My youngest convinced me that the horn in our daily driver 240 is pretty inadequate especially when someone has decided to muscle in on your right of way etc. I've bought a "raging snail" super-loud horn from Ebay https://www.ebay.com.au/itm/124107199688 and have a momentary illuminated switch to mount on the dash coming from Hong Kong (yeah I know!) but what I really want to know is whether I need to put in a relay for the new horn or not. I'm retaining the original horns on their circuit and want to be able to activate the loud ones when and if required independently. I've done a fair bit of research online but most of it is contradictory so would like to get the info from a trusted source....
Wiring in additional horn 1984 240
The horns are both wired in parallel so that the voltage drop is minimised and that one horn will continue to function if the other one is out. Simply put the relay in the new parallel loop to the new horn and put the positive from your switch to the activation side of the relay. Relay earth could be anywhere. Alternatively, run your relay power from a fused source and run your relay earth through your dash switch (same-same in this instance). Make sure your power source is from a fuse or contains a fuse somewhere. Sourcing directly from a battery can result in melted wiring (or fires).
Back in the day (high school!) I fitted a set of air horns to our 240 wagon. I kept the original horns activated by the steering wheel switch, and I used one of the older-style rocker power window switches in the centre dash console to activate the air horn (with a relay as the air horn compressor uses a fair bit of power). I found that when you really NEED the loud horn, you fumble around to find the switch and it's too late. If I were to do that again (dual horn types) I'd put a 2-position toggle switch on the dash and have it always set to activate the LOUD horn via the steering wheel. In the case where I might want to give a gentle "toot" I'd flip the switch and use the steering wheel horn button to give the toot, then switch back to the loud horn as default. Just a thought. Regardless, if the loud horn draws much current, you'd be better off to put a relay in the circuit. TBH you rarely need a quiet horn, so if it were me I'd just put the loud one in place of the existing horns and if need be put a relay in to be sure. You can do a very quick press on the horn button to get a "toot" even with air horns, which is what I have on my current 240 wagon.
On the 240 EV, I've retained the existing single (1993 model) toot toot horn because I sometimes have to use it in car parks with pedestrians not paying attention, and I don't want to blow their brains out with an air horn. :)
Nothing brings a smile to a drivers face more than that little trickle of blood emanating from a pedestrians ears after a well deserved wake-up call (that and the shattered screen on the dropped phone).
Thanks muchly for the info guys, I've changed my mind slightly and I'm now thinking after reading some of the above that I will sound the new (hopefully fricking loud) horn through the steering wheel boss and wire the old ones up to the momentary switch when it arrives. I'm definitely going to fit a relay but again, there's a bit of dissension online about how best to approach the wiring, I'm think I could walk into Jaycar and buy one of these https://www.jaycar.com.au/30a-horn-relay-spst/p/SY4068 or if that's deemed too el cheapo one of these https://www.autobarn.com.au/narva-relay-12v-5pin-30amp-68024bl I've also read a bit on the UK forum about a "flywheel diode" found in some relays causing grief so don't know whether that is an issue or not, I can get my head around the wiring otherwise but any extra input regarding what goes where would be great....
The jay car 12v 30A is fine for what you need. Neither of thoes relays are diode protected so dont worry about that. always have a good 12v test light or multi meter handy a test light is easier to work with i find at least. I found some crude drawings on the Interweb to show you how and why a relay is used (i couldnt find any paper to draw you a diagram)basically a relay is an electromagnetic switch wich takes the high current load as normal switch cant handle high current and will burn out a normal automotive switch is good for 10A or less if your lucky
Common 12v normal relay pins are
30= Batt Pos
85= ground to chassis
86= trigger
87= load or output
And to work out what size fuse u need is this simple formula good for spot lights too
Wattage ÷ Voltage = Amps
Eg as 2x100w spota lights (200÷12=16.6A) so a 20A fuse would be needed for example.
Here is some good reading and pictures
https://www.12voltplanet.co.uk/relay-guide.html
Thats was how to wire up horns to an external switch.
I am not 100% sure as i don't have a 240 but most automotive horns earth to work meaning the relay gets its 85 earth from the horn Pad on the steering wheel wich is the switch and connects to the chassis (ground/ earth) when pressed so it turn the relay on like that. Hopefully that makes sense
Thanks Lloydy for going to the effort of posting all the valuable info above, I have only just read it now but managed to successfully?do the hook-up yesterday. I unhooked the pos feed wire to the horn press as the trigger to 86 on the relay and ran the earth 85 back to the terminal on the steering wheel boss that connects to earth to complete the circuit if that makes sense. I've still got an illuminated 'horn' momentary switch on its way and will run the old dual horns through that, the new horn is louder and more shrill but if it puts out 180db then I'm a monkey's uncle!