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Took the 78 R100/7 for a short ride to work this morning and when shut her off there was battery fluid pouring out. Turned it back on and I checked the volt meter on the bike, and at around 2500 rpm the voltage is 18V!

recent work I have done... tear down and cleaning of starter. Replaced both positive, and negative battery leads with Motorrad Electric wires (beasty). Replaced instrument Cluster with digital speedo w/ integrated volt meter.
Wired a 50 Ohm 10 watt resistor as per "method 2" in Snowbum's article on adding the functionality of the generator circuit using an LED and resitor found here (http://bmwmotorcycletech.info/genlampresistor.htm)

The LED lights up when key is on and fades out around 1500 rpm.
That seems to be working as hoped/expected.

Volt meter on new gauge reads 18V when reving engine! but settles down to just over 12 at idle.
Kinda stuck and need to get the bike home to the garage and diagnose! (bout 7 miles away country road)
she runs but I need to get those volts down so as not to fry anything and keep the battery from boiling more.
Any ideas?! What might cause the sudden spike in voltage?
Thanks in advance.
-Chris

Usually a runaway voltage points to the voltage regulator. There are several auto-types that will work.

Kurt in S.A.

If not a wiring fault it will be a shorted voltage regulator applying full current to the rotor. 18 volts is what you get when that happens. To get it home turn on every electrical appliance you can which will drag the voltage down and keep the revs as low as possible. Top up the battery if you can as it may yet survive.

Thanks guys!
I have a scary thought that creeped into my brain... Bear with me.
The plug to the voltage regular has 3 spades.

Black wire to DF
Green/blue wire to D+
Brown wire to D-

In Snowbum's description I had to tie one end of the resistor of the updated generator light circuit to the D+ (green/blue) wire at the Voltage regulator. To do this, I removed the wires and spades from the little black plastic plug.
If I had accidentally reversed the wires connection (ex. Green/blue to DF, and Black wire to D+) when I reconnected to voltage regulator, would I get the problem I am having?

Correct me if I'm wrong, but couldn't he disconnect the regulator to eliminate the charging system and limp home on the battery alone?

Limp home 7 miles? Should be no problem, unless the battery has been damaged to some degree.

Kurt in S.A.

sinjun68 wrote:Thanks guys!

If I had accidentally reversed the wires connection (ex. Green/blue to DF, and Black wire to D+) when I reconnected to voltage regulator, would I get the problem I am having?

I think it might.

Either way no big deal to get home.

Yeah, I would just disconnect the small red wire at the battery. Cover it so it doesn't short.
Maybe unplug the headlight, and keep the revs up if you have to stop.

Thanks for everyone weighing in.
I had indeed misread the schematic on top of the regulator. I interpreted the 3 spade connectors in the schematic as though I were looking down through the regulator as opposed to what I would see if I flipped the thing over and looked at the blades directly. A switch of 2 wires and the she's running strong and volt meter while riding reads 13.8 - 14 volts!
All is good in the world again!
Thanks again folks!
-Chris

Don't trust the original voltmeter. use a good accessory one to check the charge voltage. Run it in neutral, sitting still with the meter connected directly to the battery, not to the lugs on the battery. Rut it around 2.3-3 k rpm until the reading stops increasing. That is the charge voltage and it should be 14.1 to 14.2, depending on riding style and length of ride.

For more, go to http://www.w6rec.com/duane/bmw/battery/index.htm and scroll down 3/4 way.