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Charging Sytem Question: Can a faulty rotor make a good stator test faulty?

Discussion in 'Mechanical & Technical' started by ejether, Aug 21, 2012.

  1. I'm working on a CB900c to sell and its not charging.
    I've followed some of the tests here and determined that, at least, the stator is bad (No voltage between the yellow wires) . I haven't had time to finish diagnostics yet and I plan on doing that. My question isn't related to the specific diagnostics but it makes me wonder.
    With my stator, no voltage is being created, but there is no short that I can find. And while I still plan on replacing the parts, or selling the bike (depending on cost) I'm curious:

    Can a bad rotor make a stator look bad in this way when its not?

    What do you gurus out there think?
  2. General statement to answer what I understand your question to be:

    The output of an alternator is directly related to the strength of the magnetic field. Whether the magnetism is supplied by a field winding or magnet it must be at max to see max output. Magnets can loose magnetism.

  3. That's what I was thinking.

    So if either the rotor or the stator were bad, there would be no voltage from the stator wires and the loss of voltage at the stator may indicate that the stator is bad, that the rotor is bad, or both.

    Thanks Andy
  4. Texasl

    Texasl Totally Charming Retired Moderator Staff Member

    Did it suddenly quit charging or was it a slow process? What is the resistance between the yellow leads? (bike off and stator disconnected)

    Whilst it is not impossible for a permanent magnet to lose magnetism, it is quite rare, and usually needs some sort of external source to make that happen.
  5. Well, I didn't necessarily want to get into diagnosing this particular bike, since you asked :D I don't know the answer which you seek.
    I unknowingly bought the non-running bike this way.

    0.4-0.5 ohm resistance between the yellow wires.
    No faults to ground.

    I *think* the rotor on this bike is not a permanent magnet. I've read that it is a common point of failure.
  6. There would have to be zero magnetism to get zero output. So if the device relies on permanent magnets, in practical terms, they would have to be missing or beat to death.

    Again this is an overview of all alternator devices and not just auto/moto specific.
  7. If there are no field windings it will be a permenant magnet vs semi permenant type. Might not be a very good permenant magnet but it's supposed to be. Its magnetic strength can be easily assessed using a non-magnetised ferrous object.
  8. Texasl

    Texasl Totally Charming Retired Moderator Staff Member

    That's fancy talk for get a big screwdriver near it and see if it grabs it. :secret:
  9. :mrgreen:
  10. If you were fancy you'd have screw drivers with magnetic tips :nana
  11. Texasl

    Texasl Totally Charming Retired Moderator Staff Member

    If I were fancy I wouldn't be nearly as charming and lovable. :roll:
  12. Ok, did a little Googleing. Your charging system is shared with the CB750 and CB1100 of the same era, and is NOT a permanent magnet alternator. The magnetic field is from a rotor with windings (not permanent magnets) that is fed voltage from the regulator thru brushes. So if the regulator isn't feeding voltage, the brushes are bad, or the rotor windings are bad, the end result is that you have no magnetic field moving past your stator windings to give you output. I'd suggest that you might want to search on a CB750 forum for troubleshooting info, or find a copy of the service manual - too many things involved to just try replacing one at a time until it works!
  13. Texasl

    Texasl Totally Charming Retired Moderator Staff Member

    Brushes and slip rings, huh? Check those brushes.
  14. rotor windings can be crap too.
    Measure resistance
    Specs are in the manual
  15. Yep, all those things are true, I haven't had time to work on it and I'm seriously debating whether I should fix it at all considering the engine smokes, its got 44K and fixing it is a $$ losing proposition. The right way to do it is to replace the rotor , stator and RR which would make it ~$300 more. So, unless anyone has a line on some super cheap parts... Sad :angry7:
  16. Last edited: Aug 23, 2012
  17. Sadly I could not open the above link. What year are we looking at because I might have overlooked another twist on externally excited machines. In this case you may actually have a stationary field coil that induces a magnetic field in the rotor (ie no brushes) windings. The "stator" still sits around the rotor. The rotor windings, stator windings or field coil maybe kaaappuuuut.

    Rather than diagnosis by replacement you could try unplugging the field coil and applying battery voltage to it to see if you get an output. Borrowing a dc current clamp meter will be a massive asset in finding out what element is at fault in charging systems. Just to get her running you won't need to replace everything.
    Last edited: Aug 23, 2012
  18. Not sure but I might have a bottom end.
    Don't remember if it's still here or not
    I'll check tonite or this weekend
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