Horn Wiring - check my work
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    1. #1
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      Horn Wiring - check my work

      With the motor going back in soon, I'm starting fresh with the front-end harness in my 122 this spring... I've had nothing but troubles with the lights and indicators up front since I bought it.

      Somehow, the horn was functioning when I yanked it, but looking at the remains of the harness now has me scratching my head as to how.

      It seems that the relay was wired like this, with 12v supplied to the coil through the brown harness lead, and grounded through the horn ring at the steering wheel.

      Does this look right to ya'll?


      Mike

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    3. #2
      Member LloydDobler's Avatar
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      Yes, that looks right to me.

      Although stock the cars didn't have a relay, power was run straight from fuse 2 to the horns, and the ground was run from there straight up the steering column to the horn ring.
      2003 C70 T5M Convertible - Eibachs, Koni FSDs, Enkei RSF5s, OBX downpipe, Snabb intake, RIP kit, & drop-in intercooler, Quaife LSD, 19T, Green Giants, 22 psi Hilton tune.
      2006 V70 2.5T - Ice White - Oak Arena, (almost) bone stock daily driver.
      1966 122s - Collectible project, restoration and many mods on the way.
      2005 V50 T5 AWD - Daughter's first car. No mods unless she does 'em herself.

    4. #3
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      Thanks Lloyd!

      I'll admit, I can't tell what this relay is actually doing in this circuit, since the 12v is feeding both the coil and the switch... Other than being a relatively fancy switch, this seems like an overly complex solution.

      I'll likely revert to stock wiring here.

      Mike

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    6. #4
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      123GT used a typical (i.e. OD, headlight flasher, reverse light) relay for the horn circuit.
      Power for horn came from the secondary GT-unique fuse box, power for the relay came from the usual #2 8Amp fuse.
      Advantage is it's impossible to cause a live short from frayed wire to horn swich through the steering column using the relay.
      '62 Amazon Original 65K mile time capsule
      '94 MBz E320 wagon 120K miles '92 MBz 300CE 2Dr 180K cold weather miles
      '94 MBz E420 120K miles '86 BMW 635CSi 2Dr 180K warm weather miles

    7. #5
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      Yeah... generally that's my understanding of one advantage of auto relays with potentially high amp applications: use a safely-fused connection across the coil attached to whatever switch you're operating, and your higher current run across the switch to the light, OD, etc. That way, even with a frayed wire, you're going to pop an isolated fuse instead of pulling 350 watts across your fist when you try and honk at some geese.

      Additionally, you're (ideally) minimizing long high current wire runs and saving on expensive high current rated wire and switches.

      With this setup as wired above, since the high-current line is feeding both the coil and the switch, it seems self-defeating... the relay coil source is actuating itself.

      Thanks for the confirmation that I'm not crazy. Back to splicing...

      Mike

    8. #6
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      Mike, just to be clear, there is nothing faulty about supplying both the relay coil and the controlled device (horn, OD, etc.) from the same source, as long as it can supply adequate current for the device alone. The relay itself draws very little when activated by grounding the switch...

      Fusing the circuit to the relay coil is unnecessary, the fuse needs to protect the device controlled by the relay outflow, as that's where the high current flows.
      What's activating the relay coil is grounding the switch...That allows the current to pass to the horn.

      Wired without the relay, when you push the horn button, there's a tiny spark inside the steering wheel, at the horn contact.
      With the relay, the spark is in the relay box...

      Michael
      Last edited by northNH; 03-14-2016 at 09:28 PM.

    9. #7
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      Michael - agreed that it's not faulty to supply coil and device from the same source, but it seems to be negating nearly any potential advantage that the relay could offer. "Inflow/Outflow" is not really a function of a relay if the switch supplying the device is shared by the coil, since the relay is really only a switching device for the current path.

      High current is passed through the entire supply circuit - from battery to device to ground - when called for by the device. In many relay wiring schemes, this high current path (through the relay switch) is specifically isolated from the relay coil. The coil path, with associated human-operated switch, is a unique 12v supply - when actuated, it completes the high-current circuit and operates the device (in this case, the horn). Two distinct battery-to-ground connections, married by the electromechanical properties of the relay.

      In the diagram above, as my horn was wired, all I'm doing is using the human-operated switch (at the horn) to trigger another switch (at the relay), all sharing the same current path to ground. Everything in the diagram "North" of the relay (to the battery) is seeing the current draw from the horns through the relay switch.

      Mike

    10. #8
      Member LloydDobler's Avatar
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      You already said it earlier, the point is to not have high current up the steering column or through the switch in the wheel.
      2003 C70 T5M Convertible - Eibachs, Koni FSDs, Enkei RSF5s, OBX downpipe, Snabb intake, RIP kit, & drop-in intercooler, Quaife LSD, 19T, Green Giants, 22 psi Hilton tune.
      2006 V70 2.5T - Ice White - Oak Arena, (almost) bone stock daily driver.
      1966 122s - Collectible project, restoration and many mods on the way.
      2005 V50 T5 AWD - Daughter's first car. No mods unless she does 'em herself.

    11. #9
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      Mike, I think I understand the source of confusion...

      The two different circuits inside the relay i.e. the control circuit (tiny electromagnets) and the high-amp "points" for the controlled circuit e.g horn, are ENTIRELY separate and electrically isolated from each other. What ultimately allows current to flow to the horn is the magnetic force of the energized electromagnets physically pulling the points to a contact position (CLICK) allowing current to flow from the battery/dynamo to the horn. Hence the term electoMECHANICAL relay...

      Although your posted diagram shows a (dashed) line between the circuits, this represents the magnetic force poduced by the low-voltage coil (you only need ~3 volt battery to energise) that closes the points; it does NOT represent electrical connectivity.

      And remember, all the electrons came from the same battery, tho' through different paths, and everything takes only what it needs and no more, unless miswiring or short-circuitry allows them a quicker way home.

      Hope this helps, or straighten me out,
      Michael
      Last edited by northNH; 03-16-2016 at 06:12 PM.
      '62 Amazon Original 65K mile time capsule
      '94 MBz E320 wagon 120K miles '92 MBz 300CE 2Dr 180K cold weather miles
      '94 MBz E420 120K miles '86 BMW 635CSi 2Dr 180K warm weather miles

    12. #10
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      Mike;

      Lloyd and Michael have it right, but if I may clarify...you need to consider the current paths...and there are two: Control (or Relay Coil) and Load....the point is not to subject the poor horn contact to the high Load current, only a modest relay coil current, and a three terminal relay allows this perfectly...see marked up graphic:


      Only in circuits where Control and Load circuits are NOT supplied by the same source must a relay with separate Coil and Load circuits be used (a four terminal relay)...but that is not the case here.

      See also: http://www.sw-em.com/122_horn_notes.htm

      Cheers

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