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MG MGB Technical - 12v socket wiring question

Electrical novice here. I want to wire up a 12v socket (67 GT). Car is negative ground (that much I do know). Can I connect the positive terminal of the socket directly to the + battery terminal, or is it better to go to the fuse box?

Bernie
Bernie Lowe

Bernie - The fuses in a 67 MGB are pretty well loaded, so you would be better off taking the 12 volts from the terminal on the starter solenoid where he battery leads connects. You can get an in line fuse holder and fuse from your local auto parts store and install it close to the solenoid so that the wire to your plug is protected. a 10 amp fuse should be adequate. Cheers - Dave
David DuBois

Bernie, If you want to have the outlet hot all the time you can run a female spade terminal to the back side of the bottom fuse. If you want it to come on with the ignition then connect it to the back side of the top fuse. Dave's suggestion of installing a 10 amp fuse inline is a wise addition. RAY '67 Tourer
RAY

Bernie

There is a brown hot wire in the middle of the wiring harness under the dash on the 67. Was told by the group that this was for the radio.

After advice from the group, I went with an added spade on the white wire off the ignition switch with a solenoid switch off that brown wire. I added a fuse ahead off the solenoid

This makes the plug hot only when the key is on.
Bruce-C

Thanks for the advice gentlemen. Followed your instructions, got it right!
Bernie Lowe

The trouble with brown hot wires is that unless you add a fuse as near the supply as possible as Dave suggested any short could total the car if it happens while it is unattended.

A purple wire behind the dash is the easiest source, already fused, and already there for 68 and later. That's always on, and so very handy for plugging a charger into.

Late 64 on had a brown behind the dash for a radio but would ned an inline fuse added.

If you want the socket to be controlled by the ignition key and it is a 68 or earlier you have to come off the ignition. Green is fused but it's not a good idea to add more load to that so you would need an inline off a white, again behind the dash.

69 and later had an accessories position powered with they key turned to the first position but doesn't need the ignition to be on. From 71 on this had an inline fuse in anyway so pickup from the green/pink for the radio. Earlier again will need an inline added to the white/green behind the console.
Paul Hunt

When I added a outlet to my negative ground 67 MGB I used a relay. Brown from hot side of fuse box was run through a fuse and then to the relay. The switch side of the relay was connected to the fused side of the fuse box at the top or white wire. That way the load does not run through the ignition switch and the outlet turns off with the ignition switch.

If you have not already done it, put relays on your headlight circuit. The load through the switch will fail as the switch gets old.
JML Jim Lema

JML

That makes good sense. Think I will reset mine to your recommendation.
Bruce-C

Now I'm beginning to second guess myself.

I followed Ray's model by simply connecting the 12v outlet positive terminal to the 'always on' side of the fuse box. I connected an in-line fuse holder and a 15 amp fuse - didn't have a 10. I found a solid ground point for the negative side. The 12v outlet works fine.

But reading all the subsequent advice, I think I may have done it..well, not as right as I could have.

Does the way I have done it pose any hazard?

Bernie
Bernie Lowe

JML - you should have powered your relay from the green side not the white, that way the relay operate circuit would have been fused as well. As it is it is unfused and so potentially (ho ho) at risk of burning the harness if it shorts.

Bernie - the fusebox has 'always on' both sides, it depends which side of the bottom fuse you go. The front side (brown) is unfused and would need an inline. The rear (purple) is fused and doesn't. Since the loading on the purple circuit is insignificant (apart from when you are sounding the horn) there is no real point in going to a brown and having to provide an inline if what you want is an 'always on' socket'. This is how the later cigar lighters were wired from the factory.
Paul Hunt

This thread was discussed between 11/03/2009 and 13/03/2009

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