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MG MGB Technical - Electrical problem help please!

Hi - I have a 1967 MGB which suddenly lost lights, horn and headlight flasher (on indicator stalk) on Friday. There is also an old Motorola radio wired in on this circuit which glows very dimly (about 10% of normal) until you try and activate any of the other aforementioned items at which point it doesn't glow at all.

This started when I pulled the radio from the dash on Friday to see if I could connect a new aerial - first thought was that I had disconnected something but it doesn't seem so - there are some loose wires but they do not appear to have any connectors to go into! (white with a brown stripe has a female spade connector - only others are two green wires with earthing connectors on the ends but they make no difference if I connect them to the chassis).

After establishing that there was 12v to the light switch, and that it was working when pulled out, I shut the car up. Moments later, I got a call to say that my lights were on, but the switch was pushed to the off position. Pulling to the on position turned the lights off again! Removing the switch to check the connections resulted in everything stopping working again.

My knowledge of car electrics is terrible so any help would be much appreciated. I think it sounds like an earthing problem, but am unsure why it suddenly started happening, then produced the odd situation described above. The fuses have not blown and an ammeter shows no voltage getting to the wiring for the lights at the front of the car.

Thanks in anticipation.
Martin Port

Martin - What is the polarity of your car? As I remember, the very early 67s were still positive ground, does yours fall into that catagory? In reading your post, my thought was that you have a positive ground car that someone put a negative ground radio in by insulating it from ground. If this is the case, then when you connected the aerial, you put a ground on the radio. The other posibility is that you have shorted something under the dash while pulling the radio out and reinstalling it, since it mounts right above a number of the switches in the dash and right nest to the ignition switch. The very first thing that I would do is to disconnect the radio completely and see if this corrects the problem. The next thing to do, if disconnecting the radio doesn't improve things is to take the electrical print for your car to a copy shop and have an enlarged copy made of it (2X works well) and then have it laminated because you are probably in for a bunch of troubleshooting and having a large, laminated print that you can use a grease pencil to trace lines is going to be real helpful. If you have the situation of a negative ground radio mounted in a positive ground car by isolating it from ground, get it out of there and either convert the car to negative ground or find another way to listen to radio. That kind of installation is nothing but a disaster waiting to happen. Good luck, and let us know what you find - Dave
David DuBois

I'm not aware of a white/brown on a 67 with or without a female spade connector. Likewise greens are a fused ignition supply and should not be connected to ground regardless of what connector is on the end, and I can't think offhand of anywhere where they would. Is this original wiring or a POs bodge? Even if in a pukka loom it could have been rewired with the wrong loom, even off another car altogether!

Dim lights that go out when something else is switched on is the classic symptom of a bad connection back towards the supply. The lights should be fed from the brown circuit which is unfused and is always live, as is the headlamp flasher on a 67 of either polarity, and the radio. The horns are fed from the purple circuit which is fused from the brown and so will suffer from any problems in the brown. Although there are bullet connectors in the brown feeding the headlamp flasher and the radio which could be causing a problem, the fuse for the purple circuit is wired direct back to the solenoid. The brown connection for this at the fusebox also goes on to feed the flashers and radio, so the problem could either be at the solenoid or the fusebox spades. Careful testing with a voltmeter should tell you which, but you might just as well remove both and give them a good clean, remembering to disconnect the battery ground strap from the battery first and reconnect it last.

The problem with the headlamp switch sounds like either a faulty switch or something shorting out wires as the switch is pushed back into the dash. If the latter it could short to ground which will cause major loom damage.
Paul Hunt

Any chance the wire color could be other than white with a red stripe - such as a brown stripe? Some of those colors get difficult to distinguish on older wiring.
Also, if that spade connector is exposed, it might be touching something it's not supposed to when you close things up. If it isn't attached to anything, I would recommend wrapping it with electrical tape, or use a wire nut, to keep it from touching anything.


Thanks for your comments and help. Turned out that there is a dry solder joint where a wire has been split in the engine bay. A wiggle brought everything back again and working fine. That part of the loom has been wrapped in electrical tape at some stage so I will take it apart and redo the joins properly. As for the wiring - it's in a bit of a messy state where things have been added and taken away over the years by previous owners. I'm trying to return it to original(ish) spec so may well have to put a new loom in at some stage.

This thread was discussed between 23/10/2004 and 27/10/2004

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