R100RT generator light stays on dimly as revs go up

[Archie]

Folks, I have another question please. I have a 1979 R100RT and have just put in a brand new AGM battery which is fully charged on my battery tender. When I took my bike for a ride yesterday after many years of it being in storage, I noticed that the red generator light stays on. As I increase the revs, the light goes dimmer, but it doesn't go out completely. I noticed that the voltmeter on the dash is showing 10.5 - 11 volts, and less than that when the turn signals are on. The voltage increases as the revs increase, but it never gets up to 12 volts. When I got back from a 20 mile ride mostly on freeway yesterday, I hooked up my BMW battery charger and had to leave it on overnight to get the battery back to full charge again. It seems quite clear that my battery is not getting charged.

What should I do immediately to check and see where the problem is? Should I take off the front cover (disconnect battery first) and probe around and see if anything there is loose? How do I clean the connections if there is corrosion - with a fine sandpaper? Any suggestions would be much appreciated! Thank you!

[Jean]

It sounds like one of your diodes (on the diode board) has died, or your alternator brushes are not working so good. Your voltage regulator is also suspect. There are some simple tests for a faulty Voltage regulator...I think Duane's site tells how to do it.
There's also a little diode on the back of the big board that can couse the light to stay on but you would be charging the battery anyway. Your description says the battery is not charging!
Using your "bench" voltmeter, see what the charging voltage is, at the battery posts, with the engine running around 2500 rpm...it only takes a few seconds to do this. It ought to be 13.5v at least.
You WILL have to remove the front cover for the other investigations:
Check brush length...and spring tension. Should be "snappy"...
Then there's the diode board. You can disconnect it and check the "forward" and "back" resistance on the diodes to discover if one of them is bad.
They CAN be replaced. I just don't recall the part numbers to do it. I think Radio Shack had the diodes that can be used if yoou are adept at electronic soldering.
Once again, the directions for this are either ON this site somewhere, maybe on the IBMWR tech hints site, or Duane's or Snowbum's....
Good luck. REport back.

[Kurt in S.A.]

Buy Rick Jones' book on Boxer charging from Motorrad Elektrik. It takes you through all the steps to isolate the problem.

Snowbum does have a section on troubleshooting the charging system.

Kurt in S.A.

[Jean]

I forgot...there's a couple of other things that could be wrong, too...The fact that light dims suggests SOME voltage is getting back to the battery so broken wires can pretty-well be disregarded as a problem.
Your battery is new and charged, so that is also eliminated.
The thing I forgot is a faulty rotor, however that usually manifests itself in a "NO-charge light" condition.
FIRST thing to check is the voltage regulator tho. You'll probably have to remove youor tank to get to it, but with clever fingers, you might get away without doing this. Unplug the wires from the regulator. Connect a jumper across the Blue and Black wire terminals in the plug. Run the engine and see if the light goes out. You also might glance at your voltmeter, too.
IF the light DOES go out, replace the voltage regulator. If it stays lit, you can go into the next round of testing...brushes, diodes, etc.
Start simple.

[Archie]

Jean, thanks for your input. I removed the tank and located the voltage regulator on the right side, it's kind of below the brake fluid reservoir. I then pulled out the black plug from the "silverish" regulator. You said to connect a jumper across the blue and black wire terminals in the plug. There is no black wire - only blue and brown. Is it OK for me to use some regular thick gauge red wire between the blue and brown terminals in the plug? Also, there are 3 blue wires and 1 brown wire coming out of the harness. The brown wire goes to the front terminal, two blue wires go to one of the other terminals and a single blue wire goes to a 3rd terminal. Do I jump them between the single blue wire and the single brown wire or does it not matter? I can send a photo if you'd like....Sorry for the caution and questions - this my first time tackling something like this!

[Kurt in S.A.]

It must be a blue and black wire that you jumper across. The plug connectors are shaped like a pyramid. The two that you want to jumper at the ones at the base of the pyramid.

See the "Voltage Regulator" section here:

http://bmwmotorcycletech.info/trbleshootALT.htm

Kurt in S.A.

[Archie]

Kurt, here is a photo of the plug. As you can see, there are only blue and brown wires, not black. So do I jump the two blue wires at the base of the pyramid, the one side of which has one blue wire going to the terminal, and the other side of which has two blue wires going to the terminal?

Having a problem downloading a picture, but am working on it...

[sprints@pldi.net]

Rick Jones has the best little BMW electric book (actually CD now - he ran out of books). They are $15 and worth every penny. If you're going to keep the bike, invest in you library.
BEFORE YOU REMOVE THE FRONT COVER - DISCONNECT YOUR BATTERY!
If you don't you will have even more problems and more expense - Rod

[Archie]

Just wanted to report to everyone that I did jump the terminals in the plug and the generator light stayed on. It did start dimming slightly as I increased the revs, but the light didn't go out completely and remained fairly bright.

So, with the voltage regulator eliminated as a possible problem, I'm now on to removing the front cover and checking things in there.

Thank you to all who have helped me so far - it is much appreciated!

[ME 109]

When I have had similar symptoms, a diode has been the culprit.
I've had two diodes replaced on my original diode board over the years.
An amazing run out of it @ nearly 275k klm's, especially with the crap corrugated roads it's been down.
Original rotor and stator as well.