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Thread: LED Frustrations

  1. #16
    --Tony AnnapolisAirhead's Avatar
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    May 2007
    Watkinsville, Georgia/Annapolis, Maryland
    I found this site to be a big help calculating resistor values:

    Measure the existing resistance with the incandescents, then plug in the intended LED, power source (12v), etc. and it will give you the resistor value you need to add (I soldered the resistors to the positive lead).
    Last edited by AnnapolisAirhead; 12-19-2009 at 02:56 PM.
    1971 BMW R75/5 | 1975 Moto Guzzi 850T Cafe | 1983 BMW R100RS | 1988 BMW R100GS
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  2. #17
    Cage Rattler wezul's Avatar
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    Nov 2006
    Glen Ellyn, IL
    I decided to pop in an LED tailpiece on my F8ST. Turns out the Canbus tests resistance on all bulbs on the bike and even though there are numerous LED's on the replacement board the resistance is still lower than the incandescent so the bulb test fails and I get a bulb failure indicator on the OBC.

    Not a major big deal but I have not been able to figure out the resistance difference nor obviously a solution. So the board sits idle.

  3. #18
    Honey Badger Semper_Fi's Avatar
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    Aug 2008
    Kingston, NH
    If you add a couple of jumper wires (in series ) to your exisitng resistor and the buld load you caan get a range of resistors you could plug them in one at a time until you get to a resistance value the computer will accept.

    A little clugey (sp?) but it will get you where you need to be and then once the value is identified replace it with an same as or better value.

    Default to higher value if you can't get common resistor value that will work.
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  4. #19

    Some clarification

    Just in case anyone is attempting to measure LED module resistance. These are semiconducting devices which don't behave like resistors. Measuring them with a meter will not provide useful information applicable to this problem.

    I suppose it's possible the bike is measuring resistance, if so, there will be trouble fooling that feature into thinking there is an incandescent load. It's far more likely that the bike is monitoring load current.

    This schematic shows what you need to do to match the LED current to the incandescent current.

    My previous post(#15) tells how to determine the value.
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    Mike Marr
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  5. #20
    Cage Rattler wezul's Avatar
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    Nov 2006
    Glen Ellyn, IL
    That's helpful, thanks Mike!

  6. #21

    Thumbs up Yup, more load current

    Motorrad Mike's got it. On K bikes, I've successfully used 2 x 22ohm 5watt resistors in parallel; yields 11 ohms OR ~1.+ Amp. Stock is 21W or~1.6A per bulb (not accounting for incandescent characteristics: inrush V stable on).
    I cheated on my HR (OilHead bike pix #36): took a Dremel cutter & shaved narrower the current shunt strip in the flasher. Still looking into an equivalent "fix" for K bike flashers (easier to add resistors, less elegant). Good Hunting... <<<)))

  7. #22

    Here's another option (posted before). I din't like the idea of wasting power just to heat the resistor. The plan for me was to increase the amount of light using more efficient options. Using the existing light bulbs helped the system work as designed...

  8. #23
    Registered User bmweuro's Avatar
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    Oct 2009
    Northern Cal
    Check out these LED Turn Signals

  9. #24
    The local dealer wasn't interested in messing with the bike, and a local independent who used to be a BMW mechanic didn't want to try, either. So I found a guy who would "try". Additional reisistors in any configuration only dimmed the bulbs. I printed and brought him both diagrams above, to no avail.

    Back to square 1, or removing the LEDs and replacing the incandescents. Still open to suggestion.


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