Alternative horn switch

[Mr Spigot]

The horn on my 1952 MM is the original, but emits a rather high pitched peep. While it is adjusted properly, I had thought of adding a second horn with a beefier tone, but on a switch so I can to swap between them still using the horn push on the steering wheel. Conscious of the high current involved, I wondered what type of switch I should use.

[mike1864]

Consider fitting a nearby changeover relay to toggle the battery feed between the "hot" terminals of the two horns.
The drive to this relay's coil can then be in thin cable and fed from a simple, low current, switch; to cosmetically match other switchery on your dash.
Match the rating of the relay to your beefier horn's spec. (perhaps 10+ amps?).
Though you won't be stressing (arcing) its contacts too much as long as you don't operate it while pressing the horn button.

[Mr Spigot]

Thanks for that. I'm not good at electrics, but looking at the circuit diagram below am I right in thinking the following connections are correct?
30 - live feed
85 - live feed to switch (thin cable)
86 - switch connection to earth (thin cable)
87 - feed to horn 1
87a - feed to horn 2

[Bill_qaz]

85 and 86 are the coil of the relay one to earth and one to switched supply, either way around works.
Or put supply to one and switch the other to earth.
87 will have an output when coil energised (relay activated)
87a will have an output when relay coil is not energised (relay at rest)
Hope this helps

[Mr Spigot]

Great. Many thanks.

[geoberni]

I'm just curious as to if you're overly concerned.
The original wiring was sufficient for two horns on many cars intended for export....

[Mr Spigot]

The only reason I asked is because the original horn is rather pathetic and I would like something a bit more effective, but did not want to take out the original horn. I can mount a modern horn up under the bonnet which can be used in modern traffic but a flick of the switch reverts to the original 1952 horn.
My 1960 car has twin Windtones and they can blast the paint off a car in front!

[geoberni]

The only reason I asked is because the original horn is rather pathetic and I would like something a bit more effective, but did not want to take out the original horn. I can mount a modern horn up under the bonnet which can be used in modern traffic but a flick of the switch reverts to the original 1952 horn.
My 1960 car has twin Windtones and they can blast the paint off a car in front!

OK, and I'm just asking questions here, neither expressing approval or disapproval of whatever you choose to do to your car.

So you fit the car with 2 horns, which you can switch between...
To what purpose, a 'new one' for normal use and a 'pathetic' old one that is only used at shows to show that it's 'pathetic'?

You're not going to have the opportunity to change between them when the use is actually required, to warn someone of immanent danger...

I wonder whether the original one is as serviceable as you say.
Is the 75 year old diaphragm as good as when it was made....

Have you seen this document? https://www.mg-cars.org.uk/imgytr/pdf/altette_horn.pdf

[Mr Spigot]

To what purpose, a 'new one' for normal use and a 'pathetic' old one that is only used at shows to show that it's 'pathetic'?

Essentially, yes. The original horn is quite high pitched and sounds like the sort of thing you would have on a moped, more of a 'peep' than anything else. The modern horn would be permanently on, but the old one could be used if needed. This would maintain the originality, but have something more practical for modern use.

[geoberni]

Therefore I wouldn't fanny around with a relay; just a simple change over switch.
So many people seem to complicate matters.
Relays are useful in the right circumstances. I used 2 relays to let me enable/disable my trafficators, so I'm not against them when needed.

[svenedin]

I don't have time to look for it now but it is quite easy to find the workshop manual for the Lucas horns, either the earlier type or the later 9H which had low-tone and high-tone variants. I have a bit of experience of refurbishing these horns and part of the setting up is to set the horn so that the current is within the correct range according to the manual. If they are incorrectly adjusted that can draw an excessive current. I seem to recall that the setting up is largely to do with the points gap and throw of the diaphragm. Small adjustments make a big difference to how loud the horn is.

Stephen

[myoldjalopy]

Yes, I wonder why the OP's horn is apparently so feeble. They are usually pretty loud. I substituted one for the doorbell once, but it had to come down as every time someone pressed the doorbell button I nearly jumped out of my skin! Like Stephen says, it may just need a bit of TLC.

