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MG MGA - Horn relay siting

Does anyone know where it was recommended that the relay (6RA) be mounted when using a pair of Lucas horns?.................Thanks Mike
m.j. moore

Mike, the horn relay is best mounted close the the horns, but in a place where it is always kept dry. I mounted mine under the cross section metal plate just under the bonnet front end, where the guide plate for the bonnet lock pin is fitted. Relays can be mounted anywhere but preferably in close proximity of the unit/s for which they are intended. Hope this helps.

Frank
F Camilleri

Thanks Frank, That seems to be a good dry place to avoid accidental knocks but do you happen to know if it is the place originally recommended by the factory or by Lucas? There was an original Lucas aftermarket fitting kit for the added second horn on Ebay last year which included the relay, wires, connectors, instructions,etc. but which unfortunately went for a sky high price and beyond mine. I was only after the instructions/relay position because the relays themselves are quite easy to get.
It's interesting to note that in the Workshop Manual wiring diagram there is mention neither of a horn relay nor a twin foglamp relay even though there are dotted line circuits for both the two horns and two lamps..........................Mike
m.j. moore

""Relays can be mounted anywhere but preferably in close proximity of the unit/s for which they are intended""

Frank where did you read that? or why do you think that? what is the advantage?
Robert (Bob) Midget Turbo

I've probably missed it, but I don't recall a relay being used for the horns. I installed the second horn using a jumper wire from the original horn. The wiring diagram shows a relay for the flick switch, which I installed many years ago. I could never find the correct relay until later, so I installed a common aftermarket relay. Marvin
Marvin Stuart

I don't understand the perceived need to mount relays close to the intended device on the MGA. In theory the idea is to reduce voltage drop caused by resistance in the wire. In practicality however, it's not necessary.

10ft of 14ga. wire has about .025 ohms of resistance. To put that in perspective, your average set of copper contacts has about .2 ohms of resistance. The very old DC switch on your dashboard with worn contacts probably carries about 1.0 ohms of resistance. Simply by eliminating the switch from the circuit using the relay, you are eliminating far more voltage drop than you will ever come close to by reducing the resistance of the 12V supply wire.

Putting the relay down by the horns, assuming you are using the original wiring to switch the relay, requires running an additional 12V fused supply wire to the horns. Because our batteries are mounted in the back, this is extra work and also looks unoriginal unless you take time to completely hide the wire.

Using Velcro and very good adhesive (or a permanent DIN rail mount and the proper style relays) you can mount your relays for heavier current drawing devices under the dash, and draw your 12V supply from the voltage regulator or fuse block. I mounted a fused terminal block under the dash for this purpose.

Coincidentally, I had some free time at work today and started drafting out a wiring diagram to modernize the MGA's electrical system via fuses and relays, mounted under the dash and using as much of the original wiring harness as possible. I think it can be done with 10 small relays and a 12-terminal spade type fuse block, as well as a couple inline fuses.

Mark
Mark J Michalak

Wiring harness in the MGA used larger wires for the horn circuit, so it is capable of running dual horns with no relay. For the fog lights, both 1500 and 1600 circuit diagrams show a single fog lamp only, and the wire here is the normal small (18 gauge) size. To run dual fog or driving lamps you do need a relay and a separate larger power supply wire.

A relay can be mounted anywhere in the vehicle. It is commonly located such as to minimize length of wire runs, especially for larger wire sizes. When using a relay, power supply wire before the relay needs to be sized to carry the full circuit current, while wires after the relay need to be sized to carry the load current. If you use two separate wires between relay and lamps, the wires can be smaller. Once you have the correct wire size, length of the wires is almost irrelevant No wire will run 50 feet in the MGA.

