Modern Vespa

Hi,
I have a problem with charging and I'm not sure if it's a rectifier problem. So I posted here asking for help.

I tested alternator, they all measure <0.1 Ohm between the pins, and no short to ground. So not a problem with alternator.

I run the engine without rectifier connected to alternator. The alternator read 15.3V AC between pins.

The problem starts when I connect the alternator to rectifier. The voltage from alternator drops to 3.3VAC. the voltage output from rectifier is also 3.3 VDC.

What confused me is when I pulled out the rectifier. Forward bias all show 0.467V as it should be, and reverse is Open Loop (also good). All the test I followed were from here: Is it possible that the tests were insufficient to tell whether a rectifier is working correctly?

Thanks

The MP3 does not use a alternator, but a stator.

As long as the Ohms between pins 1+2, 2+3 & 1+3 are nearly identical then the problem is not with the stator. However, there could be a problem with the regulator.

The real acid test is what you measure in Volts DC with the engine idling at full operating temp. You should see around 13-13.5V DC and go up to around 14.5-14.7V DC max under load.

The stator is the winding part of every alternator
The rectifier/regulator only outputs 3v, not 12-15v probably bad

Sabaert, thank you for correcting me. Technicality aside, stator looks good for me. Problem is only when rectifier is connected. The voltage drop from both stator (3.5VAC) and from rectifier/regulator output (3.3VDC).

And again this confuses me, because when I pulled out the rectifier, I tested all (as instructed in the video) and found no problem with rectifier. So not stator, not rectifier?

Garth, that's also my initial thought. But I did the diode-test and rectifier seems ok! 😆

Don't forget that if you are measuring voltages with a high impedance meter (which means most digital meters, especially cheap ones) then you can read high voltages in circuits that also have relatively high impedance, so as soon as a low impedance load is applied the voltage will drop drastically.

I'm sure you're aware of this, but I've seen others on here caught out by that several times, having missed high resistance ground connections etc, etc.

Thanks Jimc for your note. It is good to be reminded these little details.

But I am confused how this fact applies to what I measured hear.

When I measure voltage, I isolated the stator (measured 15VAC).
Then after connecting rec+reg to stator (both disconnected from the circuit), voltage of stator dropped to ~3VAC, and output from rec/reg also dropped to 3VDC.

Do you mean that my measurement on stator could be measuring the ghost voltage which is much higher than this should be? If so, I did a resistance test and they were all 0.2Ohm, within specs 0.2-0.8Ohm from manuals. So now I'm sure the stator is at least not in he problem.

Still, the question is the rectifier seems fine, as I tested with diode functions. Maybe it's the problem of some regulator circuit inside? Hell, I'm not even sure what's inside this rec/reg unit.

so you think you have a charging issue?

what's the voltage reading at idle across the battery?

how old is the battery?

did you load test the battery?

No maybe, it almost a certainty given your voltage readings (and good explanation ).

Just replace it.

Change the rec/reg. Works great! Thanks Jimc

One more question, is there any hidden fuse from the positive (+) rec/reg to battery? I read a few post about hidden fuse but it seems for starter relay.

The line from rec/reg to battery is open, no connect.


Edit: found the hidden fuse. And yes, it's the culprit for non-charging issue even with new regulator.

This mystery is now solved. Thanks Jimc and Oldasdirt and sabeart and others.