Vintage BMW Motorcycle Owners

R90/6 alternator light stays on. It has a brand new battery. What steps should I go through to find problem?
Thanks.

Lots to read here:

https://bmwmotorcycletech.info/trbleshootALT.htm

Thanks!

GEN light glows bright and steady at any RPM . . . .

Battery is brand new.

Installed new voltage regulator (adjustable voltage type but didn't adjust since it claims it is pre-set at 14.3v).

Removed diode board and tested, everything tested good.

All wires on these components look good.

Removed rotor, tested 3.9 ohms resistance between slip rings. However, Haynes says should be between 6.21 and 7.59 ohms. Then read that a 3.9 reading shouldn't make any difference.

Does anyone with experience know if 3.9 ohm reading on rotor might be the problem?

Any ideas of where to go from here?

Thanks.

For a /6, 3.4 ohms is what is called for...so your 3.9 is basically OK. The earlier rotors 1970-1973 were supposed to be around 6.9 ohms...maybe that's what you were reading in Haynes.

So what's the history? Is this bike new to you? Or did it work fine at some point but now it doesn't? Why the new battery...just old or did it test old?

Snowbum has some steps that should be followed. You'll need to isolate the problem. Rotor sounds OK. I wonder about the wiring connected to the alternator and the diode board...any corrosion there? There are wires that go from the front of the alternator which takes each of the three phases to the backside of the diode board. If corrosion sets up in those wires, the bike won't charge.

The light in the instrument pod is telling you that the voltage on either side of the light are not the same. One side of the light is basically the battery voltage...the other side of the light is what the output of the alternator is. If the voltage difference between these two is significant enough, current will flow which cause the light to come on.

Thanks Schrader,

First, Haynes clearly says that /5 and /6 should be 6.21-7.59 ohms across slip rigs and "all other models" 3.4 - 3.74 . . . .

Bike is not new to me. I've had it since 2004. Was riding it a few weeks ago and noticed the GEN light not extinguishing. I had taken it out a couple of months before that (before winter arrived) and it was fine. So it had been sitting in the unheated, humid garage for a while. Anyway, I suspected I'd be troubleshooting so started off with a new battery since the existing one was 6 years old anyway . . . I had a new voltage regulator laying around so I threw it in to eliminate that problem.

I didn't want to reassemble alternator and rotor if the 3.9 ohm reading was a problem, so that's why I inquired here first.

Also, I don't suspect the brushes because they look OK and I don't think the light would suddenly come on bright and steady if it were the brushes but I may be wrong.

If you're confident the rotor is OK, I would clean the alternator terminals as you suggested and put it back together and hope for the best.

Thanks.

I'm sure of the 3.9 reading...I have the Classic Boxer Charging book from Motorrad Elektrick and it is a good thing to have. Haynes...meh...sometimes it's OK, but this time it is wrong. Only the /5 is the higher reading...beginning with the /6, it is more like 3.9 ohms.

You say you threw a new voltage regulator in the bike. Did you do Snowbum's test which bypasses the regulator? I did that once during some troubleshooting. Basically when you jumper the regulator and start the bike, if the alternator is working you might expect to see the voltage at the battery rise quickly towards 15-16v. Don't let it go that high, but if it does increase to well above 14v, then you know the problem is the regulator. If the voltage doesn't increase beyond say 13v, then you have to go into either the diode board or the alternator.

I'm not sure how much you tell about the brushes. They are being pushed down by some small snail springs to keep them up against the slip rings. If the brushes have worn significantly, then the snail spring could hang up on the holder and not get good contact with the brushes. You probably could find something to carefully push down on the brushes from the top...maybe a piece of paper towel...if the light goes out, that might be the problem.

Problem resolved!!!!!

I believe there was a broken solder on one of the three wires coming out of the stator (that go to the diode board) . . . Soldered it up and everything is good to go!

Thanks for your assistance Schrader!

Nice! The diode board needs all three phases in order to fully charge. Now get some riding in!!