To avoid adding extra wires, it is common to mount the relay close to the load devices and close to the original switched wire. Then you run one long new power wire of larger size to the relay, trigger the relay with the original harness wire, and connect output wires to the loads nearby.

headlamp and driving lamp relays may be installed near the RF corner of the vehicle near the original 3-way snap connectors where the original switched wire arrives and lamp wires split. Purpose of these relays is to take the load away from the switch and to provide a larger supply wire. Wires between RF corner and the existing lamps may remain as original.

A horn relay might be installed in the same area or perhaps closer to the horn(s). Original supply and switched wires for the horns do not terminate or split at the RF corner, but continue on uninterrupted between bulkhead and horn(s).
Barney Gaylord

I was really only after the ‘original’ siting for the relay recommended either by the factory, assuming that two horns was a factory option, or by Lucas, if an aftermarket option.
If it wasn’t for originality I think the best siting would be on the firewall near the fusebox and then only a few inches of extra wire would be needed plus the bit required to connect the second horn in parallel with the first.
However, not seeing a relay on the 1600 Wiring Diagram even though the two horn option is clearly shown makes me wonder if a relay is essential. The only doubt I have is whether the horn push switch will handle the extra current. The original Lucas horn WT618 draws about 4 amps so is the switch capable of handling 8 amps into the inductive load? The horn cable in my harness is 28/0.3 which is rated at 17.5 amps so there’s no problem here.

I think the fact that Lucas produced a kit of parts, including a relay, for adding the second horn probably answers the question on switch rating.

(see http://cgi.ebay.co.uk/Lucas-WT618-Windtone-Horn-Relay-Kit-NOS-/260681350868).

As for the second fog lamp again there was no relay in the BMC Wiring Diagram for this option. I’ve just checked the foglamp wire in my harness and it is the same size as the horn wire viz. 28/0.3. The original lamp bulb size used in the Lucas SFT576 was 48 watts so again only 4 amps is drawn, 8 amps for the two and well within the cable rating of 17.5 amps so there is a similar question : is the foglamp switch capable of handling 8 amps this time in a non inductive load? I would have thought that it could but I don’t know for sure. If the foglamp switch rating is 8 amps or more then surely there is no need for a relay?.........................Mike


PS I’ve just found the original Lucas switch current ratings and for the Horn switch SPB160 it is only rated at 5A max. into an inductive load so it is not man enough to handle two horns and a relay would therefore be essential.

For the Foglamp switch SPB104 this is rated at 10A into a resistive load so it would be capable of handling two fog lamps and a relay is then not necessary provided the original 48 watt lamp bulbs are not exceeded. However, if you don’t plan to use a relay it would certainly be worth checking on the foglamp wire size in your harness just in case some were supplied with a lower rated wire than in mine.

m.j. moore

Robert, in his post Barney has given a very good explanation on where to mount relays and the advantage of mounting them close to the load. I did read this info in a book entitled "Auto Electrics" by Joss Joselyn and Bob Kraft. In the section about relays, the authors suggest to mount the relay close to the accessory to reduce voltage drop and to avoid unecessary extra lengths of wire. But,I also mentioned that relays can really be mounted anywhere,

Frank
F Camilleri

Generally speaking, there is no electrical advantage to placing a relay any one location vs another anywhere along the OEM wiring. The electrical loss is the same regardless on where it is placed. It is a series circuit. Just don't rewire and add extra length to locate the relay in an "out of the way" location. This adds losses due to the resistance of the added wires.

If you are of such a mind, there is an advantage to placing a relay near the horns in the MGA. The MGA has a 12V wire running forward to the horn from the fuseblock and then back to the horn switch, which grounds the circuit to energize the horn. If adding a relay, the path from the horn back to the switch doesn't have to carry the horn current. Just use the relay to switch to chassis local to the horns. This eliminates the return path in the harness,essentially increasing the voltage seen by the horns. Oddly enough, this increase in electrical efficiency adds slightly to the current seen in the 12V line feeding the horns. It is of no real consequence.

This being said, I have run dual windtone horns on the 50+year old OEM horn button and factory wiring scheme with no detrimental effects. It is not necessary as is shown in the factor wiring diagram for the optional horn.

