Help! Lights only working when accelerating

Help! Lights only working when accelerating

This is a discussion on Help! Lights only working when accelerating within the Technical forums, part of the Forums category; Hello everyone! Im new to the forum but not to cafe racers! Some background info: Started with a Kawasaki KX750 then moved to a Yamaha ...

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  1. #1
    Junior Member jamesderek's Avatar
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    Apr 2019

    Help! Lights only working when accelerating

    Hello everyone!

    Im new to the forum but not to cafe racers!
    Some background info: Started with a Kawasaki KX750 then moved to a Yamaha xj550 seca and now I Currently have a 1982 Yamaha XV750 Virago and a 1980 Honda CX500.

    Im currently stuck on my 82 XV750 and need help. So everything on the motorcycle was upgraded and works properly until you start it. Once I start it, the speedo and headlight shut off and then start flickering. As soon as I start giving it gas they stop flickering and start working properly. Battery is brand new. Good grounds everywhere. Is it the stator or rectifier? Any way to check before I start ripping it back apart? Any help would be great!


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  2. #2
    Senior Member TrialsRider's Avatar
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    Jan 2011
    Ontario, Canada
    Curious, because if your battery is really good, then it should be able to provide a consistent voltage to your lights when your alternator can't keep up to the demands.

    Check your grounds (with a meter), electricity is always trying to get to ground and if it can't it's not happy. Motorcycles have lots of ground wires that return to the battery ground, because so much stuff is rubber mounted.

    Yes, you have an alternator which includes a stator and that likely puts out 1, 2, or even more likely 3 phase AC current via what would typically be 3 yellow wires coming out of the engine casings somewhere. Your service manual contains an electrical schematic diagram, which you should obtain immediately to help locate those wires as well as the regulator and rectifier which is required to convert the AC current produced by your motor into a nominal 12volts DC that can work on your battery and electrics.

    You don't likely need to take much apart, you just need a multi meter and an understanding of how to test with it.

    ... and this is where you tell us; oh ya, and by the way, I rewired the whole bike and installed LED stuff in place of the original resistive load lighting :|
    Last edited by TrialsRider; 04-18-2019 at 06:07 AM.

  3. #3
    Ews is offline
    Senior Member Ews's Avatar
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    Jun 2015
    Lewiston Idaho
    As TrialsRider said, lights should run off the battery, so if it's good then there is an issue with a ground, or the wiring to your headlight. I am not super familiar with how Yamaha does things, so is there possibly a voltage regulator in line that is bad? Check your charging system too, when running put the ol' voltmeter to the battery, it should be reading approx 13-14 volts DC at idle. Anything less is not enough to power the system and charge the battery at the same time.
    Dream as if you live forever, Live as if you die today.~ James Dean

    I feel the need, the need for speed.~ Mavrick (Tom Cruise before he was a big ol' bag of crazy)

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  5. #4
    Senior Member woodsman's Avatar
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    Mar 2016
    It sounds like a wiring issue. If you have the juice to start the bike it should run the electrics regardless of whether the charging system is working. It looks like you have a connector not seated by the breather, may not be the problem. Where does the power for the lights and speedo meet up with the source?

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