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timhecht's picture
VBMWMO #9103
Joined: 05/05/2016
Posts: 7

Seriously. When I wrench on a motor vehicle the job isn't done until: (a) I draw blood, (Cool I need to buy a tool I didn't have (or couldn't find), and (c) I lose a nut, washer, or other small parts piece in the most inaccessible location.

The problem bike a 1977 R100S that someone turned into an RS. Previous owner installed two circuit breakers; hard-wired a single prong auxiliary cable for a Radar Detector.

The problems are electrical. None of the following voltages have been confirmed with an external volt meter or multimeter...yet.

With the headlight and parking light off and engine running at 1000 RPM volt meter registers just above the 12 volt line; RPM increase up to 3000 RPM volt meter shows almost 14 volts. Above 3000 RPM (3500 - 4000+ RPM) volt meter bounces all over the place - 0 - 18 and everywhere in between until RPMS drop back to 3000 or so.

With headlight on and battery maintained on a trickle charger daily, at around 2000 - 2500 RPMs volt meter shows about 12.5 Volts; some higher readings, but never close to 14 volts.

So now, after the bike has been unridden, having sat in the garage in MI for a year + we moved to San Antonio, TX; and its time to get the bike on the road and enjoy the almost year round riding weather. Those of you who live in MI know what I mean!

An additional problem, also electrical: front half of bike, all lights, including instrument cluster (and oil pressure gauge light) all work - front turn signals, headlight, parking light, etc. all work. With the exception of the oil pressure gauge light, all "dash" lights are not bright - not really dim - just not as bright as the oil gauge light. Rear half of bike - no lights... no brake light, no running light, no turn signals; the sun has set...

OK so with my limited skill sets for the lighting issues I probably need to start with fuzes, electrical plugs, and ground wires...right? I can't imagine all the light bulbs have burned out unless there was a shorted circuit that popped them all.

With regards to the voltage fluctuations - time to learn how to use my multimeter and see exactly what it reads for voltage. Could it be a combination of loose ground wires and a bad diode board?

I bow to the collected wisdom the Vintage Forum Members; any and all constructive advice will be deeply appreciated!

Here's a few photos of the bike for your enjoyment; someday I'd love to replace the RS fairing with correct S fairing.

Darryl.Richman's picture
VBMWMO #6285
Surf City, USA (Santa Cruz, CA)
Joined: 10/27/2006
Posts: 2185
When a whole area of the bike

When a whole area of the bike stops functioning electrically, I would immediately suspect that you have a bad ground - a loose fastener for the ground or corrosion or both. You'll want to find a color coded wiring diagram for the year and model bike, and then trace down the ground side of all the lights not working.


VBMWMO Webmaster,--Darryl Richman

khittner's picture
VBMWMO #8223
Joined: 02/15/2012
Posts: 371
Nice Bike!

Sweet bike, Tim. I'd guess a loose or corroded ground wire in the taillight fixture for your loss of all rear lights.



schrader7032's picture
VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6713
Tim - As suggested you

Tim -

As suggested you probably have a loose ground on the voltmeter. And is this the fairing voltmeter? Those are notoriously bad for making actual measurements...need to use a handheld meter at the battery terminals.

Sounds like you may have some corrosion on the pins that supply power to the "dash" or possibly there's some internal corrosion. Interesting, the oil pressure light gets its ground from the oil pressure switch on the left side of the engine. All of the other lights tie to a common ground through the plug-in at the back of the "dash"...that common ground is on the frame.

As for the back half of the bike, does the clock light work? You say the parking light works...both of these get power through a fuse that is common with the tail light. So, it could be a ground problem for the back half of the bike.

I'm here in San Antonio...if you would like a second set of eyes, let me know.


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

timhecht's picture
VBMWMO #9103
Joined: 05/05/2016
Posts: 7
Thanks guys...

I appreciate your suggestions and advice; when I get a minute from getting the house ready for family visitors I will concentrate my efforts to check wiring connections. I have a wiring schematic (downloaded from another post on these forums) and well at least I know what the "ground" symbol looks like!

Schrader7032 - if I really get stuck I'll PM you for your help - thanks for the offer!

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