The fact that Lucas did produce a kit for wiring windtone horns doesn't necessarily mean it is meant, or required, for use on the MGA. It may have been meant for any other of hundreds of vehicles which may not have had the proper wiring to start with.
Chuck Schaefer

I cannot track every aftermarket wiring harness ever produced for MGA, but the original issue used 18 ga for the single optional driving lamp, same as the parking lamp wires. For running dual driving lamps, either the power supply wire needs to be larger (similar to headlamp and horn circuits), or it needs a power supply relay with a larger feed wire.

As for the second fog lamp, there was never any PROVISION in the BMC Wiring Diagram for this option, relay or not. The relay is not shown because the second lamp is never shown in these diagrams. I don't think there was ever any standard location specified for the dual lamp relay, because it was never a normal factory option for MGA.

I have seen some MGA running dual driving lamps without a relay, and I have never known one to burn the wiring harness solely because of lack of a relay. The original fog lamp switch will likely carry enough current to run dual driving lamps. The original power feed wire may or not not get noticeably warm. The resulting voltage drop (and reduction of light output) may be relatively small and acceptable.

However, I have seen some very hot lighting switches (and some burned out). Normal lighting load is 4x6W corner lights, 4x4W dash lamps, 1x4W map lamp, 2x55W head lamps, total = 154 watts or 13 amps at 12 volts. I have installed two 10W halogen bulbs for large dash instrument lighting, increasing total current load by 1-amp. Some people may install halogen tail light bulbs with higher current draw, pulling a couple more amps. If you then add a pair of 60W driving lamps that can be on at same time as the high beam headlamps, add another 10 amps, and the main lighting switch may be carrying 25 amps. Neither the switch nor the power feed wire to the switch was intended to carry that much current. Addition of a relay for any more than a single fog or driving lamp is one place where I endorse use of a power relay in the MGA.

As to the Lucas Horn Relay Kit on eBay, there is no mention of any specific car model application. The kit appears to contain everything required (except the manual switch and horns) to install dual horns on any car, even a car that perhaps never had an electric horn. The model number WT618 is in fact a single Windtone horn (high or low note) that will draw 2.3 to 3 amps at 12 volts. I suspect the eBay ad is misleading in applying that model number to the relay kit. The ad notes that there is no external cover or label on the box. I will guess that this kit may have originally been part of a larger packaged kit including two horns.

I would like to see the instructions for that kit. I seriously doubt there would be any mention of the original power wire size in the MGA harness (or any other specific model car). This appears to be a generic horn relay kit, and it is up to the car owner or service shop to determine if a relay is needed. For the MGA, a 25/50amp fuse and larger wiring is original issue, so no relay is required.

As to the original MGA horn push switch, I have seen the guts of the thing. It is a heavy bar on button contacts arrangement capable of carrying high current for short duration. Being manual push with a wobbly contact bar it will be self-cleaning. The idea of 10-amps continuous duty may be irrelevant, as it is used for short term intermittent contact. My MGA still has the original horn switch, 53 years old with 386,000 miles accrued on the car, and it still works. For the first 50 years it ran dual horns with no relay.

In August 2008 I installed dual air horns with instant-on motorized air pump and a power relay with 14-gauge feed wire (as I had in mind to use the same power feed for additional loads). Instructions recommended minimum 16 gauge wire and 20 amp fuse (noting that original MGA wiring for horn is already 16 gauge wire with 25/50 fuse). The cube relay supplied with the kit crapped out a few days after installation. As a field fix I eliminated the relay by connecting the new fat power wire directly to the air pump and the return grounding wire connecting to the original switch wire. It works fine (still much louder than dual Windtone horns), and I'm happy to have deleted this relay.
Barney Gaylord

Frank not sure of the advantage Barney was refering too except as Chuck says the circuit is a series circuit and the current is the same from the battery to the load via the relay. The voltage drop at the load is the same irrespective of the position of the relay.

