Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - My Foglight Relay

Autosparks have provided a relay for the foglight, wired like this:-

fat brown
fat red/yellow
thin black
thin red/yellow.

I've rung out the two thins with corresponding wires coming out of the loom, so I assume they are the switch loop.

Quaere - does the switch just get those two thin wires, with the whole relay system powered by the fat brown?

I really am this thick.
Nick and Cherry Scoop

Assuming your car is negative earth - I can't remember what you said about that...

Fat brown should be connected to a permanent live and the other fat wire to the fog lights.
Skinny red/yellow will come from the light switch on the dash and skinny black goes to a chassis earth.
It goes like this: you close the dash switch which sends power to the relay coil via the skinny red/yellow. The other side of the coil is earthed, so the coil closes the contacts in the heavy duty switch part of the relay. This in turn connects together the permanent live supply and fat wire to the lights.

If it's positive earth it doesn't make any difference to the connections; just connect the fat brown to the permanent -ve pole on the starter solenoid as if it was +ve and the other fat wire to the lights, plus the skinny wires as described.

You're not being thick at all. Some people just don't do electric. I like to think I'm reasonably bright, but I can make no sense at all of piano keyboards.

It might help to understand if you know that the relay consists of two completely separate circuits. The primary which handles the low power circuit from the dash switch and the secondary which handles the heavy current to the consumer (lights). All the primary does is mechanically pull the secondary contacts together, but the electrical circuits are separate.

Thanks, Grey.
What I still don't get is (i) what I do with the other terminal on the switch, and (ii) how does power get into the switch circuit?

Positive earth, by the way (and the switch comes from a time BR (before relay)).

Nick and Cherry Scoop

Okay Nick.
The answer to (i) is the same as the answer to (ii)
Assuming (yeah I know) the switch ran the fog light previously without the relay it will have had a fattish wire going to the light from the switch. The other wire on the switch can come from a number of different places but the most sensible is the output from the headlight switch that goes to the dipswitch. That's because the foglight will then only work if the lights are switched on, but it won't matter if you're on high beam or dipped lights. After all you won't want the foglight except when the lights are on and it can't be left on accidentally - it'll go off with the headlights.
All that changes with a relay is the wire that used to go from the dash switch to the light is replaced with the skinny one to the relay primary. So the switch now powers the relay coil instead of powering the light directly. If the old fat wire is still in place on the switch there's nothing wrong with using it to go to the relay instead. It doesn't need a big wire, but it won't do any harm.
You don't need to worry about the current for the relay coil because it's tiny - basically negligible.
The fat wire that used to carry the juice into the light is replaced by the fat red/yellow from the relay and the fat brown to the relay can be connected to any convenient permanently -ve, for example on the starter solenoid which is itself only a relay on a grand scale. Although I'd be tempted to fit some in line fuses but as BMC didn't trouble with them they're not strictly necessary.
Actually that's a bit misleading because of the positive earth, but in principle is the same as neg earth in effect. Just keep in mind that there are now two circuits in place of one, but the only thing the skinny circuit does is switch on the fat circuit.
The advantages are: reduced current at the dash switch which is good for it's survival and (less obviously) a brighter light because the resistance/voltage drop across the relay is tiny compared to the original switch.

BTW I enjoyed the little bit of Latin in your original post!

I would hazard a guess you will have a spare ‘red’ wire to power the switch (from sidelight circuit) then the red/yellow from the switch to the relay.
Dave O'Neill 2

One further thought...
It's possible your dash switch has three connections instead of the two I've been talking about. I only thought about it when I just found a switch in my jacket pocket that I bought for my campervan, which has three connections.
If it has it's because it's an illuminated switch that lights up when switched on, to warn you that the light is on.
Strictly speaking it doesn't need to be connected - the switch will still work, it just won't light up unless the third pole is connected. But that's a different question!

Thank you very much, both. As a matter of fact, I do recall a spare red among the leftover wires from the loom. I might amuse myself by seeing where it comes from. But otherwise I will run a new one as you suggest.

I read "quaere" somewhere - maybe Patrick O'Brian - and liked it. I hope it means what I think it means.
Nick and Cherry Scoop

BTW, I have destroyed my hooter switch - a sprung toggle which sat on the dash, top left of the steering wheel.

Would this be man enough for my twin horns?
Nick and Cherry Scoop

To be certain you'd need to know the current draw of the horns. If they're the originals then that 25A switch ought to be okay.
Some modern horns can draw scary current - 30A or more, but the Spridget - spec (Lucas??) horns are more modest in their demands.

