Generator light on R100/7

[almostblue]

If the light comes on after the engine is started, does that mean the generator is bad? Or could it just mean the battery is bad? My battery currently is not holding a charge. It was low on acid and obviously needs replacing. But do I have bigger problems with my charging system? Or will the generator light go out when I install a new battery?

[donbmw]

At idle the Gen light may come on or flick at you. Does it go out as you come on the throttle some were above 1500 rpm.

Don

[bikerfish1100]

since it seems that you know the battery is bad, and it's such an easy swap-out, do that, then see what happens.

[JohnP]

If your bike was made in the early 80's bad diode boards were a common problem. The only bike that ever stranded me (including several British bikes) was my '84 R100. Diode board crashed on the way back from Mid Ohio. Nearly made it over the Delaware River when the motor stopped cold. If I'd disconnected the headlight I might have made it the whole 400 miles. A bad rotor is also a possibility. Airhead charging system trouble shooting has been a subject substantially written about in the BMWOA articles by Mathew Parkson.

[20774]

Airhead charging system trouble shooting has been a subject substantially written about in the BMWOA articles by Mathew Parkson.

...uhhh, Matt Parkhouse is the esteemed guru...but you knew that anyway...!

[BuckeyeClark]

I owned my 1980 R100RT for a little under 5 years and 2 different batteries. From day 1 when I was at idle the generator light would flicker on and off and as soon as I began moving down the road it would go out. Other than that it ran beautifully.:dunno

[almostblue]

I put a new battery in it. generator light still has faint glow. what to do? the bike in question is a '78.

[20774]

Since you've eliminated the battery, your charging system doesn't seem to be working properly. The light in the instrument pod senses the different voltage potential on either side of it - one side is the battery and the other side is the alternator system. If the voltage between the two sides is not equal (or relatively so), then you see the light glow.

Using a known good voltmeter, measure the voltage across the battery terminals with the engine running around 3K RPM. What do you get? I'm guessing it's going to be around 13v...maybe less. You need at least 13.5v but closer to 14v for proper charging.

Make the measurement...then you'll need to do some more diagnostics back up the line. Voltage regulator, rotor resistance, brushes...could be corroded contacts/wiring in the charging circuit. Rick at Motorrad Electrik sells a book which walks you through this process. Also, several links to testing the charging circuit have been posted within the past two months. Seems there's been a spate of charging issues this summer...

[sumran]

I put a new battery in it. generator light still has faint glow. what to do? the bike in question is a '78.

Does the light stay on constantly or only at idle? If it is only at idle you are fine. If it stays on all the time something is wrong. I read back through your posts and couldn't tell what RPM's you were at when the light was coming on.