I can see a slight benefit of less wire if the load after the relay is in 2 places and the wire splits after the relay but I wouldn't class that as a particular advantage. There is more advantage to be had by siting the relay in a more servicable position I would suggest.
Robert (Bob) Midget Turbo

Robert, to be honest I am not an expert on auto electrics, but I do have a variety of books on the subject. If and when I do any work on the car electrics, I always refer to WSM and other written material authored by electrical experts. As to the sighting of relays, to me it's a matter of finding a suitable place, normally either in the engine bay or under the dash.

Frank
F Camilleri

My car started out with the usual single 12v horn. Looking at the manual's diagram it showed the second horn in series fed by the same horn wire. This means to have two horns wired this way they must both be 6v. I found a pair of horns at an autojumble to discover they wouldn't work in series, but sounded great in parallel so they were obviously a 12v type. Installed in the "A" in parallel and connected to the standard horn wire all I got was a little 'grunt'! The horn switch and its wire just could not hack it. Once I had fitted a readily available relay near the horns, fed by a nice big wire, it has worked perfectly ever since. I have to admit to mounting the relay, as a bodge at the time, to an existing hole at one end of the oil cooler... I must look into hiding it one day.
That's my two penneth.
Pete
P N Tipping

As well as the two fog lamps in my 1600 wiring diagram (Barney, it's page NN2) they are also down in the BMC 1600 Parts List and, would you believe, there is a relay included as well (BHA4075) but the interesting items for me are the two screws that are described as ‘Screw – relay to bulkhead’. So that’s where it went. There’s also a harness for the relay!!

This is backed up by Clausager according to whom anyone ordering a foglamp on their car from the USA and France was supplied with two because of ‘local regulations’ and when two lamps were supplied a relay was included (SB40-1). He says that the lamp relay was added after August 1958.

However, the Parts List makes no mention of a second horn let alone a horn relay.
I believe that two horns (WT618) were used with a relay on the Magnette but here the horn switch was completely different and might not have been the same rating as the MGA button switch. All interesting stuff.........................Mike
m.j. moore

All the MGA wiring diagrams I have show the horns wired in parallel. I wired my horns as per the diagrams and they've work just fine. Marvin
Marvin Stuart

You can never connect two conventional "trembler" type horns in series, because as soon as the contacts in one of the horns breaks, it will kill the power to the other horn. In short, only one horn of the pair will work. A six volt horn works very well on twelve volts (extra loud!), I have used the original 6v horn on motorcycles that have been converted to 12v to good effect.
Lindsay Sampford

Lindsay, That would make a good question on a school Physics examination paper viz. connecting two 6 volt Windtone horns of different frequency in series across a 12v supply. I reckon the high frequency one would break current first causing both horns to sound at the HF note but the LF horn wouldn't be as loud because the diaphragm amplitude would be less.
The fact that Peter's two series horns didn't work at all probably means I'm wrong and I would have failed the exam!.........................Mike
m.j. moore

The horn are NOT in series but are in Parallel, both see 12 volts. If someone did mount them in series they would not work right. On all of my cars I have never used a horn relay.
This whole thread seems pointless. As originally installed the horns work great. KISS theory prevails. To add the second horn you simply run TWO wires from the right horn to the new left one. As shown in my wiring diagram.
Originally the horn is fused alone so no need to add a fuse. The wire is heavy enough for the load so no need to add a relay.
The power wire goes from the A terminal of the voltage regulator straight to the horn in the RF corner via a brown wire to the fuse then a brown/green wire. The ground wire is green/black and returns straight to the horn switch with only one connector. If this wire were to accidentally touch any thing it would NOT burn the wire but simple honk the horn.
This is one area where MG absolutely got it right and all we can do is make it worse through over complication.
R J Brown

This thread was discussed between 21/01/2011 and 26/01/2011

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