6 replies [Last post]
redavide's picture
Joined: 12/04/2007
Posts: 81

The charging light on my R69 was staying on constantly. I thought it might be the generator brushes, so I bought new ones and new brush springs. After installing them, the charging light was going out just over idle RPM and I thought the problem had been solved.

After about a hundred miles, exact same problem returned -- charging light staying on all the time . . . This time I held a piece of fine sand paper against the armature where the brushes contact it while the engine was running and the problem seemed to be solved again -- charging light was properly going out and everything seemed ok.

Another hundred miles, problem returned. I took off front cover and inspected the brushes. Strangely, in only 200 miles since they were installed, they had worn down to the length of the previous ones that had been in service for thousands of miles.

I'm sure (I think) that the springs weren't too tight. I only coiled them about 3/4 of a turn and that didn't seem like too much pressure at all.

When the charging light is on, the battery is clearly not getting charged since it drains completely after about 2 hours of driving with headlight on.

Any ideas? What steps I can take next?


malmac's picture
VBMWMO #8751
Toowoomba, Australia
Joined: 06/29/2014
Posts: 300
check the runout on the front armature

This is not from personal experience BUT I seem to remember reading that if the runout becomes excessive then not only will the brushes not stay neatly in contact with the armature but the brushes would wear more quickly.

However 100 miles seems really extreme.

How much side clearance is there when the brushes are installed? Could the brushes have room to vibrate - thus increasing wear rate?

Lets hope someone chimes in who has the absolutely correct answer. Really I am just guessing.



mal - R69s
Toowoomba- Australia

schrader7032's picture
VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6805
Hmmm...you said that after

Hmmm...you said that after applying the sandpaper, the brushes have now quickly worn down...likely cause and effect, I think. I probably would have used a hard rubber eraser first off just so that the surface isn't too rough.

I wonder if the grooves between the metal core "fins" (not sure what they're called) has filled up with debris. You might need to remove the rotor and clean it up. There's a special hardened bolt needed to pop the rotor off the front of the crankshaft. Be careful doing this.


Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

johnpst's picture
Joined: 03/22/2016
Posts: 77
Aloha, Kurt and Mal are on to


Kurt and Mal are on to something to be checked out.

1. If you have a dial indicator, check the runout on the commutator (the proper name of the part the brushes come in contact with). If no dial indicator, mark a line on the one of the brushes (lightly) at the holder. Rotate the engine by hand and watch for relative movement. If yes, get professional help. It may be as simple as removing the armature and resetting it onto the shaft up to grinding the commutator.

2. Yes, sandpaper is the wrong thing to use on a the commutator. There are hundreds of opinions and this one is mine but, the end result must be a clean, smooth, consistent contact between the brushes and the commutator. Here are some ideas:

a. Using a non-chlorinated solvent, completely clean the commutator. Make your thumbs sore from rubbing with a clean cotton or canvas cloth. Dry it completely, with time or air. Vacuum it if you can to remove as much dust as possible.

b. Closely inspect the commutator. You are looking for 1. a clear, clean, undercut space between every segment of the commutator. The mica must be slightly below the copper segments with no interconnecting conductive material. 2. there must be a slight chamfer in the copper on both sides of the gap. 3. The commutator must be smooth to the proper roughness. There is a technical standard to this but, if you see anything which would even slightly exceed a slightly hazy/honed finish, it's too rough. See comments on this below. 3. roundness. It must be round. 4. grooves. It must not have grooves where the brushes contact the commutator. 5. discoloration. Look for overheated copper. This is an indication of possible damage to the armature. If you see the above, you will need to undercut the segments and service the armature. This is a professional job and should be turned over to a motor/generator shop unless you have the tools and training. It won't be too expensive.

c. Insure your brushes are the proper material (BMW part number will suffice) and are not contaminated. Carbon that has been sitting around in a bad environment can deteriorate. They're cheap. Replace them. Make sure the termination on the pig-tails are clean and tight. When running, are there sparks?????

Okay, this may sound daunting but, I cannot say that anything will work as designed if not installed and operated as designed. That said, you should be able to get away with polishing the commutator to a shiny satin finish with 400 or so grit non conductive abrasive cloth. DO NOT CUT THE COPPER DOWN THE TO MICA. Use a soft, stiff bristled brush to clean between the bars. As long as the runout is not excessive, you should get more than 100 miles out of it.

Let me know if you have other questions. It's my opinion that removing it, and handing it over to a shop to work on is the best answer. Just make sure they have some specialty in DC systems.



55 - R50, 06-R1200RT, 96-M900, 10-TU250x

redavide's picture
Joined: 12/04/2007
Posts: 81
Thanks! I appreciate the

Thanks! I appreciate the responses. I'm going to clean the commutator as advised, install some new brushes and see what happens . . .

BTW, yes there are small sparks visible between the brush and the commutator when the engine's running.

Slash2's picture
VBMWMO #9015
Joined: 10/17/2015
Posts: 301
That pesky red light.

This problem developed immediately following a mechanical restoration on my R60/2 and drove me bananas for several months. Once I determined the brushes were the obvious problem I replaced them with new and thought all was well. In a hundred miles or so the generator light would begin illuminating with increasing frequency until it simply remained illuminated at all times. I found the trailing edge of the concave surface of the brush itself was deteriorating which resulted in the loss of continuity IE charging. Finally I replaced the brushes AND the springs and was very careful in the installation of said springs and have gone thousands of miles with no more hungry armiture devouring my brushes.

As always, your mileage may vary.


64' R60/2 - 66' R27 - 51' R67 - 68' R69S
88' R100RS - 06' K12R

motorradfanatiker's picture
VBMWMO #7961
Joined: 04/01/2011
Posts: 6
rapid generator brush wear

redavide, did you ever get your generator brush wear problem solved? I'm having the same problem with one of the brushes on my 1961 R69S? What was the problem and solution?


Jerry Hale
San Ramon, CA
R69S, R90S, R1100RT

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