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Thread: 78 R100S Horn-Neutral Switch not working. Are they related?

  1. #1
    Join Date
    May 2010
    S.E. PA

    78 R100S Horn-Neutral Switch not working. Are they related?

    I removed the Hannigan fairing from my 78 R100S - had to take it off to get access to the headlight wiring to replace right handlebar switch. While it was off I decided to go naked for a while. Replaced both left and right switches on the handlebar. When I powered up the neutral light is on and does not go off and the horn does not work- everything else functions as it should. I have done some quick assessments of the wiring and switches and all looks good so I am getting ready to open things up and see what I can find. Two questions I am hoping to find out before I dig in.

    I read somewhere that these two circuits have some diode behind the board in the headlight in common - could this be the problem? I cannot find whatever article I read about that and certainly might not have my usual perfect recall about what I read as I did not have any issues at that point.

    It seems unusual that the neutral switch would get stuck on. I believe this bike has the switch that engages when the plunger is pushed in any ideas of things to check before I pull the trans and replace the switch?

    Thanks for any ideas.


  2. #2
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    Roy -

    My take is that they could be related. Check this diagram out:

    You might be right about the diode in the headlight...see 85b. The horn and the clutch switch (on the handlebar) are connected on either side of this diode. I don't have a great understanding of DC electrics, but I'm thinking this is worth checking out.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User Guenther's Avatar
    Join Date
    Jan 2007
    Colorado Springs, CO
    For to check the neutral switch disconnect the wire at the bottom of the switch which DOES NOT go to frame/ground. If the light goes off while the transmission is in neutral then it is the switch.

    The horn may be broke. Check with a voltmeter at the horn connectors that you get 12V while you press the horn button (with your 3rd arm ). If you get 12V it is the horn else...

    The Horn, the brake lights and the voltmeter are all tied together at their 12V supply. Do the brake lights and the voltmeter work? If not then check the 16 Amp fuse else it might be the horn switch or the wire connection to the left side handlebar switch.

    Last edited by Guenther; 10-15-2015 at 03:55 PM.

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