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MG MGB Technical - Electrical nightmare

To sum up, last week I was trying to fix the wipers. They have not parked in a couple of years and I knew it would be a pain to fix. Turned out the accessory setting on the ignition was the problem and after messing with it, I found out the hard way some a**hole had rewired the headlights thru the brown, unfused circuit. Anyway I spent a week taking the hole dash apart took the entire dash wiring harness apart and replaced the melted wires and cleaned it up, and of course the 10 galons of water I used to try and put the fire out had gottin in all the switches and gauges, so they had a cleaning too.
My point is, after I spent a few days studying the wiring diagram and actually putting everything straight, I realized the hole car is wired thru this one brown circuit, and that the headlamps, hazards, ect.. are not fused in the engine bay. Seeing how this wire will carry a massive current, I was wondering if anyone had any success with any kind of fuse/breaker protection? The car is straight now, but being 30 years old and not having the best electrics to begin with I am going to try and fuse this circuit as it comes from the alternator, so if any more chaffed wires on this circuit ground out it won't go up in smoke. Any help would be appreciated.
J Arthurs

The only solution I've seen so far is to wire in relays for the headlamps and other accessories that this wire feeds. You can then add a fuse from the switch to the relay. Added bonus is brighter lights. Check out Bob Munch's webpage or contact him direct for a diagram.
william fox

Hmmm, an ar*ehole at the factory, perhaps, since the headlights were *always* powered from the brown always hot unfused circuit.

Since the wipers are powered both for running and parking through the fused accessories circuit I doubt that a problem in the ignition switch would cause non parking but allow running, that would be caused by some other problem with the park switch itself or the wiring between the switch and motor.

Whilst powering the headlights through relays will solve one problem it won't solve the problem of any brown wire (with the exception of that between the hazard flasher and its in-line fuse for obvious reasons i.e. it *is* fused but usually behind the dash) causing major loom damage if it should short to ground. Modern cars use 40amp and 80amp fusible links as close to the battery as possible. On an MGB you could use one for the alternator and one for the remainder of the electrics except the starter, as close to the solenoid as possible, and both of those circuits can be either one or two brown wires depending on year. Alternatively you could use just one for both circuits.
Paul Hunt

Paul,
Not talking about the brown wire going into the headlamp switch, talking about the blue wire on the indicator/flasher switch. PO had decided to bypass this and run his own wire to the headlamps. I might try an 80 amp fuse for the accessory side. The problem is that the ignition is wired thru this, thus every circuit, bar the starter, is on this one circuit, and I don't want to be blowing the fuse every time I have the headlamps, radio, and fan on.
J Arthurs

I'm tring to visualize what has been done. Why not just undo what the PO did and put things right?
willia mfox

I agree with William. It might well be time to consider a new wiring harness, hooked up as per the wiring diagram. British Wiring (British Wires?) has a good reputation for supplying a quality wiring harness. Then, if you do make any changes, you can annotate them on the wiring diagram for future reference. Les
Les Bengtson

I pulled it apart and replaced/repaired the wiring harness back to original. I am just stil concerned about the brown non-grounded circuit as it is still live and unfused and there is a total of 4 feet of this wire in the cabin. If for some reason it got nicked and grounded out it would catch fire again. This is how the wiring is supposed to be.
J Arthurs

What year is your car? On the four fuse boxes the brown wire is fused at the bottom fuse.
william fox

William,
As strange as it may seem, after I studied the electrical wiring diagram for a couple of hours among other things I noticed the brown wire going to the bottom of the fuse box(mine is a 73) is not fused, it runs straight to the battery via the alternator. This is not so strange, but it does so by first going in the cockpit and right aroung the indicator flasher unit, it splits four ways, one going back to the fuse box, and the others going to the headlamps, hazards, and the ignition switch. All of these lines are unfused and if they were to be grounded out, by a loose screw or a nick ect., they would burn up.
J Arthurs

I see what you mean. I just checkedthe diagram in the Haynes manual. The brown comes off the starter relay and splits on the hot side of the fuse. I guess you could wire in a fusible link like modern cars have.
william fox

J. Arthurs:
I note your concern, and all I would add is that you make certain that your grommets and other insulating devices are in good order. Where wiring passes through metal panels or near them and could suffer from vibration wearing thru the insulation, you can go to a decent professional electronics store or even some Hot Rod shops and get special add on insulating strips which are self clamping to the metal (in much the same manner as your weather strip) and can be formed to follow odd shapes. I have added some bits of these to various places where I felt there might be some chance of rubbing. Another old trick is to fashion some protection from a piece of small hose slit lengthwise and then put over the raw edge of the sheet metal. There are many things you can do to prevent a short such as you are concerned about.
Bob Muenchausen

This thread was discussed between 29/07/2002 and 03/08/2002

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