Mysterious Electrical problem (interesting)

[gspd]

then when the engine starts and revs to around 3000, the light goes out? OUT, NOT glowing dimly..

3000 !?
If everything is a A-OK it should be completely out at anything above 1100-1200rpm.

[Deleted User 61]

Yeah, the U.S. hasn't been that way since the 50's, mid 60's at the latest.

Must have been the LSD.
Damn hippys.

[chasgrips45]

In fact I was on the 4 lane section. I was also on a pretty treacherous section of road ,going downhill ,on the East side of the mountain. After I thought about it for a while I realized that it would have been a dangerous stop. I should know I was there! Man ,that semi traffic is rough! I`ve been stranded on I-95 in Ft Lauderdale before , at least there was a little more room to move. Again ,the question, why did the gen light come on bright ,& the voltmeter peg to neg when I applied the brakes?

[Jean]

gspd, At 3000 the alternator shoould be doing it's BEST at least. My light DOES go out around 1200-1300 rpm, but Chas R90s has some very interesting history, so I was giving him the absolute benefit of the doubt. I also was considering that one phase of his rectifier might be faulty...this will cause the light to glow dimly or flicker even if "some" charging is happening. You get a bit less voltage and at low rpm, it is pulsing DC...I say this for Chas benefit...
Likewise, if the fuse board is the sort with the "hidden" diode, some very weird things can happen when that diode fails to act as a diode.
Chas...earlier, gspd mentioned that when your points wire was damaged, there was a short to ground. When you applied the brakes, you either put a load on the battery that your charging circuit could not overcome when the brake light was ON OR, due to some quirk in your wiring and the blown fuse, you were introducing a current path that defeated your charging circuit.
I think you have some connections somewhere that aren't OK.
Does your fuse board have a separate terminal point for the charge light and numerous other wires (Not associated with a fuse) or not?

[chasgrips45]

Yes Jean, I found that the top fuse was blown,this time. I`ll check the schematic & take a look. As soon as I replaced the fuse ,everything fine. I have the small book published by the guy who sells BMW electrical parts. It`s a shame that I cant remember his name,but it`s a great diagnostic manual,I`ll do some of his recommended diode board alternator tests. I`m getting pretty good at this electrical stuff. BTW Jean, this is a different bike. This machine has "normal" electrical electrics.

[Jean]

Ah-ha. A DIFFERENT bike.
"with normal electrics." (?)
Even so, that can be pretty scarey!
Normal to BMW doesn't always match what is logical.
(As in...that connection through the starter relay that always messes you up when you need to replace the relay!)
OK. You have a schematic. that's a start.
I was going on recall on the last bike...you do seem to have a thing about those R90s' don't you?
(what happened to the OTHER one?????)
Book: That would be Rick Jones...maybe? In the edition I have the drawings are not as definitive as I'd like for real trouble-shooting. They are just an easy-to-read representation of what wire goes where, sortta, and then only for the starting and charging circuits.
You have carefully avoided my question about the way the charging light works.
IF it does indeed go OUT, at what rpm does this happen?