Wildguzzi.com
General Category => General Discussion => Topic started by: robertllr on May 27, 2025, 11:31:05 AM
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2007 California Vintage. I have heard a bike sitting for a while can lose the magnetism from the alternator rotor's permanent magnet. Is there a rough "pull" test of some kind to determine whether an alternator rotor has sufficient magnetism?
I know it's possible to simply test the alternator's output--but the bike has to be running and mine is down at the moment with a fuel leak I have to solve first.
And while I'm on here, I can't find anything on the web about how to bench test a single-phase regulator/rectifier--though there are plenty of explanations of how to test a 3-phase one.
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I've heard of magnets weakening at high temperature, rather than sitting. Try reading this for single phase info https://wildguzzi.com/forum/index.php?topic=86123.0
Also search for saprisa regulator on this forum.
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If the rotor has been stored outside of the iron frame, it will lose strength. The frame 'keeps' it strong.
If you overheat it or bang on it, it may lose strength.
If you are just curious, I would wait until you get it spinning and use an ammeter.
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with an Ohmeter, measure each of the yellow wires to each other, the readings should all be the same, just an Ohm or two at most.
Measure from yellow to chassis, it should be Megohm or more.
The magnets shouldn't lose their strength unless they are stored without the stator, the stator will act as a magnet keeper.
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Thanks for the help so far. I fixed my fuel leak and did the engine-running stator output test. (The static continuity and ground reading was fine.)
Manual says 15 volts AC at idle (1,000rpm). I'm getting 25!
And with the revs up to 5k, I get over 100v instead of the specified 80!
Is this possible? Can an alternator output so much more than the specs? Are the specs perhaps just bare minimum numbers? Is my brand new multimeter not working? Is a 20 year old alternator just now starting to feel its oats?
You wisdom--please!
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I wouldn't worry about the AC Volts, once you get the regulator in circuit it should cut id down to <14 DC across the battery
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Like Roy said, not to worry.
The permanent magnet alternators are pure current sources. If the voltage appears low, you may have an issue. If it is high, that is fine. When the regulator and battery are connected that will 'hold' the voltage down where needed.
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Thanks. In further research, I have found that the readings I got are not beyond the pale for a single-phase alternator. I suspect that the specs listed in the manual are simply minima. However, I have also learned that for a proper diagnosis of the charging system it IS important to check the unregulated stator output, so you know you can discount a bad alternator if the regulated DC voltage to the battery is out of range.
So now I can safety assume my alternator is fine and I most likely have a faulty regulator. Evidently there is no way with normal shop tools to check the functionality of a single phase rectifier/regulator--unless someone here knows different?
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Thanks. In further research, I have found that the readings I got are not beyond the pale for a single-phase alternator. I suspect that the specs listed in the manual are simply minima. However, I have also learned that for a proper diagnosis of the charging system it IS important to check the unregulated stator output, so you know you can discount a bad alternator if the regulated DC voltage to the battery is out of range.
So now I can safety assume my alternator is fine and I most likely have a faulty regulator. Evidently there is no way with normal shop tools to check the functionality of a single phase rectifier/regulator--unless someone here knows different?
I have checked them often with an oscilloscope. Most people don't have a o'scope laying around, so it gets complicated. But can be done.