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Old 07-29-2009, 04:47 PM   #1
BarryT OP
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Location: Edwards AFB California (Rosamond CA)
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I got the 77 BMW 75/7 now I need help

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
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BarryT screwed with this post 07-29-2009 at 05:04 PM
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Old 07-29-2009, 05:09 PM   #2
crazydrummerdude
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Check the rotor.
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Old 07-29-2009, 05:13 PM   #3
BarryT OP
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alternator?
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Old 07-29-2009, 05:16 PM   #4
crazydrummerdude
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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).
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Old 07-29-2009, 05:18 PM   #5
BarryT OP
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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
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Old 07-29-2009, 05:22 PM   #6
TimTowtdi
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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.

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Old 07-29-2009, 05:27 PM   #7
Wirespokes
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If it aint the battery...

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.
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Old 07-29-2009, 06:15 PM   #8
Bill Harris
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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.
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Old 07-29-2009, 06:36 PM   #9
BarryT OP
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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?


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Old 07-29-2009, 06:42 PM   #10
datchew
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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)



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Old 07-29-2009, 06:46 PM   #11
TimTowtdi
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Quote:
Originally Posted by BarryT
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
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
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Old 07-29-2009, 07:25 PM   #12
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Quote:
Originally Posted by BarryT
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
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.
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Old 07-29-2009, 08:39 PM   #13
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Quote:
Originally Posted by Bill Harris
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.
X2 Buy the book, Rick is very helpful over the phone too.
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