should be. I go to the grocery store about 500 yards away n park. Come outa the store n start er up n no alternator.

That's possible but the primary thing at this point is the alternator. What i know is that both stator n regulator are working when it's cold, working all the way to the grocery store, but that's only 2 minutes. When i exit the store n start er up the stator (n maybe the regulator too) is dead.
The meter on the dash reads 12.2

I would take the dash meter with a grain of salt and verify the voltages at the battery AND the voltage regulator.

also are you testing with a known, good condition, topped up battery? if not, you may be getting out of spec readings that are skewing the tests.

Well i been putting it on trickle charge at night and it's always topped up by morning. I know it has good capacity. And have checked several times to verify the multimeter jibes with the dash gauge.
But yeah tomorrow I'll measure at the regulator too.

Or so i THOUGHT i would
But no, there's nothing to measure today, because now the dash meter is reading a consistent correct voltage after riding around to half a dozen different places shutting off n on many times.
So where does that leave me? I've seen some wierd shit, but that's how it is with a lot of intermitent electrical problems.

Thanx guys for your input i know some things i didn't know before

Sooner or later it will pop up again expect to see you back here.

yup, hopefully later than sooner. i'll just bump the conversation and continue where i left off.