[mike1864]

An afterthought. If the OP DOES go for the relay solution, the feed to the coil needs to be via the Ignition fuse, not the unfused - permanently ON - horn feed. Otherwise, depending on when you've parked the switch, the coil may be taking current all the time; draining (albeit slowly!) the battery.

[geoberni]

I don't have time to look for it now but it is quite easy to find the workshop manual for the Lucas horns, either the earlier type or the later 9H which had low-tone and high-tone ....

Stephen
I already gave him the link for the horn specified for the MM.

[svenedin]

I don't have time to look for it now but it is quite easy to find the workshop manual for the Lucas horns, either the earlier type or the later 9H which had low-tone and high-tone ....

Stephen
I already gave him the link for the horn specified for the MM.

Sorry I missed that!

Stephen

[mike1864]

Therefore I wouldn't fanny around with a relay; just a simple change over switch.
So many people seem to complicate matters.
Relays are useful in the right circumstances. I used 2 relays to let me enable/disable my trafficators, so I'm not against them when needed.

...as long as the switch chosen is rated well above the current of the highest load; I find that relays tend to come in higher ratings.
Plus you need to be confident about routing permanently live, unfused wiring into the dash area. But I suppose the headlamp wiring is already a similar risk .
I once fitted a neat - but ambiguously rated - switch into the petrol pump feed. It burned out the first time I drove off! So now it has a relay; and only inches from the load, enabling a quick reversion/bypass in the event of an issue.

[geoberni]

Therefore I wouldn't fanny around with a relay; just a simple change over switch.
So many people seem to complicate matters.
Relays are useful in the right circumstances. I used 2 relays to let me enable/disable my trafficators, so I'm not against them when needed.

...as long as the switch chosen is rated well above the current of the highest load; I find that relays tend to come in higher ratings.
Plus you need to be confident about routing permanently live, unfused wiring into the dash area. But I suppose the headlamp wiring is already a similar risk .

Current for the horn is less than 4 Amp.
IMHO not worth messing around with relays.
If worried about 'unfused wiring' you can always add a 5A inline fuse to the horn circuit.

You'd be really worried with my car; I've got a Ammeter with big brown cables to it

[mike1864]

Therefore I wouldn't fanny around with a relay; just a simple change over switch.
So many people seem to complicate matters.
Relays are useful in the right circumstances. I used 2 relays to let me enable/disable my trafficators, so I'm not against them when needed.

...as long as the switch chosen is rated well above the current of the highest load; I find that relays tend to come in higher ratings.
Plus you need to be confident about routing permanently live, unfused wiring into the dash area. But I suppose the headlamp wiring is already a similar risk .

Current for the horn is less than 4 Amp.
IMHO not worth messing around with relays.
If worried about 'unfused wiring' you can always add a 5A inline fuse to the horn circuit.

You'd be really worried with my car; I've got a Ammeter with big brown cables to it

Yeah but...the OP says he's adding a much beefier horn. 10A? 15A?
I used to have an ammeter, but now have a four LED "traffic light" voltage display, which I find more informative for first level diagnosis. Plus a DVM in the boot

[geoberni]

Yeah but...the OP says he's adding a much beefier horn. 10A? 15A?
I used to have an ammeter, but now have a four LED "traffic light" voltage display, which I find more informative for first level diagnosis. Plus a DVM in the boot

There is that.... No accounting for the extravagances of individuals....
I'm not even convinced their current horn is set up correctly, my SII single horn registers 103Db on a phone app noise meter.
Plenty loud enough for warning wayward pedestrians etc
There's few horns that will penetrate into a modern car with the sound system turned up; as evidenced by the number of numpties that don't hear Blue Light vehicles up their ass on a dual carriageway/motorway ......


Running with a Dynamo, I prefer the Ammeter. A Voltmeter is a bit late to spot the Dynamo not charging the Battery.