1. gm1236

    gm1236 Well-Known Member

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    Heres the deal, ‘75 duster, had no horns or wiring. There is a green wire/red stripe, female connector that ends behind the battery. Schematic shows this is the horn wire. It’s live all the time???? Should this only be live when the horn relay is grounded by the horn button?
    How can I test the horn relay?
    Where should I go from here?
     
  2. 67Dart273

    67Dart273 FABO Gold Member FABO Gold Member

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    I'd start by unplugging the relay. I'm surprised it is not running the battery down. You have a shop manual?

    Horn relay connector has 3 wires, violet is constant battery hot, horn is gren/red goes out to horn(s), and small black goes back to column connector and to horn switch. It should ground only when horn ring is pressed
     
    Last edited: Oct 20, 2020
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    • gm1236

      gm1236 Well-Known Member

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      Thanks, nothing is plugged into the wire, so I guess there’s no draw on it. I’ll see if I can get at it, it’s up behind the instrument cluster.
       
    • 67Dart273

      67Dart273 FABO Gold Member FABO Gold Member

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      I don't know what wire you mean. If the relay is connected under the dash, horns or no, it CAN draw if the horn button is shorted. What happens is, that with no horn, and with the horn button shorted, the relay engages and causes a draw on the relay coil and you won't be aware of that---except the battery goes dead.

      The ignition switch does not disable the relay
       
    • Northern Grit

      Northern Grit Well-Known Member

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      The horn itself lives underneath the battery tray and might still be there, probably got stuck on and someone unplugged it and pulled the wire up. The relay lives behind the glove box,
      20201021_211644.jpg 20201021_211632.jpg
      Unplug the relay and check the black wire for connection to ground, if it only grounds with the horn pressed ( normal ) then you most likely have a stuck relay. If its ground all the time then you most likely have a short in the horn switch or column, and lastly the green and red wire could be shorted to a 12v source in the harness.
       
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      • gm1236

        gm1236 Well-Known Member

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        Thanks guys, I’m going to dig into this weekend.
         
      • BobW

        BobW Curmudgeon At Large

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        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          Additional, not relevant to this thread. For you guys using "Bosch style" relays pay attention to the coil wiring. Most? All? Bosch style relays have internal spike supression or flyback diodes installed, so this makes the coil polarity sensitive and if you wire them reversed you will blow up or short the diode.
           
        • BobW

          BobW Curmudgeon At Large

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          I only posted that because most mechanics are not electricians, and think relays are some sort of Voodoo!
          Long time ago I took a course on basic digital electronics while I was unemployed. When it came to explaining capacitors, the instructors words were "Magic, pure frickin magic"!
           
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          I barely knew much, although I was a General class radio amateur, in 68 when I went to Navy electronics tech "A" school, Treasure Island, San Francisco. They had a pretty good course at that time, but nowadays, reading what they teach currently, it seems more like "black box" and very little component. Unfortunately I went through one of the last vacuum tube courses in the school, and soon after was converted to "all solid state." At the time we only got maybe 3-4 weeks of solid state theory, the rest of the school was vacuum tube stuff

          I fixed GCA (ground controlled approach) or PAR RADAR and it was pretty much all vacuum tube. The receiver had been upgraded to a solid state system, partially, but the rest of the thing was (hundreds) of tubes
           
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