Vespa 125 PK lights off when warm

Discussion in '2 smokers' started by Piggdekk, Jul 15, 2017.

  1. Piggdekk

    Piggdekk love speed, hate rush

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    Hello guys,
    a friend of mine came today with his vespa pk 125 who is showing a very strange behaviour. When it warms up all eletrical circuits (lights, horn, blinkers...) die, but the engine keeps working fine. I took it for a ride and it happened after 5 minutes or so, no lights but engine still running. This model does not have a battery, and I checked and the generator has independent coils for ignition and lights. I measured tension at the regulator and when electricity works I can measure 9-11 volts AC, but when electricity dies I measure 0.
    I checked the drawing of the wiring and I'm inclined to believe that the issue is indeed the regulator: when it warms up it shorts and stops producing electricity, but what do You think? Shall we pull the trigger and buy a new generator or is there something else we can check? The regulator isn't cheap considering the value of the vespa.
    thanks
    luca
    #1
  2. snarlyjohn

    snarlyjohn Human money repellant

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    Run the engine til it's hot and the lights die, then quickly disconnect the generator from the regulator and connect a 12v test light to the generator wires. If the test light doesn't light up then you'll know that the lighting coil is faulty; if it does light up but there's no output from the regulator (when the lighting coil is connected) then that would indicate that the regulator is faulty.
    #2
  3. Piggdekk

    Piggdekk love speed, hate rush

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    Thanks Snarlyjohn,
    I did try to measure the output yesterday with a multimeter and it looked like there was no output already from the generator. What I don't get is why Piaggio felt the need of wiring what I believe is the output of the ligthing coil (blu out of 11 in the picture) first to the horn switch and then back to the regulator?! Maybe the horn doesn't need regulated tension? So when I measure at tension at the regulator I'm already measuring after the connection in the switch. I guess I need to check that too, but the fact that it dies when hot makes me more think that is the generator.
    luca vespa pk wiring.JPG
    #3
  4. Piggdekk

    Piggdekk love speed, hate rush

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    Looking again at the wiring I'm thinking that the horn probably works on lower tension as it seems that is not wired to ground but it only sees the difference between regulated and not regulated tension. This wiring is really confusing to me!
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  5. snarlyjohn

    snarlyjohn Human money repellant

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    Some horns were designed to run on AC (they could be made cheaper that way); it appears that when the horn button is pressed the horn is placed in series between coil and regulator, making it buzz. When the button is released the switch bridges out the horn so that there's a direct connection between coil and regulator. Yeah, it's weird but that's Italian electrics for you. But I agree - the most likely culprit would be the coil. If possible test it with a light rather than a multimeter. A lightbulb (especially one that's 10 or 20w or so) will put enough load on the coil to show up a high resistance fault in the system whereas a multimeter might show that everything is fine.
    #5
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  6. Piggdekk

    Piggdekk love speed, hate rush

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    Thanks Snarlyjohn!
    I'm italian like the vespa but this electric system is too weird for me too!
    I shall test with a test light, didn't think of putting an higher resitence, makes totally sense.
    luca
    #6
  7. Piggdekk

    Piggdekk love speed, hate rush

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    the other part that worries me a bit is that there is no fuse in the system. Wouldn't a short immediately overload the generator? I'm worried that if I put a new generator in there and there is a short somewhere I may fry it quickly and it's 100 euros for a new one.
    #7
  8. Piggdekk

    Piggdekk love speed, hate rush

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    Did some more trouble shooting today. I didn't have a test light hence I kept testing with the multimeter. On the generator output there is always tension,also when the lights die, what stops working is the regulator. I don't think this would change if I was using a testing light, so the regulator seems to be the culprit right now, tomorrow I'll look for one and test as well with a testing light, the regulator should be easy to swap and cheap. However, do you think it's possible that the regulator has some built in logic that switches it off if there is too much current draw? I'm still wondering if the problem is not a short somewhere. Tomorrow I might test the resistence of the lighting circuit from the regulator output to make sure isn't drawing too much. I believe the regulator is 40W, so if I'm not wrong the resistence should be at greater than 3-4 ohms? (40w=12v*I, max current I=3.3 Amps. P=I^2*R, R=P/I^2=3.6 ohm)
    I'm clearly overcomplicating, after all is just a vespa!
    luca
    #8
  9. snarlyjohn

    snarlyjohn Human money repellant

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    I'm not sure I'd have much faith in any resistance readings of the regulator, but if you've got power going into the regulator and none coming out it's a pretty sure sign that it's faulty.

    As far as I know they don't have any current limiting features - they simply cut the peaks off the voltage curves as required. The excess is diverted to ground and dissipated as heat.
    #9