Replace gen light for resistor?

Discussion in 'Airheads' started by oJOEYo, Apr 27, 2014.

  1. oJOEYo

    oJOEYo Adventurer

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    Hey All,

    I've been working on my R80. I've mounted a acewell 2853 tacho and installed everything. Everything is working fine but the gen light. I used a regular led, wired it to a resistor and wired that to the gen light wires.

    The gen light doesn't come on. I am the first suspect as I think I've switched + and - on the LED. However, next to the light not working, can it do any harm?
    #1
  2. BMW Kurt

    BMW Kurt Philosopher

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    I'm no electrical genius, but I have been told that the gen light is an integral part of the charging system. I'm not sure an led will work. I think it needs a filament to complete a circuit.

    Someone with a lot more knowledge than I will step up shortly, I'm sure.
    #2
  3. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    The Alternator bulb is necessary. A 330 ohm 1/2W resistor in parallel or in place of the bulb does the trick.

    --Bill
    #3
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  4. oJOEYo

    oJOEYo Adventurer

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    This is how I wired it: Could it be that I switched + and - on the led? I had the bike running last week..

    [​IMG]
    #4
  5. Disston

    Disston ShadeTreeExpert

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    I also qualify as an ametuar when it comes to elec.

    I think the resistor needs to be in series. Otherwise the elec can go thru the LED without going thru the resistor.

    I believe LED have a polarity. Otherwise why call them "Diodes"? If the diode is hooked up backwards it probably blows the diode. Most test meters these days have a diode checker. You should be able top check to see if it is still good.

    Other than that your diagram makes no sense at all.
    #5
  6. dm635

    dm635 I Roll

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    Been driving old BMW cars for a while now. The Gen light works the same. No light no charge. I believe starting in '87 there's also a resistor across the bulb holder or the socket to eliminate the charge problem. Still a good idea to replace a bad bulb though.

    My son picked up an '85 635 the guy had parked for years due to a charging problem. You guessed it, bad Gen bulb. That wasn't the only problem with this car.
    #6
  7. oJOEYo

    oJOEYo Adventurer

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    I used this scheme [​IMG] from the airhead acewell thread. My scheme doesn't look that much different right?
    #7
  8. Padmei

    Padmei enamoured

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    I've got an LED wired in with my elctronic Danmoto speedo as the idiot lite. It is wired in parallel with my park lamp in the headlite as the charge lamp/resistive load as I can actually see it (if I get off the bike & look at the headlite) if it is working. A resistor won't tell you if it is faulty & LEDs I've found aren't the most reliable indicator.
    The LED is dull when charging unless I turn the lites on when it then disappears as it should as they take very little voltage to retain a certain amount of light output. Secondly the quality of the LEDs I've been using hasn't been that good. I would try another LED or failing that try a lamp in its place to see if you are getting voltage.
    #8
  9. motor_chris

    motor_chris Been here awhile

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    I picked up a 4-watt incandescent bulb and wired socket from Oreilly's for 10 bucks, and it works just the same as the instrument light. I can see the bulb turn on and off based on engine RPM.
    #9
  10. FrankR80GS

    FrankR80GS Been here awhile

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    You're right! Moreover, LED are not simple ohmic loads but need a specific voltage to allow a current flow.
    #10
  11. Flash412

    Flash412 Elk Biffer

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    Put the 330 ohm resistor in the circuit as pictured to excite the field of the generator. To use an LED as a GEN light, you need to put the LED and a 1K-ohm resistor IN SERIES with each other and then put that series-combination IN PARALLEL with the 330 ohm resistor. If the LED doesn't light up, turn it around because you got the polarity bassakwards.
    #11
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  12. oJOEYo

    oJOEYo Adventurer

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    It's weird that I've never seen it wired in the acewell thread with an extra 1k ohm resistor!
    #12
  13. chollo9

    chollo9 Screwed the Pooch

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    I used an LED from Radio Scrap with a resistor molded into the unit, wired it as you would the original, and everything has been working fine for thousands of miles.
    #13
  14. regomodo

    regomodo Been here awhile

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    The 330ohm resistor value doesn't have to be adhered to. I believe there is a range in which you can can pick your charging point w.r.t RPM. I didn't have 330ohm in my kit so I used 370. No issues.
    #14
  15. StmbtDave

    StmbtDave AKA Invisible Dave

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    Understand that the 330 ohm resistor being in the circuit has nothing specifically to do with using an LED. The resistor being in parallel with the bulb, be it LED or incandescent, is to provide a circuit to the generator should the bulb burn out. The 330 ohm size was originally calculated for a circuit with an incandescent bulb so I'm not sure whether it is correct for an LED circuit or not :dunno. I would suggest initially not using the resistor in parallel, get the LED to work properly, then wire in the resistor and double check that the LED still illuminates.

    Dave
    #15
  16. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    Provided the wiring is done in accordance with the diagram(s), the LED polarity needs to be reversed for the LED to illuminate.
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  17. nothing

    nothing Been here awhile

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    Said resistor is required when using a natively-low-voltage LED in a 12 volt system, as this will limit the current so as to not blow the LED. Whoever is making diagrams without that resistor specified is assuming that you will use an LED with an integrated resistor sized appropriately for 12 volt use - these LED are usually labelled as "12v LED".
    #17
  18. oilandblood

    oilandblood n00b

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    hey do you happen to have any info on that light?
    I have quite a problem trying to look up lights and resisters at most auto parts stores unless I have a item number. or something.
    #18
  19. motor_chris

    motor_chris Been here awhile

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    I think I still have the package it came in, I will check for you.
    #19
  20. Biebs

    Biebs Been here awhile

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    Remove the led and just have the resistor in the circuit.

    Then start it up hook voltmeter to battery and rev bike 3000 rpm see if battery voltage goes up.

    The resistor replaces the bulb but when you add the led in parallel you change the resistor value.


    I believe you will need the 12v led with resistor built in on a in line resistor.
    #20