I’ve bought some horns to replace the nasty oem ones and the fitment is more complicated than I realised, even though they are simple electromagnetic snail horns.
Any members nearby with a spare hour or so??
Jeremy
If your original horns have only one terminal then the horn has a built in earth which is created when it is bolted to the chassis (if there isn’t a paint barrier).
If i’m right snail horns have two terminals, then you need to make a short earth lead from one of the terminals to the chassis, a spade terminal and 8mm ring terminal onto the bolt mounting the horn to the chassis.
None earthed horns usually come with the earth lead.
It doesn’t matter which way they are wired unless there is a specific + and - marking on the horn unit.
Thanks.
I have seen that about making up a short lead to earth but that appears to be unnecessary as the mounting bolt acts as an earth already, even though there are two terminals. I think the second terminal is to take the power from one horn to the other of the twin horns.
That should make it simple but the suppliers say that the oem leads may not carry the current the new horns will draw, so a new lead has to be made up from another 12v source and incorporating the higher rated relay supplied with the horns.
This is starting to take me out of my depth…
Hi, As our previous post, seriously it’s not difficult but just a “Bit fiddly”.
(shame I am too far away).
Save it for a nice whole day and just go for it. 
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Did you get a relay with your horns
Hi see this post. 
I did indeed get a relay with the horns
According to MX5 parts, the bolt acts as earth and that’s what the wiring diagram indicates too, so no need to add an earth lead.
I had hoped that these little TM80 horns would draw similar enough current for the original relay to be adequate but MX5 parts are telling me that even the original leads are inadequate. I would have expected that for a compressor horn but not for these little electro-magnetic ones, so that’s really disappointing. I am crap at soldering and not much better at crimping, so making up a new power supply and sorting out incorporating a relay into that is out of my comfort zone.
I have to admit I am tempted to just use the original power supply as I expected to and restrict myself to short stabs of the horn button…
Come on people there must be someone around the Exeter area to help this chap out for a few beers? 
Haha, thanks.
Maybe I should just follow your ‘lead’ 
and use the original wiring.
Your car has presumably not caught fire as yet…
No I didn’t use the original wiring, as said I used the relay.
The only original wire (I think from memory) was the feed to the relay and then all new wires from there.
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MX5 parts list a horn relay for the MK2.5’s and MK3’s, so if that is indeed the case, there should be no need to fit another.
https://www.mx5parts.co.uk/relay-multi-application-late-mk25-mk3-35-p-2253.html?utm_source=google&utm_medium=googleshopping&utm_campaign=googlebase&gclid=EAIaIQobChMIvrTkxvKF6gIVRLTtCh3AnwC-EAQYASABEgJemvD_BwE
Yes, that is what I was hoping but the original relay is allegedly not a high enough rating.
Sorry, I misunderstood.
Mines been fine for 3 years, although, how often does one use the horn ?
Twice between MOT’s ???
I’ve been looking to see what amperage the standard relay might be, but not finding anything.
If anyone is that worried, why not replace the standard relay for one with a bit more oomph.
And if the car does have a relay, what’s the worst that can happen, you burn out £1.70 relay.
https://www.beal.org.uk/automotive/automotive-relays/micro-relay-4-pin-12v-20a.html
The horns draw 11 amps according to the box.
So you’ve just plugged and played, using the oem wires and relay
Fires tend to cost more than £1.70.
Just an opinion but if you are going to do it do it properly.
Each individual obviously can make their own decisions. Many thanks
Hmmmm, maybe I’ll just leave the oem nasties, they don’t exactly get a lot of use anyway.
I suppose I could make up a lead direct from the clamp bolt on the battery terminal to the relay. and another one from the relay to one horn and then from that horn to the other, using the second terminal.
I do have some red and black cable reels and a crimping set. Can anyone tell me where to find the oem relay and if I can find it, can I just copy the terminal pattern from the old relay to the new one?
I am grateful for the advice, really I am
Disconnect the wire from the OEM horn, get someone to operate the switch, follow the sound of the relay.
I’m not overly concerned about what Scarletpimpernel mentioned, in all fairness to him/her, he’s probably just being cautious.
I’ve had mine fitted at least 3 years, also did the same on my daily car about 5 years ago.
I dare say there are lots of other member fitted the same.
I found this. Not an exact science granted, but may help.
https://www.hella.com/hella-sg/assets/media/Hella_Horns_Brochure_LoRes.pdf
Consider that the Hella ‘rich tone’ or ‘full tone’ consume between 66 and 72w each.
These are similar in design to the standard horn.
66w at 12v = 5.5 amps, 72w = 6amps
The ‘Black Twin Tone’ similar in design to the ones I posted draw 6a each
You could potentially be increasing the current draw from 11 to 12 amps.
Hardly worth losing any sleep over, also I’ll put money on the relay being 20a.
The fuse should be a 15a fuse.
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