If you have the mechanical voltage regulator still on the bike, they can get corroded points and start to misbehave. (I know it's hard to believe on a conservative bike like an R100, but it's true. 😆 ) You can spot the mechanical regulator as the 3 inch long tin can on the right side of the frame, under the fuel tank.

There are all sorts of solid state (all transistorized, with no mechanical parts to wear or go out of adjustment) regulators that can replace these older models. If you find one for a BMW motorcycle, you'll pay $50 to $100. But Bosch put the exact same regulator on older Saab, Volvo, and VW cars too. They look and mount exactly the same, except they typically have plastic cases. I picked up a German-made, third party copy, brand new in the box, off Ebay for $9 + shipping. Plugged it in and my voltmeter has been rock steady at 13V ever since. See current Ebay auction 222109865767. You local parts store may have one even cheaper.

Also see my other posts concerning corrosion on the electrical connectors and the use of No-Ox to return to perfect electrical connections. No-Ox will work wonders and eliminate all the impossible to trace iffy connections caused by 40 years of riding in the rain. I never unplug a connector on my '79 R100 without a dab of No-Ox going on before being reinstalled.

Hope this helps.

Thanks! I took your suggestion and bought the regulator. Hope that takes care of it.

I think the regulator will get you 60-80% of the way there, but never discount the effect of minute amounts of corrosion on the connectors either. If the contacts inside a steel can corroded, then what must the connectors outside the sealed cover look like?

No-Ox is available on EBay and Amazon, or if you have an electrician pal then bum some off him. The smallest, least expensive container you can find is enough to do 20 motorcycles. It just takes a tiny dab on each connector contact.

I highly suggest that the first use should be when you plug in your new regulator !!!

🙂

I ordered the No-Ox. Thanks for suggesting it. I need all the help I can get!

I have a 1979 R100 RT which has been teaching me all about electrical issues. Besides the regulator, I finally had to disconnect every connector on the bike and treat it with No-Ox....
All the relays on the frame, opposite the regulator.
All the connectors on the colored circuit board inside the headlamp.
The connector on the underside of the speedo/tach housing
The tail lamp harness connection over the battery
Even the 2 fuses

Now it's all good. My pain; your gain ! Enjoy.

I've got it all apart so I might just follow your example and do every connection. Then I can move onto my R75/6 which has a problem with the front caliper! That's a pain! Brake fluid...
Thanks again for the tips.

► Do all your new friends here a favor. Go back and edit your first post within this thread and change the title from "Electrical Problem" to "Regulator / Battery Problem". That way all your pals can find your lessons and wisdom in the coming months.

► Again, my heart goes out to you. I hate brake work more than anything. Especially when it involves bleeding DOT 4 fluid. It makes my skin crawl, and it always manages (somehow) to end up splashed all over the bike. But again, since I just finished this service on my 1979 model, I'll have all sorts of tips ready for you to use.

All the best !