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Old 11-22-2014, 03:57 PM   #1
Vagabond_Explorer OP
Joined: Aug 2012
Location: Southern Cross Land
Oddometer: 134
1995 R100R Mystic Electrics questions

I went for a ride on my Mystic yesterday and after a refuelling stop, the bike won't start. It sounded like a weak battery so my friends push started me (thanks Greg and Grant!!). The bike went another 2 km then came to a complete stop. Very weak electrical power, faint headlights, weak indicator lights. Trailered the bike home, bought a brand new BMW battery, dated it, filled in the acid, charged it, then installed it.

The first half dozen times, it would engage, however engine won't start then it'd disengage and goes quiet even whilst thumb was still on the start button.

After that it started perfectly. This morning it also started perfectly. I used a multimeter to see if the generator is dead. Here are my measurements.

1. Battery 12.8V
2. Idling 12.52V
3. Rev to 3000 rpm 12.56V
4. idle + headlights on 12.3 V
5. Rev to 3000rpm with headlights on 12.3V
6. Switch off keys 12.71V

When the key is on, neutral lights up but GEN and Oil lights are both off. Do you think the alternator is dead? If it is, why isn't my GEN light on? I am worried about point 5 above as I repeated the same exercise with my R100GS and the voltage increased when rpm was brought up to 3000 rpm.

Last I look it still had a BMW badge instead of BSA (Bastard Stopped Again) but I am grief stricken my beautiful Mystic might now be sick and I have a long ride planned next week.

The old battery shows 8.2V on the multimeter.

Please be gentle - I am a rookie when it comes to anything more than an oil change, and straightforward servicing. Thanks.
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Old 11-22-2014, 04:25 PM   #2
Bill Harris
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Joined: Sep 2008
Location: backwoods Alabama
Oddometer: 7,101
You've got an alternator problem.

Some reading:

And you really need to get that oil pressure light functional.

'73 R60/5 Toaster
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Old 11-22-2014, 04:36 PM   #3
Dutch fool
Joined: Apr 2008
Location: Holland, land of tulips and wooden shoes
Oddometer: 746
Going by your readings, your charging system is dead.

Now to find out why, or what's broken.
First, you say the 'GEN' lt doesn't come on with key on, could be as simple as a dead bulb, or a corroded connector.
You need a working lt to get a working system.

Then the next thing to check is the continuity of the rotor, remove front cover, lift the 2 brushes from that rotor and measure resistance between the 2 copper rings, that should give you 3,5 -5 Ohms.
An open circuit means no charging.

Next thing to check is the length of the brushes, anything below 6-7 mm means they are worn.

If that all checks out ok, then you must check the voltage regulator and rectifier and wiring.
That may be outside your comfort zone, but with some common sense and a good manual you will get there.

Geht net, gibbet net.
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Old 11-22-2014, 04:38 PM   #4
Horizontally Opposed
Joined: May 2008
Location: U-puku-ipi-sing
Oddometer: 6,411
How old is the battery?

51/3 , 60/2 , 60US , V7 Eldorado , R100M
It is the mark of an educated mind to be able to entertain a thought without accepting it. Aristotle.
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Old 11-22-2014, 05:17 PM   #5
Vagabond_Explorer OP
Joined: Aug 2012
Location: Southern Cross Land
Oddometer: 134
Thanks gents.

Danedg, the old battery was undated, not a BMW battery, a gel based Duracell, but it looked old - faded labels, etc

I am wondering though, why is it when the new battery was installed, the engine wouldn't start straight away but instead the power turned the starter motor, then disengaged and then nothing. Shouldn't it just keep turning and turning whilst my right thumb is on the starter button?

Thanks Bill for the articles. Appreciate the pointers PaulRS, I will strip the front cover off and do those tests this evening. The weather is forecast to be 44 degrees C today or 111 deg F!
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