The power to the horns is pretty direct, and isn’t the issue (as it sometimes seems to be for the headlights). Your 73 MGB has a ground sided switch in the steering wheel. The idea is to take the full current out of that switch and instead run just milliamps through it, triggering the relay, which in turn is a whopping big switch and can handle the amps of the horns.
There are four terminals on a typical relay. 85 and 86 energize the coil, and 30 and 87 are the big switch that actuates the device.
MGB and other popular British cars had their horns wired two different ways. Both were similar. Both were easy. Both can use the same relay. They simply need to be addressed differently.
The difference was how the horns were grounded. Most early cars usually had the horns fed with power all the time. All they needed to was ground to come to make noise. Later, horns always had ground and needed power to make noise.
‘How can I tell the difference?’ Easy. Look at your horns. If they each feed with two wires that disappear up into the harness, you probably have horns that are fed power all the time and get ground from the horn button.
If your horns have a short wire that goes to ground right there by the horns, they have ground all the time and the get their power from the button.
Before we wire in your relay, please note. When we say ground, we mean contact with the body or the fame of the car. When we say power, we are talking about what is running through the wires of the car. As far as our relay is concerned, the polarity of the car is not relevant.
If your horns have power all the time, and get ground from the button, this is how you wire the relay.
Interrupt (cut) the wire from your horn button to your horns. Put the wire to the horns on the 87 terminal of the relay.
Connect the other side of the interrupted wire (from the button) to the 85 terminal.
Connect a new clean ground to the 30/51 terminal.
Connect the 86 terminal to a source of power.
If your car has horns with their own ground all the time…
Interrupt (cut) the wire from your horn button to your horns. Put the wire to the horns on the 87 terminal of the relay.
Connect the other side of the interrupted wire (from the button) to the 85 terminal.
Connect a new clean source of power (Fused. 20 AMP American style fuse. 35 amp British slow blow fuse) to the 30/51 terminal.
Connect the 86 terminal to a clean ground.
'How to wire a relay for horns on MGB and other British cars' have 6 comments
July 31, 2014 @ 5:51 am Scott Hindman
Will this work on my ’78 MGB?
February 24, 2016 @ 2:18 pm Oladimeji
Can the relay be used for fullights
June 5, 2016 @ 3:02 pm Colin Hammond
Your diagram specifies “new horns not OEM”. Is there a reason for that? I’ve read on other sites that relays vastly improve OEM horns. Add to that cleaning the contacts another should be a nice difference, yes or no?
Also, used Mercedes Benz Horns, pros & cons? (Aside that Moss would like to sell new horns.)
June 5, 2016 @ 3:04 pm Colin Hammond
That should read “…contacts, and there should be…”
September 19, 2020 @ 5:25 pm Dean De Pew
I have no horns but the 2 wires to run the horns are on either side of the car. All 4 wires are hot all the time, even when they are disconnected from the horn. I understand there are 4 wires total, what in the world or how in the world causes them to always be hot?
June 5, 2023 @ 4:29 pm Mike Andreola
Dean De Pew, did you ever get an answer? I also have four wires. Actually two hot wires come to the first horn and two go to the other horn. I installed aftermarket horns and i can only get one horn to work at a time. I need help with wiring the relay , which wire do i use and which do I disconnect?