BMW K bikes (Bricks)

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1Back to top Go down    alternator light on Sun Jun 26, 2016 8:38 am

Ian Drinky

active member
active member
Hi All,
i have a question in regards to my alternator, The light comes on very faint at idle and the neutral and brake warning light dim, when i rev the bike the light will go out and the others lights will brighten again, i have cleaned the brushes on the reg and checked battery voltage when running 13.6 v.

any suggestions would be appreciated.




2Back to top Go down    Re: alternator light on Sun Jun 26, 2016 9:08 am


Life time member
Life time member
The charge light coming on at idle is not uncommon but may mean your alt is not outputing to spec which can lead to a bad battery.

If that's 13.6v while running at 2000rpm then your alternator is not really outputing enough voltage to charge your battery properly. Ideally it should be around 14.0v

If that 13.6v was measured with the engine idling then that's not a valid measurement.

You can also measure your battery's static voltage. That should be more than 12.45v but ideally around 12.8 or more. Static voltage is measured with engine off, after your battery has been properly charged and had a rest for a few hours.

Your battery should also be measured under load. Without the right equipment, that becomes very subjective, so best to take your bike to a battery shop and have it and your alternator tested correctly.

Did you measure the alternator brushes? They can wear down or at an angle so they don't contact properly.

1988 K100RT VIN No. 0094680
1989 K100RT VIN No. 0097367 (naked)
1996 K1100RS VIN No. 0451808
Fuel: 95 Octane
Engine Oil:Nulon Full Synthetic 15W50
Gear Box Oil: Nulon Synthetic 75W90

3Back to top Go down    Re: alternator light on Sun Jun 26, 2016 9:57 am

RicK G

13.6V is not high enough, it should be very close to 14.2 at 2500rpm. As Hollister said the brushes probably need attention but if the alt light keeps glowing faintly there is a fix.

"Every normal man must be tempted, at times, to spit upon his hands, hoist the black flag, and begin slitting throats." from Mencken's 1919 Prejudices

Bikes 1993 K1100 LT, 1998 K1100 LT, 1993 K75 RT, 1996 K75RT, 1986 K75 GS, 1979 Z1300 Kawasaki X 2 for now

4Back to top Go down    Re: alternator light on Sun Jun 26, 2016 10:38 am


Life time member
Life time member
I'd say it doesn't hit the 14.0-14.2 until 3000 or so but 13.6 is "normal" at idle.

Current stable:
86 Custom K100 (standard fairing, K75 Belly pan, Ceramic chromed engine covers, paralever)
K75 Frankenbrick (Paralever, K11 front end, hybrid ABS, K1100RS fairing, radial tires)
86 K75C Turbo w/ paralever
94 K1100RS
93 K1100LT (x2)
91 K1
93 K75S (K11 front end)
91 K75S (K1 front end)
14 Yamaha WR250R

5Back to top Go down    Alternator on Mon Jun 27, 2016 9:15 am

Ian Drinky

active member
active member
Thanks guys for your input, i have removed the alternator and dropped it into the auto elec to test on the bench, waiting on his report.




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