OK charging system guru's, WTF is up with my GS...

So, here is the deal.... Rode to work yesterday, no problems... parked the bike and didn't hook up the battery tender that I always do, jumped on the bike and rode to work this morning no problems again. At lunch I jumped on my bike (no problems) rode 30 mile run on the freeway to drop off my Triumph crankcase for welding and when I got on it to head back to work it almost didn't turn over.... wtf? Rode back to work parked for four hours and when heading home it barely cranked over again... at home with the bike turned off the battery read ~10.5 volts on my Fluke, and running at 3K it was still at ~10.5 volts.

The generator light is lit when the key is on, bike off. It does not come on when the bike is running or at idle. This didn't really attract my attention because it hasn't lit at idle ever since putting in a new diode board two years ago... and btw the battery is a three year old Westco sealed unit that is always on the battery tender when the bike isn't in motion, well except for last night.

So, I plan on testing the resistance of the slip rings this weekend (can't run the bike at night as it keeps the kids awake)... any other advice?

And yes, i will buy Rick's book in the morning... what a crummy way to end a crummy day

Hate to say it solo, but it sounds like your battery bit the big one. That's my gut feeling from the start. I've had those Westcos do that - just die suddenly for no good reason. One only lasted a year. Now I stay away from them and go with the Odyssey or Panasonics from Digikey.

Disconnect the ground and hook up the battery charger. See if the voltage comes up, and stays up. Betcha it won't.

It's a tough call on this end but since you own a Fluke, I'm going to assume you know how to use it.

Have you considered that the battery may have dropped a cell? The alternator can charge its little guts out but if one of the cells is shorted internally, it'll never get the system over 11-12V. I had a Gold Wing that did this and it charged the battery until the case was bulging. :eek1 I was limping it home on the battery thinking I had a bad alternator (11V indicated). A new battery and I was on my way.

I think under those circumstances the generator light would go out because the alternator is doing its job--it just can't raise the system voltage of the bad battery.

The other check would be to charge it at home. Charge it, then take it off the charger and let the battery rest (stabilize) for about 10 minutes. Measure the voltage. If you don't have 12.4+ it's toast.

My other knee-jerk guess is a failed diode ground harness (if you're using the stock BMW diode board).

Of course those are edumucated guesses but 10 minutes with a voltmeter and that front cover off should give you some answers. It may also be one of the normal suspects (rotor, diode block or brushes) but I assume you know how to check those.

Damn, 205'd while editing my masterpiece.

Looks like it's a slam dunk Solo! Got a favorite place you get your batteries at?

Don Kehotey - where'd the expression '205d' come from? Sorry for sneaking in right under you, but I agree whole heartedly with your diagnosis. Well, except that it could possibly be the diode bd, rotor, etc. No, it's the battery! Hell, I wouldn't even take the front cover off, what's the point??

LOL!

Thanks guys... I have the battery on the tender right now and will let it run up a charge until after work, let it "stabilize" and remeasure the standing voltage.

If I can choose one, dead battery ain't too painful!

Then, I'll measure everything anyway

I'm confilcted a bit here, what exactly is the trigger for the light to light? or not light as the case may be..... I would have guessed a certain voltage output above, no light below, light on.... am I correct? If so, what is the mechanism?

Down in the basement I'm supposed to tell you to STFU and get me a beer but considering you've got more years here than me and this ain't Jo Momma:

A few years ago there was the story about a guy getting arrested for 205 MPH on the freeway. LINKY It got retold as 'news' so many times that somebody stuck the term '205' to anything that's already been posted.

Or so I've been told.

I'm pretty sure that it's simply a diode and the direction of current flow.

• If the alternator is at lower voltage than the battery, current flows from the battery back to the alternator windings which lights the indicator.
• As the revs come up, they balance, no current flows so the light flickers and goes out.
• When the alternator voltage is higher than the battery, a diode in that circuit keeps the lamp from lighting up again.

If the battery is shorted and sucking all the current (acting like a big resistor), the system voltage could be 6V and the alternator could be on fire trying to keep up. As long as the alternator has higher voltage than the battery, the light will go out.

The alternator light will not come on with the key on and the engine off if the rotor is open. The same is true if you are screwing with the wiring and forget to reconnect the voltage regulator and ride from San Francisco out to Concord on just the battery. I had to ride out and rescue a service writer who did that.

very interesting.... Thanks!

Put the voltmeter on the charged battery. When you press the starter the voltage will rapidly drop if there is a cell shorting under load.

How the hell did you get so many posts in such a short time??? You've got me beat by a mile.

Thanks for the explanation. I kind of get the idea. So I'm supposed to get you a beer now?

Solo - the idiot charging light works like this: When you turn on the key it energizes the charging system. Because the charging system can't just generate electricity on its own just from the rotation it needs a flow of electricity to the rotor to energize it. The charging light is after the ignition switch and before the rotor.

When the motor starts and the alternator gets spinning real good, it starts charging. At that point it's putting out as much electricity as at the ignition switch, so there's no way for the little electrons to flow through the light. It goes off. As long as the alternator is putting out more juice than the battery the light will be off.

To go one step farther, the regulator turns that flow off and on, because without that flow to the slip rings and the rotor windings, there's no electricity generated. When the battery needs a bit more charge, the regulator turns on the flow to the rotor.

A good question would be why the light doesn't go off and on all the time. Don't know - but I guess it's all happening so fast once the battery is charged up that the filament never gets hot enough to luminate.

Damn! Now I'm the one that got 205d - and not by just one guy but by four!!!

I've got my quota of 'four per day' in just this thread.

Not exactly - a priori, it has nothing to do with "a certain voltage" but only with a voltage difference between the two connectors of the bulb.

Btw, that's one of the reasons why I added an extra voltmeter to all my motorcycles ...

Two of the best explanations that I'm aware of: http://www.buchanan1.net/charge.shtml and/or http://www.thunderchild-design.com/info.html#chargsys ... with a circuit diagramn, it's much easier to understand (at least for me )

Pearls of wisdom right there to a whole lot 'o' no0bs. Thx for the explanation. A system balance explanation makes a ton of sense to me.

Solo... any successful diagnosis yet????

If it's any consolation, I went out to the bike this morning for the first time since the new baby (been too tired to be alert enough to ride) and my battery is dead. Only been sitting for 1 week. Sheesh. It's at least 3 years old, so after a little testing, I'll be sure, but I think we're both gonna end up with a battery funeral.

So i drove the truck today.

Nope, just got up myself (6:30am) and unhooked the bat tender... battery reads 13.3 volts right now (and unhooked the neg bat cable as well). I'll recheck when I head out the door in a hour and post the reading when I get to work... stay tuned!

Thanks to everyone chiming in, I appreciate the advice