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charging diode

Discussion in 'The Garage' started by skillzman1, May 27, 2012.

  1. so bear with me,I'm no electrician.
    I had my buddy look at my wiring harness today to answer a few questions.
    While looking the harness over I found a melted green wire about 4 inches long plugged into the white wires in the harness. He explained that the white wire is part of the charging system and the melted wire was a fusible link of some sort. He said there should be a diode in there to allow the bike to be turned off. Otherwise the kill switch/key etc,.. will not shut the ignition off. What likely happend is somebody pulled a ground some how to shut the bike down and melted the wire?

    at least thats what I think he said....my question is, he showed me in the wiring diagram where it designates a diode....and said I need to find one to install..anybody know what this looks like and where to get one?
  2. TimmyT

    TimmyT XS400 Addict

    I have a '78 and on my wiring harness the white wires go from rectifier diode to the alternator. The dioded is also grounded with the voltage regulator which is also connected by a green wire to the charging system. On my bike both the voltage regulator and the diode are the only 2 components bolted to the battery box. You should check them both out.

    The easiset to test is the diode, use a continuity tester and connect eachcontact to the brown and green wires, it should only pass current one way... the lught or buzzer will be off when connected one way and only be on when cennections are reversed. If the light is on or off for both arrangements then the rectifier is faulty. You will need a voltmeter to test the v-reg.

    check there is still continuity with the white wires, and either replace them one at a time or or if its just melted the poly coating you can save them with a couple layers of heat shrink.

    oh yah sorry i dont have a pic of these parts but the v reg is held by 2 screws and the rectifier diode is held on the battery box by one screw. Good luck
  3. ok...clarification....I found this diode on ebay.
    http://www.ebay.com/itm/390189607762?_trksid=p5197.c0.m619..it looks like the same one I'm dealing with.
    However the 'green' wire is actually a hot wired splice somebody did previously as the diode obviously went bad.

    On my harness it plugs with the factory connectors into the white wire at both ends.
    I also saw a recommendation to substitute this diode fro radio shack.

    my question would be...what would fry the diode in the first place?
  4. I'm familiar with the 2 rectifiers you mentioned as I just relocated them under the tail section today...I will have my buddy test them next time he's over..He told me to be sure to mount the rectifier (the one bolt one) in an area of good air flow to prevent overheating.
  5. bcware

    bcware Well-read Top Contributor

    Too much current passing through will kill a diode (heat). Maybe the replacement was too small or simply overheated because it wasn't cooled adequately.
  6. well the replacement was just an aluminum wire with green shielding...which melted for likely the same reason. perhaps bad ground?
  7. jmd_forest

    jmd_forest Bldg 1980 XS400 Cafe Bike

    Is it possible the diode was installed backwards? That would be like a direct short to ground that will easily burn up the diode and the wire.
  8. BBS360

    BBS360 XS400 Guru Top Contributor

    Old thread, but since it was brought up...

    FYI to anyone reading, the single diode in the wiring harness has nothing to do with the charging system.
    It activates the headlight relay when the motor starts spinning.
    LeBouc likes this.
  9. LeBouc

    LeBouc XS400 Member

    And its function is to make sure that the headlight gets opened ONLY when the alternator is producing an electrical current, usually when the engine runs. That way, the headlight does not drain the battery when the engine is not running and the headlight is effectively opened when you need it!

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