4 replies [Last post]
Oldpaint's picture
VBMWMO #5088
Boston, MA
Joined: 10/27/2006
Posts: 12

My running lights had stopped working a couple weeks ago, first the back then a couple trips later the front. Yesterday I opened up both the tail light and head light buckets and tested the bulbs, all were working. I took out the battery and fully charged it. Re-installed the battery and ta-da, the running lights lit up. But by the time I got home, they weren't lighting up anymore. This morning I opened up the buckets again to take a look. At one point while I was checking the contacts, the power just shut off. I must have accidentally shorted something.

Is this likely an easy thing to remedy? Simply change a relay or something? Does an R27 have fuses? I read through the electrical sections of my owner's manual and repair manual but didn't see anything about fuses. Have I created an unfortunately complicated problem for myself?

Any feedback is welcome.

schrader7032's picture
VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6698
Being a battery-coil ignition

Being a battery-coil ignition bike, it needs a good battery to even run...unlike the /2 twins which are magneto ignition and the battery is only used as a reservoir to power the lights when the RPM drops down at idle.

Just to be clear, when you say "running lights", are you saying that is headlight parking light, the tail light (ie not the brake light) and the license plate light?

I would first think the grounds if it affects all of these lights at the same time. From the diagram, it appears that the ground for them all runs through a connector or junction box probably under the seat near the battery. It could be there's corrosion on this connection. If you currently have the situation where the lights aren't working, use a multimeter to check for continuity between the ground at the lights and the frame. If you don't have continuity there, then begin to trace the ground wiring.


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

R.D.Green's picture
Joined: 12/04/2008
Posts: 281
So - - You took out the

So - - You took out the battery and fully charged it. Following this, you had lights but only for a short time. This might mean that the battery is either not holding a charge (in which case you would probably need a new one) or the bike is not charging the battery (in which case you need to find out why). For the latter, I would start with the easy stuff, that being a check of the generator, brushes, and connections. Make sure the commutator is nice and clean and the brushes are not worn out and that they're making full contact with the commutator. Kurt's right - - the battery and the grounds are critical. The battery is very small on an R27 and, from my own experience, they don't necessarily last all that long. I would put a new one in about every 12-18 months when I had my R27. They're cheap. As I remember, the junction box is located inside the battery box. No fuses. Make sure all the connections in the box are clean and secure.

Grant R26
Grant R26's picture
VBMWMO #8145
Toronto Canada and Long Island, Nova Scotia
Joined: 03/28/2011
Posts: 189
original wiring issues

Hi Old Paint
I currently have a 56 R-26 and and 63 R-27. both have original harness and I hope to keep it that way but both have given my minor troubles

Gradually as time permits, I have gone through every connection in the headlight bucket and at junction box in the battery box. As you pull out each wire, you will probably see heavily tarnished copper and probably at least half of the fine wires sheared off.

I have cut back each wire slightly, stripped then tinned each end. then re-insert and clamp down again. By going through everything you will undoubtedly find several funky terminal connections hanging by a thread.

Do this with the hot lead on the battery disconnected.

there is no fuse. My 50s Brit bikes are always "on". Just kick them over and they go ( at least in theory)

so I've installed a fuse at the battery; hopefully prevents a funky connection from burning the bike to the ground. (grin)

The headlight ignition key on the BMW...at least in theory...makes it safer when sitting but I've put fuses in both of mine.

Last year, when a hot lead in the headlamp sheered off from vibration and touched ground, the fuse undoubtedly saved me lots of grief.

Have fun !!

Grant in Toronto

Oldpaint's picture
VBMWMO #5088
Boston, MA
Joined: 10/27/2006
Posts: 12
Thanks, all, for the comments

Thanks, all, for the comments and suggestions. It's sorted out (although that status might be temporary). I checked the battery and fully charged it. It held a charge fine. I looked at the wiring at as many posts and junctions as I could see without messing with anything. There seemed to be one cable in the headlight that only had a couple threads left so I pulled it out, cut the end off, stripped a 1/4" section clean and put it back into its connection. No change. I looked around the junction box in the battery compartment and didn't see a fuse. As you can probably tell, I don't know much about this stuff so I stopped there.

I ended up bringing it to a shop about 15 miles away where I was scheduled to bring it anyways to have a broken odometer pulled to be repaired. They found that someone had put a fuse in the headlight bucket at some point and the total lack of power was due to a blown fuse.

There is a little wrinkle to this, though. I rode it out of the shop yesterday and everything was working fine. Near home, I stopped to have coffee with a friend. An hour later, I hopped on and the generator light wouldn't go off. It seemed to pulse at first, from lit to very bright, then it just stayed on and looked pretty steady. I've had this problem years before, the generator or something in the generator was losing polarity all the time. After a couple fixes that proved to be temporary, Chris Betjemann at Barrington Motor Works solved the problem. I'll have to check my records to see what the solution was. It sounds like it might need another round of whatever he did.

I got home fine last night and got up this morning ready to tackle the polarity problem. I read up on the forums about re-polarizing the generator, got out my laminated colorized R27 wiring diagram, etc. I started it up and at first there was a little pulsing but then after a few seconds the generator dimmed and finally just shut off, indicating everything was working as it should. No doubt this is a temporary condition.

Sorry about the long post. Just wanted to thank everybody for the advice. And yes, you guessed correctly what I meant by "running lights." I guess running lights are what they have on a boat, not a motorcycle. If there's a new chapter, I'll update this.

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