• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Horn question follow up

Share this page

longbridgehealey

longbridgehealey

Obi Wan
Country flag
Offline
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)
 
longbridgehealey said:
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)
Click to expand...

Sounds like needless complexity.
 
It did to me too, and I was hoping you would agreeđź‘Ś. And you were right, there was a bare wire grounding the system on the horn circuit, which was not in the trafficator.
 
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.
 
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.
 
Michael Oritt said:
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.
Click to expand...
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.
 
It doesn't matter whether it is the ground or the hot wire--if current flows through it the considerations are the same.
 
Or use this--nothing needed at all:
 

Attachments

  • HORN.jpeg
    HORN.jpeg
    7.7 KB · Views: 145
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.
 
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 !
​
 
Last edited by a moderator:
steveg said:
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.
Click to expand...
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.
 
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)
 
Back
Top