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'78 R100/7. The charging light stays on now brightly, in varying degrees. But with a voltmeter on the battery and engine revved up to around 3k, the meter rises to near 13.75 volts, which is good. What gives with the idiot light staying on? BIke has new battery, brushes, diode board and rotor.

Randy, are you sure that it is the charge light? Some have confused the two lights.

That voltage is too low for proper charging, but that isn't why the light is on. Do you have the voltmeter probes on the battery terminals and not on the lugs that are bolted to the battery? It should make no difference, but sometimes it does and then you have a problem.

Did you make the modification for better grounding while installing the new diode board?

With the bike not running, do you have enough battery to run the lights while operating the starter with plugs removed to keep it from starting? You are just adding as much load as possible while watching the voltage at the battery. It should not drop below 11 VDC, or so. If it drops so low that the starter won't turn over the engine, then the new battery could be bad. Yes, sometimes a new battery is bad.

From Haynes: Alternator and regulator quick test; This test should only be carried out if the charge warning light remains permanently on while engine is running. Remove fuel tank and engine front cover. With the engine stopped and the ignition switched off disconnect the regulator 3-pin plug. Using a spare length of wire connect the D+ (blue wire) to the DF (blue/black wire) terminals on the plug. Start the engine and increase speed to 1000 - 2000 rpm. If the charge warning light goes out, the regulator is faulty, if it stays on, the alternator is faulty.

The new battery spins the starter like a new battery should. The oil light is orange, the gen light is red. Never have been confused with that. Don't think I've ever seen 14 or more volts over the years on any bike I've owned. Put on new parts about 5k ago and just parked the bike last night after a 500 mile weekend ride without a problem. Temps dropped near freezing outside last night. Got on the bike to ride to work this a.m. and the gen light would not go off as usual. I'll try Timo's idea. Thanks for the help.

I looked for that diagnostic test on my website, but forgot where it is located. A few times a file has disappeared and I have no clue how that could happen.

dwerbil wrote:Don't think I've ever seen 14 or more volts over the years

Neither of my bikes will make 14V and they work fine.

Just got around to look under the front cover. Everything look plugged in; nothing out of the ordinary. Hooked up a volt meter again and started the bike. Gen light still stays on, and with revs, meter would climb to near 14 volts.

I unplugged voltage regulator and plugged in another one of unknown condition. Same thing happened as above.

Started it again a few minutes later (on the premise that something may have changed by itself) and with revs above idle, the light went out and volts climbed up to 16 volts. I unplugged the unknown regulator and plugged back in the bike's regulator. Started bike, revved it, light went out properly, meter peaked at near 14 volts.

Weird. Maybe I should buy a new regulator. Is there a cheap Borg Warner that will work from Auto Zone or the like? I forget the part number.

EUBMW has them for $39.95 + free shipping: http://www.eubmw.com/Electrics.html Did you try the test mentioned above?

I've got this on record

Borg-Warner R-588

Bamboo812 wrote:EUBMW has them for $39.95 + free shipping: http://www.eubmw.com/Electrics.html Did you try the test mentioned above?

Was about to last night, but think the issue is oxidized lugs on the reg. The gen light started to behave with the system charging properly. I'm going to brush them up with a nail file. Picked up another regulator and will keep on-board one as a backup.