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Kikemon 01-08-2013 04:12 PM

Generator light flickers at idle
I just sold an '89 R100GS to an inmate who reported to me that he noticed a flickering generator light on the bike at idle. This is nothing new to me, the bike has been doing it since I owned it. I was told by someone knowledgable about airheads that "They all do that" so I never gave it another thought and the bike always ran well and charged fine based on riding, not testing voltage at the battery etc. The battery lasted as expected, bright headlights etc. The light goes out as soon as the throttle is brought up just over idle (idles at about 1000 RPM if I remember right). Anyway, now that it is someone else's bike I was wondering about this advice a little more and don't want to mislead him with bad advice passed down second hand. I tried searching and could not find anything pertaining directly to this issue and I was wondering if anyone could give a definitive answer on this.

Clay Spinner 01-08-2013 04:31 PM

I've only ever heard of concerns when it doesn't do that! I'd say the bike is fine!

Warin 01-08-2013 04:46 PM

The red light says - the battery has more voltage than the alternator.

As the alternator is only some 280 watts at maximum, you cannot expect it to put out much power at idle speed. And it certainly won't put out enough to cover the 'normal' load at idle. With a more powerful alternator or less load this does not happen...

boxerboy81 01-08-2013 05:50 PM

It's more than "they all do that"'s a matter of they all should do that. It's how the charging system is designed to work. If the GEN light doesn't come on with the ignition on or doesn't go out as the rpm rises above idle, then there's something wrong.

Yours, or now his, sounds right to me.

The usual question from people new to an airhead is along the lines of misinterpreting the GEN light idle glow as the oil light coming on.

Knowing how it should work is important as the signs will be there to make an early diagnosis of a charging problem.

Kikemon 01-08-2013 05:57 PM

Thanks guys. That makes perfect sense.

Buddyv 01-09-2013 10:29 AM

Kikemon -

Thanks for the concern - and follow-up. Pleasure doing business with a responsible seller!

I am the new owner.... bike runs extremely well. The light threw me because my 1988 r100rs does not do this.

Should I be concerned about the RS???? Or are we talking different parts entirely??

Thank you all for the advice.


Big Bamboo 01-09-2013 10:34 AM

My generator light does not "flicker" at idle. Something you might try, check the slip rings on your rotor. If they are tarnished brown and not a bright copper color, hold a Scotchbrite pad on them while the engine is running. This might eliminate the flickering. If not, check that your bushes are within spec.

ignatz72 01-09-2013 10:39 AM

As long as the charge light on the RS comes on with the key ON but bike not running, you are fine.

The difference between Airhead charging light behavior usually boils down to the state of repair and/or cleanliness and tightness of a particular bike's wiring harness connectors. There is the possibility of component age and failure, but a corrosion free harness can make all the difference.

The best way to know what's up is using a multimeter on the various parts as you go through the harness to clean and dielectric grease all the connectors. Rick Jones' Classic Boxer Charging book will help here, and there are charging system test procedures in a Clymer's manual or the BMW repair manual too.

Also, the idle on the GS might be a hair low - try an idle speed of 1100. The light may very well go out. My 92 GS idles at 1100 with no GEN flicker but it only has 15k miles on it, so the system is practically new.

disston 01-09-2013 10:39 AM

Does the light work? When key is turned on the light should be on before the bike is started.

Maybe the RS is idling to high? The light goes out on a lot of the Airheads at only 1300 rpm. Does on my R90/6 and many others. Will almost always go out by 1500 rpm at the highest.

The ultimate test would be to check sitting voltage of the battery and charging voltage of the system when idling and when the rpm is raised.

spo123 01-09-2013 03:58 PM

I agree with ignatz72.....Try to INCREASE your idle slightly....The flicker should disappear.

Bill Harris 01-09-2013 07:49 PM

I interpreted "flicker" to mean that the idle is adjusted so close to the cut-out point for the ALT lamp that the lamp sorta bounces on-off as the idle fluctuates. That's how I've always set mine-- sometimes the idle is a tad high, sometime a tad low, it'll vary. It's not a computer-controlled fuel-injected engine with feedback loops to give 800.0 RPM all the time.


Wirespokes 01-09-2013 08:53 PM

Set the idle where it feels right - a nice steady thump thump thump. Don't worry about where the charge light goes out - but I would say that if the light is out while idling, it's too high.

DoktorT 01-09-2013 09:47 PM


Originally Posted by Wirespokes (Post 20442306)
Set the idle where it feels right - a nice steady thump thump thump. Don't worry about where the charge light goes out - but I would say that if the light is out while idling, it's too high.

If you get all the wiring and terms clean and tight. It makes a significant difference in efficientcy of the charging system. The later Airheads have an auxilliary grounding harness. Got to go in and clean those terms up as well. If you have an earlier machine, do add that aux grounding. You will instantly pick up a half a volt on an otherwise good looking system 3 decades old or more.

I set idle at 1100 and never see the gen light. But a glow at idle is not bad, it is just fine. Just saying you can do better by reducing resistance with a few refinements and maintenace to keep the electrons flowing free.

supershaft 01-10-2013 12:41 PM

The light flickering. It varies depending on the type and specific voltage regulator to some degree.

Everyone suggests checking your wire connectors. Check the wires themselves too! The wires get cooked and don't work right anymore. Usually it is within an inch or two of the terminal. If the wire is hard, it needs changing. The B+ wire gets overheated on a regular basis and usually needs to be changed every 30k miles or so. BMW's aux. grounding wires do too. I see that mess cause trouble on a semi-regular basis. I highly recommend bolting a ground to your diode board and running it to the front starter mount on the case. Keep your hot wires away from ground wires and sharp edges as much as possible. I recently saw a picture and the B+ wire was all over the place. Wrong! Short and to the point!

With a voltage meter at the battery I have never seen an airhead with a stock regulator read over 13.9 volts. Many a guru says that if if isn't charging at least 14 volts something is wrong. That will get you a lot of business! If you are running a wet cell battery you don't want your charging system to be charging over 13.9 anyway or it will cook your wet cell dry in no time. It should be charging between 13.7 and 13.9 if all is well with a stock system.

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