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MG MGB Technical - Horn Relay - not cutting the loom

I have seen the Hammer and Spanner ( my go to resource) diagram for the 2 wire horns – reproduced below - and the advice on putting the relay behind the dash, but have also seen the Brake relay solution which is really neat and does not involve any cutting of the loom.

I wondered if a similar approach might be possible for the horns, accepting the relay will have to be under the bonnet. I did find this on ‘MG Exp’:

1. Remove all wiring from the horns:
2. On the relay, place a jumper from the + coil input to one of the load terminals, then tie the purple wire on to this jumper as well. The - wire from your horn switch will go to the relay's other coil input terminal.
3. Run a ground wire from both horns to chassis gnd.
4. From the remaining switched load contact of the relay, run a wire to the other horn input terminals.

Any views? Will it work with a 2-wire system?

To be honest I am not certain what is meant by the ‘load terminals’ and other ‘coil input terminal’ Has anyone got a picture ideally with numbered relay terminals, I find diagrams and pictures a lot easier to follow!

Thanks

Mike



Mike Dixon

I'd guess that 'load terminals' would be the contacts 30 and 87, but then step 2 is more like it being a horn terminal.

It complicates things by making the horns operate from a switched 12v supply from the relay and an earth return at the horn, which is how they became in about 1977 with the single terminal at each horn.

To have a relay in the engine compartment - and behind the radiator would be better protected compared to right by the right-hand horn - without cutting wires and with two horns you would have to remove both connections from both horns, run two wires from the right-hand horn wiring to bring the purple and the purple/black connections back to the relay, then take a wire from the relay to one side of both horns, and take the other terminal from each horn to a body earth somewhere.

It might be easier to run a purple from the fusebox to the relay 85, remove the purple/black from both horns and bring that back to the relay 86, pick up an earth from the relay mounting point for its terminal 30, and extend its terminal 87 to the now empty horn terminals both sides, leaving the original purple wire on both horns.
paulh4

Are these additional relays a want or a need, I don't get it, unless your wiring or connections are iffy, in which case could they be sorted, why add in additional wiring, connections and component after 40+ years of successful standard operation, where's the improvement. Factory wiring seems untidy enough without adding to the knit-one-purl-one.

Nigel Atkins

The horns on my 1969 GT are wired like your description from MGexp. The "load terminals" are 30 and 87, the "coil terminals" are 85 and 86. Mine is wired like this:

Purple from fuse box to 30.
Jumper wire from 30 to 85.
Connect 87 to one terminal on both horns.
Earth the other horn terminals.
Connect 86 to the horn button.

I wasn't bothered about altering the wiring as the whole car was being rewired anyway, but hopefully my description may help you figure out which terminal does what.

The only slight issue with my set-up is that the steering column must be earthed, but since you only need to pass the milliamp current that triggers the relay it shouldn't be an issue.
Mike Howlett

The problem with the horns can be that the earth for the horn button has to come from the body, through the crossmember mountings, through the rack mountings and its bushes and bearings, and through the UJ to the column. There are no electrical paths, unlike the Triumph TR4 for example which has braided earth straps from body to rack and round the UJ.

The V8 came to me with a moped horn which was all the system would power and I fitted a relay to that straight away for the twin horns I put on. The roadster seemed OK for a long time but then seemed weak compared to the V8 and that was losing voltage in the earth circuit as well, greatly improved with another relay. If I didn't have the V8 with the relay I might not have noticed. Nothing to do with fuses, wiring or electrical connections in either case. As ever, YMMV.

Whichever way you wire the horns until 1977 it's still the column that provides the earth, but for the very small current the relay takes by comparison to the horns, that shouldn't be a problem as Mike says, unless things are very heavily corroded.

paulh4

People should do as they please or most convenient or sensible in the circumstances.

But Paul it can't be that it was nothing to do with fuses, wiring or electrical connections in either case if they previously worked and then didn't, or they deteriorated, otherwise it can only be the components.

Relays in this case are additional wiring and component helping out or taking the 'load' off much of the existing wiring and connections and putting in additional wires and connections.
Nigel Atkins

Another problem can be the collapsible column. The horns on my brother's BGT didn't work and I found there was no continuity between the top and bottom of the column. I ended up running some solder into one of the collapsible joints.
Dave O'Neill 2

Don't laugh at my solution to column earthing on my 1969 GT. My horns were very intermittent even with a relay because the path to earth from the motolita wheel to the body was so tenuous. I took a length of 13A house mains cable and stripped the insulation off, leaving me with a nice flexible piece of copper wire. I wrapped a couple of turns round the steering column shaft just on the bulkhead side of the universal joint and fastened the tail under one of the bolts just above the column on the horizontal part of the bulkhead. Even though the copper wire barely touches the shaft as it turns, it has worked reliably for many years now. I'm sure it wouldn't work at all without the relay.
Mike Howlett

Nigel - I proved to my own satisfaction by testing with a voltmeter that the problem was in the earth path to the upper column, and hence to the horn push. Nothing to do with fuses, wiring or electrical connections, but mechanical. It's not uncommon.

My MotoLita had a slip-ring on the back of the wheel the same as the standard wheel. The brush that rubs against that as the wheel is turned can be iffy. The MotoLita had a simple wire soldered to the back of the slip-ring going to a through-hole terminal on the horn-push as in the first image, whereas the original arrangement uses a sprung 'pencil' (incorrectly called a brush) that presses against the back of the slip-ring and a contact pad on the horn push as in the 2nd image.