My wiring diagrams just showed me something interesting that I didn't know. Apparently Mk1 Sprite should have a horn relay as standard, which was not included on subsequent models. I wonder if that is because of having a dash mounted switch instead of later types of horn-pushes?

Is your car a Mk1, and if so do you have a relay lurking somewhere?

Either way I reckon the 25A switch will be fine. Electrically anyway - aesthetically I have no comment!

Frogeyes didn't have any relays, I believe. Mine certainly didn't. But the dash horn switch is non-standard anyway.

With the proper horn push in the steering wheel boss, the electrical path is so fragile that I reckon a relay would be a really good idea there.

Interesting that the wiring diagram shows 2 options, with relay for twin horns, none for the single. Maybe I should get another relay.

More generally, bad news:- I've got 4 wires left over: 2 greens, 2 blacks.
Nick and Cherry Scoop

» I've got 4 wires left over: 2 greens, 2 blacks.«

What - in your new loom?
Perhaps they're accessory wires? I have no idea. Black is generally "earth" FWIW.

If you're in the mood to buy relays I recommend them for headlights. One for Hi beam, one for Lo, with added fuses. It's a simple, cheap and very effective upgrade.

There are those who disagree with me about this which is perfectly legitimate, but my experience of them is that they are very much worthwhile.

» I've got 4 wires left over: 2 greens, 2 blacks.«

What sort of terminals? Bullets or Lucars?
Dave O'Neill 2

Both blacks are un-terminated
One green bullet
One green double female bullet (though all the females in this loom are doubles, I think).

The wiring diagram shows a cigar lighter! Maybe a circuit was added for that. But I would have thought that that - and my radio - would be on the unswitched side.

Am I right in thinking that greens are usually from the fuse box, on the switched side?
Nick and Cherry Scoop

My drawing reckons that the cig lighter supply is switched by the ignition keyswitch, but it should be white. It also has a red/white going to the panel lamps. This suggests it can be used in either the Acc or Run key positions, powered by either wire.
Green wires appear to be confined to those coming from the fuse box eg: to brake light switch, fuel gauge, heater fan motor, wiper motor and flasher unit. I'm working on the Mk1 Sprite drawing, the Mk2 is not much different but the cig lighter has been deleted.

Probably not much help to you :-(

Any tracer colours on the greens or just straight green
William Revit

Whereabouts in the loom are the greens located? You need to see if they are both live, or if they should be connected together.

Similarly for the blacks - location and are they already grounded.
Dave O'Neill 2

Just green, and quite thin. I will test them this morning.

I've been working along the dash from right to left, and they come out of it round about the wiper switch, because I wired that from this remaining bunch.
Nick and Cherry Scoop

Well, the two blacks don't ring to earth, nor to each other.

The female green rings to the green wiper motor feed.

The bulleted green doesn't ring to anything unterminated: I haven't grovelled up behind the dash to check terminated greens, but I think fuel gauge is the only plain green.

So I think the bullet must go into the female, and then - when I power up - probably the fuel gauge will work.

Looking back at the diagram I did for my Mk2, I think the answer lies in number 21, lower right quadrant.

Nick and Cherry Scoop

. . . . . still doesn't work, though, because neither green rang to any wire on fuse box.
Nick and Cherry Scoop

Third post in a row - bad form - but I finally got there (I think).
The double female is actually the main distribution point for greens:-

wiper motor
fuel gauge
heater switch

I had cleverly taken the feed (which had pulled out, I suppose) direct to the heater switch, leaving the other two items unfed. Just got to make a short wire to the heater switch, and the 4-way will be complete.
Nick and Cherry Scoop

I knew you’d get there in the end ;o)
Dave O'Neill 2

Bravo Nick.

(I'll ask you about the 2 blacks tomorrow)
Nick and Cherry Scoop

The greens sound about right
Straight green should be Acc. power supply from the ign. switch.

I've been looking for a chart I had and have actually found it-yipee-
Probably won't help but it's handy to have-
British Standards wiring colours
William Revit

Thanks, Willy. That's interesting AND helpful.

Still got two blacks - and one of them is earthed after all. When I said they weren't, I was ringing them to the dash, which at the time was supported on a cardboard box at one end and a plastic crate at the other.

