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Thread: Another charging question

  1. #1
    Little Egypt Airheads
    Join Date
    Jan 2006
    Beautiful Southern Illinois

    Another charging question

    I just bought a 1995 R100RT. At time of purchase voltmeter was showing about 12.5v at road speed. Prior owner had checked with Lap and was told as long as voltage stayed at that level things would be ok, at least for a while. After purchase I immediately road the bike almost 800 miles home. After the first 200 miles and upon start up after sitting over night voltage only showed 12.1 and, slowly, slowly, over the next few hundred miles, voltage dropped to a low point of about 11.3. Being still 200 miles from home I began to think I should visit the local WalMart and buy an emergency battery charger in case the system failed completely. Then suddenly the voltage increased again to 12.1, then back down slowly to 11.3, then up, then down. I made it home ok. The bike never failed to start and never showed signs of electrical malaise. The generator light operated normally, only coming on occasionally at low idle. Voltage would drop to under 11 at idle. Does anybody have an idea what might cause this? My feeling is it might be the voltage regulator but I don't know. Seems to me diodes would either work or not, too. The readout on the gauge was also compared to a test instrument and found to be accurate.

  2. #2
    Guest Scroll down and you will see that 12V is a dead 12V lead acid battery. At that voltage the starter might just click but never crank the engine.

    Your test instrament must be giving a false reading. OR the reading was taken at the bike's voltmeter and the bike's voltmeter MIGHT have a bad connection causing a voltage drop. Take voltage readings at the battery's posts not the connections. Any bad connection/ground causes voltage to drop across them and these drops add up as more bad connections get crossed as the current moves down stream.. I have found bad battery cables that the connection had gone bad internally. One of the best products to use on old electronic connection. Radio Shack might carry it.

  3. #3
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    If the voltmeter is deemed to be accurate and properly reflect what is happening at the battery, anything below 13v is too low to sustain adequate charging. From the numbers you're providing, it appears that the charging system is not working or there is something wrong with the battery. Make sure the battery is known good...put it on a trickle charge to get it up to the proper voltage.

    You'll probably need to check the resistance of your rotor...I don't recall what the later rotors were supposed to read...probably something like 3-4 ohms. You light the brushes off the slip rings and slip in a business card or something thin. Then measure across the rings. You're basically measuring the resistance of a very long piece of wire that has its terminals on either ring. If the resistance is 0, then the rotor is open and you'll never get any charging.

    There are checks for the voltage regulator as well. These test bypass the regulator and monitors the voltage at the battery. If the charging system is working and the regulator is bypassed, the battery voltage could read as high as 16v with the engine RPM up around 4K.

    The brushes could be getting too short and not making contact with the slip rings. Of the springs that push the brushes down might be hanging up. Hard to believe this would be a problem with such a new bike...probably with few miles.

    Sounds like your alternator light is working OK. It should be on bright with key on, engine not started. It will continue to glow until engine RPM reaches around 1500 RPM. It should not come on at all when the engine is at speed.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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