I got the 77 BMW 75/7 now I need help

Discussion in 'Old's Cool' started by BarryT, Jul 29, 2009.

  1. BarryT

    BarryT Been here awhile

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    Well I picked it up today and I already have a question. The previous owner was wrong about the bikes issue. I jumped it and it fired up after a few seconds of cranking. All electronics seem to work fine. The "kill switch" doesnt go into position firmly, it seems like it could rattle up or down and kill the bike. The right cylinder is ticking by far more then the left one. I assume I just need to set the clearances on both, but here is the real issue. The generator light is on and even when I rev my needle on my meter doesnt indicate a charge. The pre owner said he replaced the diode pack and I have the old one wrapped up to prove it. What should I be looking for now alternator, fuse, wiring issue somewhere, retest the diode pack, bad battery?


    Thanks,
    Barry
    #1
  2. crazydrummerdude

    crazydrummerdude Wacky Bongo Boy

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    Check the rotor.
    #2
  3. BarryT

    BarryT Been here awhile

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    alternator?
    #3
  4. crazydrummerdude

    crazydrummerdude Wacky Bongo Boy

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    The spinning part inside it. The stator is the stationary part.

    When I had a charge problem, my GEN light was on at half intensity all the time. It ended up being a cracked wire in the rotor. If yours ends up being bad, you'll need the special rotor removal tool (a specific bolt).
    #4
  5. BarryT

    BarryT Been here awhile

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    How do I test the rotor though? Just pull the front cover off? Will I be able to see damage. I think I am going to run it and do the 1st eletrical test in my book. It should tell me if its the diode board or alternator.


    -barry
    #5
  6. TimTowtdi

    TimTowtdi Been here awhile

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    Berry,

    Well, since the light is lit, that means the bulb is working and the rotor is not open. The voltage regulator can be tested by unplugging it and connecting the two terminals that do not have brown wires. Check at the batt for charging. If it charges, replace the regulator. If it does not, post back here.

    TT
    #6
  7. Wirespokes

    Wirespokes Beemerholics Anonymous

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    it's usually the rotor. That's the most common alternator failure.

    And, it seems, charging and ignition problems are the most prevelent of all problems on the board.

    The stator almost never fails, and the voltage regulator - rarely.

    Since the charge light is staying on, that indicates the slip ring brushes are good because that circuit is complete to ground. That would tell me there isn't an open in the rotor windings, though maybe it's shorted somewhere.

    With the heat, rotational acceleration and deceleration, those windings take a helluva beating. Used rotors show up on ebay frequently for $35 or so.

    To eliminate the voltage regulator, unplug it and jump two of the three terminals (not the brown one) which essentially bypasses it. If the system now charges, you know it's the regulator.
    #7
  8. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    And the best place, IMO, for Airhead electrics is Motorrad Elektrik http://www.motoelekt.com/ . Don't fret, the alternator rotor goes out.

    Motorrad also has a great reference book Airhead charging systems.
    #8
  9. BarryT

    BarryT Been here awhile

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    Well i jumped the wires and the light stayed on so the alternator is at fault? I can check the windings on the stator though right? How do I prove it is the rotor?


    _barry
    #9
  10. datchew

    datchew Don't buy from Brad

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    put your multimeter (ohms setting) on the 2 copper slip rings and see what the resistance is between them.

    You have to make sure the brushes (black square things riding on the rings) are not touching it. block them with a piece of paper or something if you don't want to remove the stator.

    if you have infinite resistance, your rotor is most likely bad.

    (rotor in first pic. new and old brushes in 2nd pic)
    [​IMG]


    [​IMG]
    #10
  11. TimTowtdi

    TimTowtdi Been here awhile

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    You need to jump them and check charging voltage at the battery with the engine running around 2k rpm. If it is below 12.6 volts, it is not charging. Then check the resistance of the rotor. The current from the lamp flows through the regulator, through one brush, through the rotor, through the other brush and to ground to light the lamp. Is the lamp light brightly or dimly?

    TT
    #11
  12. BarryT

    BarryT Been here awhile

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    Very bright!
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  13. BarryT

    BarryT Been here awhile

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    I checked with the bike running according to the manual and I got a reading of about 8 volts. I can retest later. I already took the stator off.


    -barry
    #13
  14. BarryT

    BarryT Been here awhile

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    Update! I checked the resitance of the rotor. Sadly I have a cheap analogue meter, but it swung all the way to the opposite side if the meter past my zero. So basically the completely opposite side from infinite. i had it set on 1k.


    -barry
    #14
  15. TimTowtdi

    TimTowtdi Been here awhile

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    Sounds like either a shorted rotor (very possible), bad stator (unlikely), or a bad diode board (possible too). Or it could some other wiring problem. Whatever you do, you should put that battery on a charger. How can you start a bike with 8 volts?

    TT
    #15
  16. BarryT

    BarryT Been here awhile

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    The battery is on the charger but I have my boat battery in the garage all ways on a trickle I use to jump my bikes some times. Should I grab a cheap digital meter to get a more percise ohms reading. My meter pegged to its limits when I try it on the rotor. I checked the stator and I had none were open.
    #16
  17. TimTowtdi

    TimTowtdi Been here awhile

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    Just to clarify, you bypassed the regulator by jumping the wires and ran the bike around 2k rpm and the light stayed lit and only around 8v at the battery? It is not too uncommon for the alternators on these bikes to not charge at idle. That is why you need to run it off idle.

    Assuming you did that, and your readings are correct, you can check the resistance of the rotor brush rings to the rotor main body. That should read open. IOW each brush ring should show infinite resistance between that ring and the body of the rotor or engine case. But they should show very low resistance between each other. How is that?

    Then you can pull the diode board and test the diodes individually. This would be best accomplished with a means of loading the circuit, but can be done unloaded too. They should show high resistance in one direction, and when reversing the test leads should show low resistance.

    Got it? :evil

    TT
    #17
  18. dduelin

    dduelin Amazing grace how sweet the sound....

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    You say the Gen light is bright. That itself rules out the rotor. If it had a broken wire and was "open" then the bulb would not light up. Rule out the regulator by jumping the blue and black wires at the bottom of the regulator and running the bike to 2000 rpm. If the light goes out it is the regulator. Not likely. If the light stays on it is likely the diode board or if you are lucky the PO misplaced the insulating washer under the left hand stud of two that locate the brush holder on the stator. If the washer is not between the brush holder and the stator the brushes are grounded to the stator and the light stays on. The nuts that hold the brush holder to the stator are hidden inside. The outer ones only hold the wires to the brush holder. Leave them alone. If you remove the brush holder be very careful with it. It breaks easily and costs too much to replace.

    Very important! Always remove the ground wire to the battery before removing the front cover to access the alternator. If the cover touches the diode board it shorts to the block and it is ruined. The the light will stay on for sure.
    #18
  19. BarryT

    BarryT Been here awhile

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    Let me recheck my readings. i will be back in a few. Thanks for the help!
    #19
  20. BarryT

    BarryT Been here awhile

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    OK when I check either ring to the body I get infite resistance like when the leads are not touching eachother. When I check between the rotor rings the meter reads zero like when I touch the leads together. So my rotor is ok? I am sorry about all the questions this is my weak point.


    -barry
    #20