Nick and Cherry Scoop

Earths are earths---
If the black wires are straight black with no tracers they should be earths for something and connected to earth at one end
Problem is which end of the wire you have I guess--The earth end or the component
It would be a matter of following the wire back to see if it goes to an earth point or to something that needs an earth to work out what you have to do with the loose end you have


It's all back to front to how most modern cars are wired now --With the use of onboard computers several actuators on moderns have a continual power supply and the computers operate them by varying the earth supply
(useless piece of info for the day)
William Revit

That's nice and simple, Willy. So I have two skinny black wires coming out of the loom, both unterminated and therefore looking for a screw. One rings to earth, the other doesn't.

So I guess I have to find a component that needs the earthed one, and take the unearthed one to earth. But I'll check if I can find the other end of it first. A nice easy job to do right now.
Nick and Cherry Scoop

AAaargh! The unearthed black rings to the blue and white headlamp wire. Why is it doing that?
Nick and Cherry Scoop

Does the main beam warning light have its own earth, or does it earth through the housing?

If it has its own earth, try removing the bulb from the lamp and see if you still have a connection with the blue/white.
Dave O'Neill 2

Interesting. My drawing shows the high beam warning lamp (Mk1) with a direct body earth. However it also suggests two of the four panel illumination lamps having black wire earth tails.
I don't see any black wires associated with the headlight circuits at all, but as the song says "it ain't necessarily so".

» » However «« I do see a black wire earth tail for each REAR light, which are powered by the same supplies as the headlights.

Actually that last was a bit misleading, although they do get power ultimately from the same place, so depending on switch positions and what bulbs are fitted at the moment, it is possible to see a circuit path between the high beam wiring and the tail light earths.

My goodness, Dave. You nailed it. It's the little black wire to the HB warning lamp . Now I just have to find a device that needs the other black, which is earthed. I could just leave it floating until I power up, and something doesn't work, though in that case I must take care to prevent it touching one of the ammeter wires (haven't forgotten your warning, Bill).

Thanks, Dave.
Nick and Cherry Scoop

If that is the earth for the HB warning light and the other wire is already earthed--
'maybee' they are normally both plugged into a common connector
or not
just a thought
William Revit

Good call, Willy.
Dave O'Neill 2

I've been mulling that over, but it's odd that they're both just plain wires, rather than bullets or whatever those flat round things with a hole in them are called.

Anyway, I think I'll bulllet them and go with that. Thank you for everybody's help. I do enjoy your thinking aloud, Greybeard, especially when the next post says, 'no, hang on a sec . . . . . '.
Nick and Cherry Scoop

Nick, is your wiper motor switch connected to an earth? The switch works on the earth side of the motor circuit to enable the self park mechanism.

I ask, because I initially wired mine with the feed side going to the switch, then from the switch to the motor. Wrong! It works, but doesn't self park. The correct is feed to the motor, then live return to the switch, with the other switch terminal to earth.

Nick, I concur with Guy. Check the wiper connections. AIUI there should be one permanent live and two earths. One earth goes to the wiper switch, one to chassis (I don't know where). The latter is for the park function.

Am in Portugal right now so infrequent forum visits.
Bill Bretherton

Difficult to get on with your Frogeye from Portugal, Bill. That scarcely demonstrates a properly serious attitude.

The wiper switch looks right. Black wire is to earth, BG to motor. And the black to motor is earthed. Any other suggestions welcome, Guy and Bill.

As it happens, I need an earthed black for my hooter switch, which is on the dash. None of the hooter wires look big enough for my twin setup, but it worked before. No electrical information on the hooters - just "FOREIGN MADE", which dates them.
Nick and Cherry Scoop

just been thinking back to this---
AAaargh! The unearthed black rings to the blue and white headlamp wire. Why is it doing that?"

You worked out that the black was the earth for the HB light-
so just to explain-
The reason you get a circuit between these two wires is because the circuit is completed by the bulb itself--I would imagine that if the globe was removed there wouldn't be a connection between the Blue/white and the black----

Because both these little black wires have no ends on them, are they in a position on the loom where they might have been connected to an earthing point on the body which didn't get undone when the dash came out and they got accidentaly pulled out of their fittings
William Revit

You're right about the bulb, Willy.
But the loom is brand new, and the only other dash wires with no ends have been the panel light switch reds.
Nick and Cherry Scoop

There should be at least one black wire connected behind one of the knurled fastenings for either the speedometer or the rev counter mounting clamps. That should have a small ring terminal conector but it might have been left as just a bare ended earth wire.

Sorry I can't add further suggestions Nick. As for my absence from the Frogeye I'm responding to management requirements! Nicer weather than home though!
Bill Bretherton

This thread was discussed between 26/05/2018 and 31/05/2018

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.