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Topic-icon Charging problem on an '81 R100RS

5 months 1 week ago #5333 by 12545
Charging problem on an '81 R100RS was created by 12545
Was on a multiple day ride when I stopped to rehydrate at a Shell station on a very hot (94-95 degree) day. When I got back on the bike and started the engine, the Volt meter
pegged as high as it would go at over 16 volts. Bike ran normally, rode down the road for a couple of miles and stopped. Restarted the bike and it pegged again at over 16 volts. After a minute or two the volts dropped to 12 volts at idle. Riding down the road the volts held steady at 12.5 volts, no matter what rpm I was running. Stopped again to cool off and same thing happened again. Volt meter pegged at 16 volts and after riding for 3 or 4 miles the volt meter again dropped to 12.5 volts while running at speed.
What do I look at first?
Sam Creasman
RA # 12545

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5 months 1 week ago - 5 months 1 week ago #5335 by 8166
Replied by 8166 on topic Charging problem on an '81 R100RS
Neither 12.5 nor 16 volts is within spec for the BMW charging system. In the first case, I'd guess that the charging system wasn't working at all, and all you're really seeing on the volt meter is battery voltage. In the second, it sounds like the alternator is outputting at full power. When's the last time you popped off the front cover and verified that all the alternator connections were clean and tight, and nothing looked over heated? That would be a good place to start. If you see no improvements, next would be the voltage regulator. Keeping the output of the alternator within specified voltage limits is the job of the voltage regulator, so my first step would be to verify that the connections to the VR were clean and tight. If that didn't yield any improvement, the next step would be a trip to the BMW dealer for a new one. Or you can try Rick Jones at Motorrad Elektrik for a replacement as well. He also sells one that can be adjusted for higher or lower voltage, which may come in handy of you're running one of the new tech super batteries.While you're ordering, pick up a copy of Rick's BMW charging system book. In it you'll find step by step solutions for problems such as the one you're experiencing.

8166 Scot Marburger, This email address is being protected from spambots. You need JavaScript enabled to view it.
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Last edit: 5 months 1 week ago by 8166.
The following user(s) said Thank You: 12545

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5 months 6 days ago #5341 by Wobbly
Replied by Wobbly on topic Charging problem on an '81 R100RS
Scot gave an excellent response.

• I can only add that the OEM voltage regulator is full of relays and contacts that are set to open and close at certain limits. Just like points ignition, those contacts needs cleaning and renewing every decade or so. That's the number one reason to find one of the newer, 3rd party voltage regulators which are all electronic. Those newer type do the switching in transistor logic rather than with mechanical springs, arms, and contacts. The increase in reliability should be blatantly obvious to even the staunchest Luddite among us.

• Then let me underscore Scot's point about newer sealed batteries (AGMs) needing a slightly higher charge voltage. With Rick's adjustable model you can dial the voltage right to the battery maker's voltage specs. Another big plus.

Don't hide 'em, Ride 'em !!

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