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Thread: Rotor Test

  1. #1
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    Rotor Test

    One hour from the house, after 4 days of touring upon coming to a stop the bike (81 r100/t) died.
    There was no "Gen" light so I switch it with the turn signal indicator bulb (working) & still,
    the turn signal indicator worked & the "Gen" light does not.
    So the bulb is good.
    The battery showed 10.2
    I jumped the bike and got it running & the alternator was putting out 10.4

    Tested for continuity & the rotor is not "open".
    I tested for resistance


    & got this


    It didn't display any resistance.
    What am I doing wrong?
    Electrical is not my strong point.
    Thanks
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

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  2. #2
    Monza Blue 1974 R90/6
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    I think the meter is showing you have infinite resistance; or, am I mis-reading the meter?

    Barron

  3. #3
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    The "OL" reading shows when the meter is on "omhs" with the leads touching nothing.
    When contacting the slip rings with the leads "nothing" changes on the meter.

    Testing resistance on any other metal the meter displays a reading.

    This rotor is only a few months old & maybe 3k miles.
    ?
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

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  4. #4
    Benchwrenching PGlaves's Avatar
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    With the meter set as shown in the photo it is reading "open". Infinite resistance.

    The three tests I do are as follows; with a business card, match book or somesuch slipped in between each brush and the slip rings to isolate the rotor from the wiring susyem.

    front slip ring to the steel fingers or the tapered steel bore: should read as shown in the photo

    rear slip ring to the steel fingers or the tapered steel bore: should read as shown in the photo

    These two tests show that the rotor windings are not shorted to ground. Any specific number of ohms would indicate a short to ground.

    The third and final test is to read resistance from the front slip ring to the rear slip ring. Here you should see a single digit and tenths or hundredths of an ohm on the order of 3 to 5 ohms. If you see a significanatly higher reading or that OL the rotor is reading open.

    If it passes all three tests then warm the rotor with a heat gun or hair dryer. Repeat the tests. The way the windings are arranged occasionally a rotor will reak OK cold, but will flunk the slip ring to slip ring continuity tests because a broken winding separates when hot but closes when cold.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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  5. #5
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    Quote Originally Posted by PGlaves View Post
    With the meter set as shown in the photo it is reading "open". Infinite resistance.

    front slip ring to the steel fingers or the tapered steel bore: should read as shown in the photo

    rear slip ring to the steel fingers or the tapered steel bore: should read as shown in the photo
    What are you referring to when you say "steel fingers or tapered steel bore"?
    Thanks
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

    No Rules Photography at http://brittrunyon.com/
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  6. #6
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by brittrunyon View Post
    What are you referring to when you say "steel fingers or tapered steel bore"?
    Thanks
    Sorry, those are parts of the rotor that you can't see unless the rotor is out of the bike. The "fingers are the rotor core the windings are wound on. The tapered bore is where the rotor slides on the crankshaft. This one is still in the bike.

    So, instead, to test for a short to ground just test from each slip ring to the center of the rotor bolt holding the rotor on.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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  7. #7
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    Quote Originally Posted by PGlaves View Post
    Sorry, those are parts of the rotor that you can't see unless the rotor is out of the bike. The "fingers are the rotor core the windings are wound on. The tapered bore is where the rotor slides on the crankshaft. This one is still in the bike.

    So, instead, to test for a short to ground just test from each slip ring to the center of the rotor bolt holding the rotor on.
    Thanks Paul
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

    No Rules Photography at http://brittrunyon.com/
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  8. #8
    Benchwrenching PGlaves's Avatar
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    Assuming I understand the two photos correctly here is what I see.

    You have the rotor isolated from the brushes with a card.
    You have the meter set to measure resistance in ohms.
    You are testing from the front slip ring to the back slip ring.
    The meter is reading OL which is the same as if you are waving the probes in the air.

    This tells me that there is no continuity between the two slip rings. This means that there is an "open" in the wiring wound on the rotor. No current can flow so no magnetism can build up.

    If I understand the two photos correctly, if you take the insulating card out and then turn the key on the alternator light on the dash won't light. But if you disconnect the black wire from the brush (not the brown wire) and touch it to a ground then with the key on the dash alternator light will light.

    This tells you that +12v is reaching the hot brush but is not getting through the rotor.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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  9. #9
    advrider.com
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    If I didn't know any better, I'd say the rotor is shot and there is a potential problem with the GEN light circuit.

  10. #10
    Benchwrenching PGlaves's Avatar
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    I agree the rotor is reading open. And if so, the generator light circuit is behaving normally. That light will not illuminate if the rotor is open. The circuit is battery to kill switch to dash bulb to regulator to hot brush through rotor to ground brush to ground. If the rotor is open no current can flow and the light cannot illuminate.

    That light provides a two way test. If it stays on with the engine running there is a problem almost always other than the rotor. If it fails to come on with key and kill switch on engine not running that also signals a problem. Usually brushes or the rotor but it could be a broken wire or totally shot regulator. Or of course a burned out bulb. And if the bulb is burned out no current can flow to and through the rotor and that is a problem.

    My first test if the light won't light engine off is to disconnect the wire (black) at the hot brush and ground it, key on engine off. If the bulb then lights I know the system is OK that far. That leaves the brushes and the rotor in the remainder of the circuit to ground.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  11. #11
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    Thanks for all the help!

    Disconnecting the Black wire (DF) & grounding it, did result in the "GEN" light coming on when the ignition was on.
    Here's what I found upon pulling the stator


    The new rotor (installed in May) has been rubbing the stator.


    I called EME, where the rotor was purchased, and was told that perhaps the rotor was "out of balance".
    I had to purchase a new one but will be sending them this bad one for perhaps some $ back.

    I will be testing the stator today to see if it's screwed.
    Thanks again.
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

    No Rules Photography at http://brittrunyon.com/
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  12. #12
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    I do not know EME - have no connection to them

    BUT

    Looking at your picture the rub on the stator was only one one side

    That would indicate to me that the problem was caused by you not getting the stator assembly
    squarely seated and centered when you reinstalled it after replacing the rotar

    Maybe the rotar was not "bad one"

  13. #13
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    Quote Originally Posted by 44006 View Post

    Looking at your picture the rub on the stator was only one one side

    That would indicate to me that the problem was caused by you not getting the stator assembly
    squarely seated and centered when you reinstalled it after replacing the rotar
    I thought that as well.
    Wouldn't be the first time that I screwed-up.
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

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  14. #14
    Administrator 20774's Avatar
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    I wondered about that, too. I've put rotors back on two of my bikes, the /2 and the single. I wondered about how to get it centered. In the end, I just slipped it on as best as I could and let the rotor bolt do the rest. Not sure what else I could have done.
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  15. #15
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    You can be fairly certain a rebuild rotar or replacement will not hit the stator if
    you put rotar on the shaft and leave mounting bolt loose

    Then mount stator and press in on the slip rings and turn back and forth with fingers
    while rotar is slipping on but firmly pressed onto the tapered shaft - it will be obvious
    if there is a rub other than the seal friction

    If no rub then tighten the mounting bolt for the rotar

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