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Thread: Voltage Jumping Around

  1. #1
    Brian
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    Voltage Jumping Around

    I'm (still) restoring a 1983 R100RS. I think the guy before me stored it outside in the snow but that's another set of issues.

    The current problem is a voltmeter that hits the maximum voltage peg when I'm at a constant speed (around 4,000 RPM). It spikes up and sometimes gets caught up there. Physically tapping on it brings it down to the more normal 12.8V. Then it'll spike again. I'd say that it spikes about 30% of the time. I'm running an Odyssey battery that's essentially new (~300 miles on it). Interestingly, everything's fine in the mornings when I go to work. It only happens on the way home after its been under its cover (heat issue?).

    I suspect the voltage regulator or it could be worn out brushes (it's got about 52,000 miles on it). Last night I cleaned the connections to the voltmeter. Am I hurting anything if I keep running it this way? All the time the bike doesn't skip a beat and seems to run fine. Has anyone else had this sort of problem? This thing's getting to be the money pit and I'm (frankly) tired of putting more money into it - just want to ride. I'm hoping for a single point fix.

  2. #2
    Administrator 20774's Avatar
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    My first suggestion was that you should check the grounds. When you say you clean the connections...is that at both ends of the voltmeter, ie, at the back of the meter and where it plugs into the wiring?

    I wouldn't put much stock in a fairing mounted voltmeter. Get a simple voltmeter, even a $5 from Harbor Freight, and use it to actually see what's happening at the battery. Use some painter's tape and tape it to your tank while you ride around. Do it safely where you can look at it and not get yourself flattened by a bigger vehicle!!

    I'm still leaning to a burnt out, abuse fairing voltmeter.
    Kurt -- Forum Administrator ---> 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!

  3. #3
    Sir Darby Darryl Cainey's Avatar
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    I wouldn't think it was worn our brushes at 52,000 miles, I had about 350,000 on my RS before having to change mine.

    I suspect the voltmeter!
    Ambassador BMW MOA Ontario Canada
    President Niagara BMW Riders #298
    Knights of the Roundel #333
    1977 R100RS, (Retired) 1993 R100GS (just getting started)

  4. #4
    Benchwrenching PGlaves's Avatar
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    Those volt meters are notoriously inaccurate and sometimes downright wonky. The first thing I would do is verify the voltage behavior with a decent voltmeter. About $12 at most auto parts stores will get a cheap analog one. $20 will get you a decent digital one.

    Then, if you find voltage jumping and diving erratically you should look at the electrical system. But I suspect the problem is the bike's useless volt meter.

    I mostly consider them a period cosmetic accessory.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  5. #5
    Registered User lmo1131's Avatar
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    Use some painter's tape and tape it to your tank while you ride around.
    I put mine under the map "window" on my old tank bag; it worked like a charm. Then, I went into the house for an hour or so and totally fried the LCD display ... it appears they don't tolerate extreme heat :-/

    My $0.02USD
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  6. #6
    Registered User
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    In my opinion, if you are getting a change in the meter reading by physically tapping on it, the first place to look for the problem is in the meter.

  7. #7
    Registered User PAS's Avatar
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    Same thing happened to my 81 R100RT. It would peg until I wacked it then it was ok. I replaced it with a nice cheap VDO gauge from Summit racing.
    The bezel on this one is more closly matched to the BMW unit.
    http://www.summitracing.com/parts/VDO-332041/

  8. #8
    Registered User lkchris's Avatar
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    They all do that.

    The gauges, that is.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  9. #9
    Luddite Looney wecm31's Avatar
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    Quote Originally Posted by PAS View Post
    Same thing happened to my 81 R100RT. It would peg until I wacked it then it was ok. I replaced it with a nice cheap VDO gauge from Summit racing.
    The bezel on this one is more closly matched to the BMW unit.
    http://www.summitracing.com/parts/VDO-332041/
    X2...

    the VDO unit is far more stable and accurate. Does require a McGyver wiring mod to light up the face, no big deal.
    Gerald P
    The last thing I want to be is just like everyone else...
    1985 R80RT
    1969 Sport Fury Convertible

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