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 MGB Technical - Additional Brown Wire.

I was installing a power plug in my 67 GT so I can power various items.

In looking at the wiring diagram, it showed that the cigar lighter, when installed, went to #3 on the ignition switch (brown wire). In looking under the dash, I found a very short brown wire protruding from the large wiring harness in about the center of the dash. Per my Volt meter, this is live. Anyone know what this extra brown wire was for?

I know we have some electrical experts out there. Any recommendations on the best way to wire in a power plug? It will be used for cellphone charger, or radar unit or gps. Any issues with power draw? I am running a lucas 60 alternator.

What wire might be good to use that is ignition key operated. Brown wires are always hot and knowing me, I am going to forget to pull the plug out and end up with a dead battery.

I dunno what the brown wire there was for, but it might not be a bad idea to connect it to an inline blade fuse and thence to a relay controlled by either a green or white wire. Gives you lots of power to your plug, fuses it, and turns it off when the key is removed.
David Lieb
David Lieb

Bruce, that brown wire is the power lead for the radio.

Bruce, if you go to the above site you will find a wiring diagram for every MGB made. You can print it off for future reference.
Trevor Harvey

I concur that the brown wire is an always-on feed for a radio. I suggest you just run a new wire of appropriate gauge from the fusebox, with an inline fuse of the right rating as close to the fusebox as possible, along the loom and through the factory grommets. Access is quite easy on early cars and it takes about 2 minutes to route properly! You can push the wire through the firewall from the engine bay and you'll easily be able to find it on the other side under the dash.

T Jenner

Ok, Radio it is. Why would they put the radio on an always powered circuit?

Now you have me curious to find out where my radio is currently connected to. It only goes on when the key is on.

Bruce, When I bought my '67 in '72, It had the BMC Motorola dealer installed radio. I assumed that it was wired as always on since there is no provision on the ignition switch for an accessory position. RAY

I kinda like having the radio on an always-on circuit. It's not like you'll forget to turn it off when you exit the car after sitting in the garage listening to the last minutes of whatever song was playing when you killed the engine. -G.
Glenn G

"Why would they put the radio on an always powered circuit?"

Why did Renault put the intermittent wipers on an always powered circuit? Neighbour put his car away in the garage and they flattened the battery.

Originally there was no provision for a radio. A cigar lighter was an optional accessory and that was powered off the brown circuit. If a dealer fitted a radio they would determined how it was wired, but again it would have been an always on circuit so you could park up with your young lady/gent and listen without the ignition being on. It would have been moresensible to power it off the fused purple circuit than the brown though. When a radio appeared on the diagrams as an option in 1968 it and the cigar lighter were then powered off an accessories position of the ignition switch, but still unfused. In about 1970 that circuit gained an in-line fuse.
Paul Hunt

Thanks Paul

What would your recommendation be for a power plug? The suggestion of coming off the fuse box from a switched pole is probably easy to do. I know my radio is currently running off the switch side of the ignition switch. Could get power off this but I am always concerned about draw on the existing wiring. This will power a phone charger or a gps system.


Bruce - I can't imagine the devices you mention causing the wiring any problems at all - assuming by 'radar' you aren't planning to be the standby power supply for your local airport using airport transfers sevenoaks radar system :o)

If you definitely want them to be unpowered when the ignition key is out, and if you don't have an accessories position on the ignition switch, then it will have to come off the ignition supply. You could take it off a green (fused ignition) wire of which there are plenty behind the dash, but that will put more load on the green fuse which is more likely to be ther weakest link (through age and weak connections rather than the fuse itself) than actual wiring. So I would use a connection to a white (unfused ignition) wire, close to the ignition switch, but via your own in-line fuse, to protect your wiring against very heavy loads or shorts on anything plugged in to your socket. The fuses are there to protect the wiring, so if you use standard gauge wire you can use a standard 17amp rated 30amp blow cylindrical fuse. I know people like to use modern blade-type fuses but it means having to carry more and different types of spares and you already have (or should have) two spare 17A cylindrical in your fusebox.

If you want the socket powered all the time then the simplest thing is to tap into the purple supply for the horns, headlamp flasher, interior lights etc. as that is always on and already fused. If you come off a brown you will have to add your own in-line as above.
Paul Hunt

Thanks Paul. Exactly what I needed. Found a neat little item at my local hardware store that turns a single blade electrical connection in to a double. I can put that on the white connector off the ignition with a fuse to the plug.

As for Radar. I use a police radar detector. Not that I drive super fast, but our police stop you for 10 over and some of our roads are just to nice to only drive the speed limit on.

Guessed as much really, Bruce :o) Always legal to buy in the UK, at one time at least they were illegal to use as the Police claimed they prevented them doing their job i.e. fining us. Speed cameras got such a bad name as revenue raisers that the authorities renamed them 'safety cameras' only sited in locations where fatalities have occured and published databases with their locations. So the detector people turned round and said "In that that case we are merely giving advance warning of hazardous locations" and I think won their point. In France it is illegal to have one in a car, even if it is wrapped up in a box in the boot!

One thing we rarely see on British roads these days are traffic police, so it is just a matter of watching out for the cameras, helpfully painted bright yellow ...
Paul Hunt

Illegal in some states in the US. Not in Wisconsin, although I have to take it off when I go on military bases. Don't have speed cameras in my small town (yet) but saw them use them in one town where they claimed they were using them for safety, but then went and decreased the length of the yellow.

The supreme court ruled several years ago that radar detectors were legal because they were nothing more than a radio reciever. This was considered a constatutional right under free speech. Some detectors, however, send out a jamming signal and these are illegal in many states. I've been using detectors since 1983 in all of my vechiles including my motorcycle. RAY


This thread was discussed between 03/02/2009 and 14/02/2009

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.