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MG MGB Technical - Wiring Problem

Back to the Oracle...hello again chaps.

I have an annoying problem which I initially managed to rectify without actually understanding what I'd done.

1971 BGT, RHD - Passengers side rear light went out, as did number plate light. Further investigation revealed that the interior light and the headlamp "flash" were also out of action.

In the dark and rain, I fiddled with the connections at the fuse box and also with the headlamp connectors behind the radiator grille (had an earlier problem there and fiddled just to check) - this seemed to work and all was OK. Unfortunately by the time I'd got her home the problem had reoccurred but this time no amount of fiddling will get the light back on.

Can anyone advise me as to which wires I should be looking at for the faulty circuit?

Thanks in advance

Vic
Vic Synott

I should have said that the number plate light on the same side as the faulty rear light was affected. The indicator and brake elements of the lights are OK and work fine. The indicator seems a little slow when it indicates left.
Vic Synott

I would suggest that the rear light problem is unconnected to the interior light and headlamp flash.

The interior light and headlamp flash both come off the same fuse, via a purple wire. The horn is also on this circuit, so I would suspect that this would have failed at the same time. I would suggest checking and cleaning the fuse and its fittings.

As for the rear light and number plate light problem, this is likely to be a bad connection where the front loom joins the rear loom, or where the two lamps join their feed in the boot - red wire. Assuming of course that the front sidelamp on the same side didn't fail at the same time, in which case it would be another fuse box problem - one of the two fuses with red wires coming off them. One is for the left, the other for the right. It is easy to establish which is which by just removing each fuse and seeing which side no longer works.
Dave O'Neill

Thanks Dave, the front sidelight on the same side hasn't failed, so I'll have a look at the fusebox again today in the daylight.

I really appreciate the advice. Electrical problems are my nightmare!
Vic Synott

Vic

I may have confused the issue slightly...
If the front sidelight is working, then the rear light problem is not going to be the fusebox. Check the connectors in the boot.
Dave O'Neill

Hmmm...now I'm confused, I went out and checked the fusebox took them out one by one, cleaned them and reset - and voila, the rear light came back on... the fuse and holder on the purple wire is filthy and worked only for a short time but I'll clean it properly with some sandpaper and a little solvent when I get home. Could it be that there's a connection near the fusebox that would only affect the rear light and the same-side number plate light?

I had a look at the wiring connections in the boot but it all seems OK.
Vic Synott

vic
ive just sent you colour wiring diagrams for your mg
is 1.2mb in size but it shows that both are of of the same red wire joined in the engine bay by the bulk hed by the brake splitter and in the boot by the numberplate lamp
mines on the bulkhead as they do oxidize over time i would do all of them to be safe and fill bullet connectors with vasoline
ste
Ste Brown

Thanks Steve & Dave,

I appreciate the help. This site (and you guys!) are invaluable for keeping my B going!

Chris Betson is due a major vote of thanks too! He's a genius! Just got my car back from him last week following a cylinder head gasket failure and as usual it's fantastic.
Vic Synott

The top two fuses on a four-fuse box feed the parking and number-plate lights. If both the fronts light but the rear and number plate the same side don't then the problem is in a connector, not the fusebox. This could be one of the two 2-way connector where red wires from the main harness join the rear harness, of the 4-way bullet connector in the boot that feeds the parking and number plate lights.

However if any two parking/number plate lights are out it *could* be a fuse or its connection, especially if you have fout independant red wires at the fusebox, as it is perfectly possibly to cross-connect diagonal parking lights to one fuse, or indeed both fronts to one fuse and both rears to the other.

Interior light and headlamp flash come off the bottom fuse (purple) and so this *is* most likely to be the fuse of connections to it. Note that there is a riveted connection on the back of the fusebox connecting the fuse holder to the spades, as well as the fuse to holder and spade to wiring connections. This gives a minimum of six possible places where a bad connection could develop, up to eight of there is more than one wiring connector on the spades, and ten if you include the connection between the wire and the end caps of the fuse! And that is just at the fusebox, and you could have more than one!
Paul Hunt 2

Vic,

Fuseboxes are notorious on MGBs They are grossly inadequate anyway, but that's another matter.

You would do well to consider fitting a new fusebox with new fuses. Your comment about filthy fuse and holder should alert you to that possibility.

Make sure you get an MG 4-way fuse box as there are two fuses ganged together on the live, or pre-fuse, side for MG applications,. Some non-MG applications may miss this link which would leave you with sidelights etc on only one side.

FWIW

Peter
P L Hills

This thread was discussed on 24/11/2006

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