Incidentally it suddenly struck me that for 1970 to 76 inclusive you can fit the relay behind the dash without cutting wires (or making changes at the horns) by removing the purple/black wire from the brush contact spade, extending that down to relay terminal 87 in Mike's image, and running a new wire from the brush contact spade to relay contact 86. Should be the same, I'll try it.







paulh4

Well if earth path and horn push aren't electrical connections what are, I think that's nit-picking. A switch can be mechanical but it still makes and breaks electrical connection(s).
Nigel Atkins

As I thought the wires to and from the relay can be inserted at the horn brush so no wire cutting.


paulh4

If I have read Mike H’s method correctly comments I hope I have captured it in the attached jpeg.

If so, the installation might be :

1. Remove all wires from horns, put the jumper wire on the relay using a piggy back on 30
2. Use the double purple wire to feed 30, noting that the purple to the second horn is still live so tape that off.
3. Use the Black and purple the same as the purple above but feeding 86.
4. Run a new black and purple from 87 to the first horn and use a piggy back to carry it on to second horn.

Looking at Paul’s comments I have tried to capture this in the second image. Have I got that right? Might be simpler and looks more like a normal relay installation

Having seen Paul’s later comments about the under dash non loom cutting method I will go and have a look under the dash/at the slip ring to see how that might work before I set off on this.







Mike Dixon

Both should work (but see next para), but the second is simpler as it leaves the horns as they are, particularly with the relay in the cabin. I have mine mounted on the bulkhead as there is a handy tab which was used for the instrument voltage stabiliser at various times.

A dioded relay isn't required, but if one is used the polarity has to be correct of course. Current flows from positive to negative, and from suppliers drawings terminal 86 is the 'positive' terminal as in the attached. Negative earth cars would need the 12v purple supply to be connected to this terminal, with the horn push on 85, or it would blow the fuse and possibly the diode. For positive earth cars the 12v supply would need to be on 85 and the horn push on 86. There are double-dioded relays which prevent reverse connection causing damage, they wouldn't work at all if connected in reverse. So really it's easiest if dioded relays are avoided.

A 'no cut' purple can be picked up from the bullet connectors for the interior light where provided, or run in from the fusebox.

For engine compartment relays closer to the fusebox two wires would need to be run back to the horn push connections.

Prior to 1970 the horn wire should be on bullet connectors, so even easier to interrupt. I mentioned 'from 1970' previously but just for the 1970 year the horn push was on the end of a stalk and there are no connection points in either the earth or horn wires other than the multi-plug, but they shouldn't suffer from the same problem as the earth source is from electrical wiring and not mechanical components.



paulh4

Paul

Thanks for posting the photo, looks good.

Sorry to be dim but in your solution of breaking into the circuits at the switch end what connects to terminals 30 and 85?

Mike
Mike Dixon

30 needs an earth which I suggest comes from the relay mounting, 85 on a relay WITHOUT a diode would connect to purple somewhere convenient.

I realise 85 and 86 are confusing but my 02 February 2021 14:25:20 photo giving relay terminal numbers assumed a non-diode relay and was done before you posted the drawings of the relays with the diodes.

Best to avoid diodes, then it doesn't matter which way round the wires connect to the relay coil.
paulh4

Thanks - I see you have updated Hammer and Spanner. I take it everything is still the same even if the diode/thermister shown on the updated H&S wiring diagram is not present?


Mike
Mike Dixon

That's right - E&OE which means Errors and Omissions Excepted which means I hope it's right :o)
paulh4

New to doing this I have had a play and have got the relay working under the bonnet without cutting the loom as per the attached diagram. Now removed to do it more neatly and with the correct cable.

From what I can see the horns draw about 5 Amps each so I will put a 10 amp fuse on the supply from the fuse Box to 85 in the relay, using 17.5 amp 2mm cable unless I can get away with the 1mm 8.75A I already have? Probably not and I can get the purple cable.


I am a fan of using the correct wire colours if I can, but I can only find a Purple and Black wire rated for 8.7A (1mm)

Will this do for:

The cable I run from 86 into the double black and purple and purple I have removed from the OS Horn using a spade connector?

The New cable from 87 to the OS Horn?

I assume the 1mm/8.75 A which I have is OK to link the 2 horns as that is only 5 amps

Any thoughts or sources or Purple and Black rated 10A or higher if I do need it?



Mike Dixon

Blimey all this extra wiring and components are going to weight the car down.

What about, just to save weight mind, Single Core Thin Wall Cable - 1.0mm² 16.5A in Purple/black (or Black/purple if you want) by the metre or 30 or 100m reel.

https://www.12voltplanet.co.uk/16-amp-single-core-thin-wall-auto-cable.html

Don't forget this will require 1mm sized traditional bullets if you're using such.

What cable do I need? 12 Volt Planet - https://www.youtube.com/watch?v=x2eHc7kczZI

Buy your relay wisely as it seems not all are as robust and long lasting as they should be, I didn't even know there were different types of the same until I investigated rewiring the Revotec relay for the replacement electric cooling fan.
Nigel Atkins

I agree with your wiring.

Thinner cable is only going to affect how much voltage is lost on the way to the horns, and they are unlikely to be on for long enough for that lost voltage to cause heat damage to the wiring. With the wiring you have, stuff rags into the trumpets and measure the volt drop between 87 and the NS horn. If it's only a few tenths of a volt then it can be ignored, I'm sure it will be a lot less than what is being 'currently' being lost from the body to the horn push.
paulh4

Thanks for the confirmation about the wiring. The only one that is a pain is the new purple and black joining up the 2 horns and that is only because I want to cover it with loom tape so grill out again.

I had a rethink and picked up the purple live via a piggy back from the OS horn, works a treat and they are certainly louder, and hopefully more reliable.

I had forgotten about thinwall - I may get some as I need some other stuff so it helps justify the postage from AES

Mike Dixon

This thread was discussed between 01/02/2021 and 07/03/2021

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