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Thread: Horn question follow up

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    Obi Wan longbridgehealey's Avatar
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    Horn question follow up

    Why use a relay for horn? I have installed a Charlie Hart seven fuse unit, with one fuse dedicated to the horns. Is there any reason to install a relay unit in the horn wiring? My new horns came with one, but Iím not sure I need it.
    Once again, many thanks!
    Larry (again)
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    Re: Horn question follow up

    Quote Originally Posted by longbridgehealey View Post
    Why use a relay for horn? I have installed a Charlie Hart seven fuse unit, with one fuse dedicated to the horns. Is there any reason to install a relay unit in the horn wiring? My new horns came with one, but I’m not sure I need it.
    Once again, many thanks!
    Larry (again)
    Sounds like needless complexity.
    Steve Gerow
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    Obi Wan longbridgehealey's Avatar
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    Re: Horn question follow up

    It did to me too, and I was hoping you would agree👌. And you were right, there was a bare wire grounding the system on the horn circuit, which was not in the trafficator.

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    Yoda Michael Oritt's Avatar
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    Re: Horn question follow up

    Not using a relay means that every part of the circuit(s) must carry the full amperage load, from the power lead to the horn button all the way to the horn itself. i don't know what kind of horns you are using but loads of 5 to 15 amps per horn are common, and wire to carry that load is relatively heavy--I'm guessing that 14 to 16 AWG is probably what would be specified.

    OTOH using a relay allows the use of lighter wires on the circuit that activates the relay as that wire will only carry the load required to actully activate and hold the relay closed, which might be only a couple of amps. And if the relay is located close to the horn(s) the run for the heavy wires can be kept short.

    Best--Michael Oritt
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    Yoda steveg's Avatar
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    Re: Horn question follow up

    One could argue, in the case of the horns, the wires are used very seldom, and for a very short interval - too short to heat up.
    Also there's a separate wire for each horn. Mine mainly get used when I tilt the driver's seat forward.
    Steve Gerow
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    Re: Horn question follow up

    Quote Originally Posted by Michael Oritt View Post
    OTOH using a relay allows the use of lighter wires on the circuit that activates the relay as that wire will only carry the load required to actully activate and hold the relay closed, which might be only a couple of amps. And if the relay is located close to the horn(s) the run for the heavy wires can be kept short.
    The fused wire for the horn is as short as it can be going from the fuse straight to the horn. putting a relay in that line makes no sense. The horn button on the steering wheel is for the ground wire.

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    Yoda Michael Oritt's Avatar
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    Re: Horn question follow up

    It doesn't matter whether it is the ground or the hot wire--if current flows through it the considerations are the same.

    Best--Michael Oritt
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    Re: Horn question follow up

    Use a slow blow fuse.

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    Yoda Michael Oritt's Avatar
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    Re: Horn question follow up

    Or use this--nothing needed at all:
    Attached Images Attached Images

    Best--Michael Oritt
    1954 Austin-Healey 100 Le Mans
    1958 Elva Courier (FOR SALE)
    1959 Elva MK IV Sports Racer
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    Re: Horn question follow up

    I agree with Michael. If the horn did not use a relay then the wire for the button would have to be a larger gauge (even if it is on the ground side) and there is limited room for the wires to run up the traficator tube as it is. Adding girth for a larger wire only worsens the situation.

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    Great Pumpkin Keoke's Avatar
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    Re: Horn question follow up

    OTOH using a relay allows the use of lighter wires on the circuit that activates the relay as that wire will carry only the load required to actually activate and hold the relay closed, which might be only a couple of amps. And if the relay is located close to the horn(s), the run for the heavy wires can be kept short.

    True statement !
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    Re: Horn question follow up

    Quote Originally Posted by steveg View Post
    One could argue, in the case of the horns, the wires are used very seldom, and for a very short interval - too short to heat up.
    Also there's a separate wire for each horn. Mine mainly get used when I tilt the driver's seat forward.
    Same here. The wire sizes shown in charts assume the wire is in continuous operation, such as running a motor. Operating for short intervals allows smaller wires. For example, until engineers figured out that the starter would operate for only short intervals, the wire sizes suggested by the charts made starters too bulky.
    John, BN4

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    Re: Horn question follow up

    Keoke,

    Your statement presents why I installed relays on my upgraded head and driving lights. The relays are fed by a higher gauge line and connected to the main power line from the battery/starter/alternator-generator. Although I do agree that my low use of the horn (averaging less then once a year) and since the circuit is separate (hones alone) and fused, I see little reason to further complexify the horn circuit.

    My thoughts,
    Ray(64BJ8P1)

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