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MG MGB Technical - wiring problem

I am restoring a 71 MGB roadster and have a wiring problem. The brown wires which I understand are always hot are not all hot. Every one is hot except the two that go to the fuse box, which then means the bottom fuse is not hot making some of the accessories not function. Looking at the schematic all of the browns should ring out as connected, but in my case the alt. the starter solenoid the four ways and the lights all ring out as if connected together, but the two on the fuse box only ring out with each other and not with the others. Am I missing something? any Ideas anyone?
Thanks.
JH Clark

This link may help you with your wiring issue and the diagrams are much easier to follow than what's in the workshop manuals.

http://www.advanceautowire.com/mgb.pdf

Andy
Andy Robinson

Yes, the brown wires should always be live. If you have 12v on the connector on the end of the brown wire (test it) but nut on the fuse-holder that end, then either the spade connection is corroded or the riveted joint between spades and fuse holder underneath is corroded. 12v on the brown fuse holder should go to the fuse end-cap, through the fuse wire to the purple end-cap, and from there via the fuse holder and spades to the purple wires.

If the brown wires at the fusebox do not have 12v, do the lights work? If so then there is a break in the brown circuit somewhere, although this is unlikely.

If the lights don't work either then check at the starter solenoid. There should be two or three brown wires there - one or two from the alternator and the other one going up to the rest of the cars electrics including the fusebox and lighting switch. Originally these were on large spades, but then they were changed to ring connectors on the same stud as the battery cable. The ring terminals are better, but even these can corrode and you can end up with 12v on the stud but not the brown wires.

(I hate double posts)
Paul Hunt

As Paul says, they should all be hot and it will be a case of finding the break.
If you have two brown on the fuse and no starter relay fitted I would be looking for a four way bullet connector in the loom down by the chassis leg (I think) or as Paul says a bad connection on the starter.
The wiring diagram link from Andy points to an excellent resource. Much better than the others I've seen and a lot easier to read (thanks Andy).

Best of....
MGmike
M McAndrew

Thanks All, I found the culprit the whole rear harness was fussed together in the rear wheel well just before the boot grommet from a major melt down in the past. so everything was under power at all times. I also had the hot brown going to the solenoid instead of the fuse panel, which in turn would liven up the bottom fuse and the other two browns.

JH Clark

"the whole rear harness was fussed together"

That happens when the feed to to the pump shorts to earth, and has happened on both my cars before my time, as well as two that I have worked on. For that reason I've fitted fuses in the white circuit to both mine, where the rear harness joins the main harness at the front bulkhead, using an in-line with bullets on each end, and an additional 2-way connector on the rear harness.

However I do not understand "I also had the hot brown going to the solenoid instead of the fuse panel, which in turn would liven up the bottom fuse and the other two browns."

Generally speaking there are no bullet connectors in the distribution of the brown circuit, just where brown wires join to things like the ignition switch and the hazard flasher (in the case of a US 71). There is a sealed connection (see attached) with several branches off, but that has a metal cleat clamped around all the wires, then soldered, with a couple of layers of heat-shrink so very unlikely to give problems. Late model cars do have a 2-way spade connection between two browns coming up from the solenoid, and two more going off to the rest of the harness and they can give problems.


Paul Hunt

This thread was discussed between 19/08/2015 and 21/08/2015

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