Power Loss With lights on
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Thread: Power Loss With lights on

  1. #1
    Junior Member changos's Avatar
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    Power Loss With lights on

    Hi guys,

    im having a bit of trouble with my 87 TW.
    With the lights on, the engine misfires at high revs and has really low power (max speed 60kmh), but if i turn the lights out the thing turns in to a Beast and has loads of power.

    Currently when i have the lights on on idle, the lights are really low and only if i rev up the engine they glow bright.

    Any similar issues here in the forum? Could it be the generator? I also thought of buying LED headlights, but im not fixing the main issue.

    Hope you guys can help.

    Cheers, from Santiago de Chile.
    I will post a pic of the bike soon.
    SanDue likes this.

  2. #2
    Senior Member Fred's Avatar
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    Welcome changos.
    The 1987 TW's unique alternating current lighting circuit likely requires some intelligent diagnosis from an experienced owner.
    Someone here hopefully can help who has knowledge of the '87.
    Any more information on the history of this problem? Is the issue new and possibly occurred after some change?
    I would suspect poor ignition caused by headlight circuit short. Can a new, proper replacement headlight element be installed as a test?
    I would try to inspect all the associated wiring for a possible short , worn insulation, etc.
    SanDue likes this.
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  3. #3
    Super Moderator Purple's Avatar
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    Hmmm – check to see it the PO has spliced in a DC input to the lights – this can be done by testing for either AC or DC at the headlight – it should be AC on an ‘87

    I’m thinking that if he’s spliced a DC input that’s shared by the CDI, this might explain what’s going on

    If you do end up going the LED route – you’ll need to know whether you’ve got an AC or DC feed anyway ……..

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  5. #4
    Senior Member Badgerflorida's Avatar
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    How are you turning the lights off? Tdubs have never had a factory switch for on/off to my knowledge. Only low and high beam switch. Are you unplugging the connector?
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  6. #5
    Senior Member elime's Avatar
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    Normal for lights to get brighter as you rev it. At a low idle the front light will just about go out. Completely normal for an '87.

    Couldn't a switch be placed where the red arrow is pointing for an on/off switch for the headlight?
    Wiring diagram for an '87 TW:

    '87 wiring diagram switch.jpg
    Last edited by elime; 05-15-2019 at 02:33 PM.
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  7. #6
    Junior Member changos's Avatar
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    Quote Originally Posted by Badgerflorida View Post
    How are you turning the lights off? Tdubs have never had a factory switch for on/off to my knowledge. Only low and high beam switch. Are you unplugging the connector?
    Its an original switch. I think older models do have a on/off light switch, and another for low or high beam.

  8. #7
    Senior Member Fred's Avatar
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    Possibly TWs sold in Chile might have been a little different than US models thus explaining the on/off switch.
    Purple may be on to something with looking for any wiring changes though. I would look there first and remove anything non-OEM added to the light leads and re-connect if you so desire to another key-on 12v DC source.
    Otherwise might Cyclomoto or Yamaha Moto in Santiago offer either helpful advice, or a '87 wiring diagram copy? Then you could compare to the one elime provided or save for your records.
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