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Horn performance 72 BGT

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Lincspeed Avatar
Lincspeed Silver Member Clark Lincoln
Dexter, Michigan, USA   USA
1972 MG MGB GT "Greenmeanie"
Put some new horns in (one of the originals was dead) they look just like the original, but made from plastic not pot metal... Anyway after fabricating a functional grounding pin in the steering wheel hub for my aftermarket wheel, the horns worked fine - both high and low note at full blast. After driving around for a few days, I go to blow the horn and get a half assed moan out of one and the other goes full bore. So I reversed the horns and fiddled with the wires a bit ( clipping onto the horns in reverse order) and got it blowing WFO both horns. Then after some driving around, again I get the crap half honk from one and the full from the other...Sounds like crap.

Any ideas?

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BillzB Avatar
BillzB William M
Batavia, Illinois, USA   USA
1974 MG MGB
Sounds like your horn button is not carrying enough current to run the horns. The easiest solution would be to just add a horn relay (search the technical library). All modern cars have horn relays, it's a lot to ask of a small switch (even the original stock one) to carry all the current necessary to run a horn.

KozyB Avatar
KozyB George Kozak
Philadelphia, PA, USA   USA
1971 MG MGB
x 2 on the horn relay. After cleaning all the connections, my horns still worked intermittently. After installing a relay in the circuit, the stock horns not only work, they are louder than I ever imagined they could be.

Best regards,

George

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Lincspeed Avatar
Lincspeed Silver Member Clark Lincoln
Dexter, Michigan, USA   USA
1972 MG MGB GT "Greenmeanie"
Thanks guys - makes sense to me. When are you inserting the relay, and what size/spec relay? (I have a bunch from my old race car builds...)

Ahmed Avatar
Ahmed Silver Member Ahmed EL Abasiry
Chestermere, Alberta, Canada   CAN
1958 MG MGA
1972 MG MGB
1979 MG MGB
X100. you need a relay. You certainly don't want THIS to happen.

Here's how to wire it. Fairly simple if you do it in the RH side of the car by the RH headlight where the front harness splits.
http://www.mgexp.com/phorum/read.php?1,2767692,2767827#msg-2767827



"... the motor car, after woman, is the most fragile and capricious thing on earth." - London Daily Mail 1908

Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
I've got the same plastic horns but already converted to relay operation with the old metal ones still in. Both types give one hell of a sound level with a relay nearby, pulling the earthwires to mass, + feed wires from the fusebox are thick enough already.

PS Horns have a very high starting current and exhibit an inductive load, putting a high strain on the steeringwheel switch

KozyB Avatar
KozyB George Kozak
Philadelphia, PA, USA   USA
1971 MG MGB
Hi Clark, the relay I use for my horns is a Bosch-style four pin SPST 12VDC/40A. I picked it up years ago at either a Radio Shack or an automobile parts store.

Forty amps may be overkill, but the thing weighs (and costs) next to nothing.

Best regards,

George

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Ahmed Avatar
Ahmed Silver Member Ahmed EL Abasiry
Chestermere, Alberta, Canada   CAN
1958 MG MGA
1972 MG MGB
1979 MG MGB
If you mount it behind the rad support (engine side), you can use a standard automotive relay like this one:
https://www.amazon.com/Ulincos-Relay-U1914-14AWG-Harness/dp/B06ZY84HZL

But if you mount it in front of the rad support panel (facing the road), then I would suggest a weatherproof relay+base such as this
https://www.amazon.com/HELLA-007794311-Relay-Weatherproof-Bracket/dp/B004KEKYWQ/
https://www.amazon.com/HELLA-H84708001-Weatherproof-Relay-Connector/dp/B000VU5ITM



"... the motor car, after woman, is the most fragile and capricious thing on earth." - London Daily Mail 1908

Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
Peak current may well lie somewhere between 30 and 40 Amps for the double horns, but most ISO relays have 40A on NO (30-87) and 30A on NC (30-87a) contacts.
Bosch are fine, but also some others like my Goodsky and FRC. It would not surprise me if all come from China despite their German spec sheets

See Goodsky in the appended doc BTW Prices for single relays are in the 5€/$ ballpark eye rolling smiley

PS For horn and HB/LB operation normal connection is quite enough, for some other applications adding a connector aids fast exchange in case of failure
The relay preventing starting in gear (Astley's idea) and my own invented addition of a wiper switch-over relay are both on connectors. The last one saved my vacation on an unknown wiper interruption


In reply to # 3591465 by KozyB Hi Clark, the relay I use for my horns is a Bosch-style four pin SPST 12VDC/40A. I picked it up years ago at either a Radio Shack or an automobile parts store.

Forty amps may be overkill, but the thing weighs (and costs) next to nothing.

Best regards,

George

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Attachments:
504209-da-01-de-Relais_GRL_S112DF_12VDC.pdf    160.4 KB

greysailor Avatar
greysailor Silver Member Grant Hill
Cypress, CA, USA   USA
1972 MG MGB "Vroom"
1974 MG MGB GT "Pumpkin"
I just completed installing new horns and some relays.
I used the relays from Amazon with a built in fuse socket. Helps reduce the wiring,

https://www.amazon.com/gp/product/B01NBAO1SA/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

I installed them under the slam panel, on the right side. Then a plastic cover to protect from moisture.

Good Luck.


Attachments:
BGT Fuse.jpg    54.1 KB
BGT Fuse.jpg

Power Relay.jpg    48.3 KB
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Lincspeed Avatar
Lincspeed Silver Member Clark Lincoln
Dexter, Michigan, USA   USA
1972 MG MGB GT "Greenmeanie"
Thanks to all for the relay tips. Found 3 old race car build relays (all 30 Amp) wired one up and damed if I didn't nearly break the windows in the garage!eye popping smiley.

So what problems might I see not using a 40 Amp relay? I matched all wire sizes in the hook up.

Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
Maybe none, constant current is not so high and activation of the horns is hopefully not so frequent. My peak current estimate is not based on any measurement, moreover not only height of the pulse, but also its duration is relevant to relay contact longevity. And manufacturters normally factor in some derating anyhow, 30 need not be the exact longtime guaranteed value..... eye rolling smiley

In reply to # 3592451 by Lincspeed Thanks to all for the relay tips. Found 3 old race car build relays (all 30 Amp) wired one up and damed if I didn't nearly break the windows in the garage!eye popping smiley.

So what problems might I see not using a 40 Amp relay? I matched all wire sizes in the hook up.

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