MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Wire Harness Meltdown

Sunday evening I was crusing around in my 1500. Suddenly smoke started pouring out from under my dash. It was my wiring. My 12V battery had slipped forward enough to allow the positive terminal clamp to come into contact with the frame cross member. My headlights were on. The only wire that appears to have melted was the small solid red wire that runs from my light switch back to my right rear tail lamp. Could my relay switch be compromised as well? How can I test my relay switch to see if it's damaged? Has anyone else ever experienced a similar problem?
S. H. Shanks

Assuming your car is converted to negative earth then the positive terminal contacting the frame would have resulted in the battery possibly exploding which it did not.

If your vehicle was still positive earth then the positive terminal touching the frame would have caused no problem whatsoever.

So what did happen?

The red wire is if I remember correctly the sidelight wiring so obviously the wire which has melted must have shorted to earth somewhere before its destination sidelamp!
Robert (Bob) Midget Turbo

Yes, the car has been converted to negative earth.
S. H. Shanks

I only suspected that the positive terminal came into contact with the frame. I'm not certain. But your right, the battery would have possibly exploded. I'm trying to diagnose exactly what happened before I install a new wire harness. Thank you for your response.
S. H. Shanks

Bob is right a battery grounding out will not cause the damage you describe. This, the lights, are an un fused circuit. There is a short to ground in that circuit. I add a circuit breaker between the big brown wire and the starter switch. The picture shows a 10 amp one used in a piece of short finding equiptment. They are avaiable in many sizes I used a 30 amp one for my MK11.


R J Brown

I had a similar poblem under my dash when I trapped a panel light wire under the oil pressure guage clamp!- turned on the lights and smoke appeared !! luckily I quickly turned off the battery isolator and restricted the damage. I needed to rewire just the panel light wiring. I have since fitted a 20 amp line fuse in the lighting circuit from the control box, I have also separately fused the circuits from the fuse box with a 6 way fuse box from maplin -belt & braces
Paul
P D Camp

Had a similar incident happen to me, the first night drive after getting the car back on the road after a 12 month rebuild. I managed to crushed the wire going to the number plate light with the metal cover, when I turned on the lights straight circuit to earth.

A new loom burnt out, learnt that you really do need a fuse in the lighting circuit.

The neighbours got a good laugh after watching the rebuild happening and then seeing the car getting brought home by a tow truck that night.
S P Rossetto

Had the same happen to me coming back from MG Silverstone in 1999. It was dark, raining and I was on the motorway near Gent. Smoke and instant flat battery.
The cause was cracked wire insulation shorting out at the back of the ignition switch. A bit of hot wiring and a jump start from a benevolant motorist got me back to Brussels but it was the nearest I have come to being on the back of a tow truck.
Investiigation showed that the wiring under the dash (and elsewhere) was so brittle that any movement would crack the plastic insulator. This is worth to check.
A new loom is not tough to install (at least the front part) although the dash does need to come out to do it properly.
Neil
Neil Purves

I have seen Paul's belt & braces - a very neat job but beyond me I fear. But I hadn't appreciated that the lighting circuit was completely unfused. Is the simplest way to rectify this to put a line fuse in the brown & blue wire (1600Mk2) between ignition switch & lighting switch. If so, what fuse rating is recommended?
Thanks
GrahamMV

I've always had problems getting the 12v in and out of the bracket. On my rebuild, I just finished wrapping the section of the round frame in front of the battery with some heavy rubber material. I was thinking about taking it off, but I guess I'll leave it now.

Thanks, Fred

Fred H

Graham, I was thinking along the same lines as you with regard to fusing. At first I considered putting an in-line fuse between the regulator A1 terminal and the blue brown wire that goes to the ignition switch, but that would mean that a short on either the ignition circuit or the lighting circuit would result in the loss of both these vital things; the thought of losing the engine and the lights at the same time makes me shudder! So I am planning to put a simple (old Mini) 2 fuse box behind the dashboard but easily reached. The blue brown wire from the regulator will be disconnected from the ignition swith along with the wire to the lighting switch and connected to one side of both the fuses. A wire from the other side of one of the fuses will go to the ignition switch via possibly a 25A fuse. A wire from the other fuse will go the the lighting switch and I was going to try a 15A fuse here. I will downgrade both of the existing fuses to 15A and see how it goes. It certainly is frightening to think that with the standard arrangement, a live wire falling off inside the number plate light can burn your wiring all the way back to the lighting switch and possibly beyond! At best you need a new wiring harness, at worst your car goes up in flames. I don't consider adding any more than these two fuses necessary. When I have done the job I will post another message.
Lindsay.
Lindsay Sampford

Many years ago, on a dark and rainy night, while driving my father's 1951 Plymouth the headlights went out! Then they came back on agin, off again, on again, this continued until I was able to stop at a service station. (Remember those?) It seems that a wire had shorted to the body in the trailer light wiring. The Plymouth had a circuit breaker in the headlight circuit instead of a fuse. Since then I have always thought that was a good idea. When a fuse blows, it turns the darkness on and it stays on until you fix the problem. With a circuit breaker you are able to continue to a more convient location to address the problem. You would not go far with the lights blinking on and off, but some light is far better than no light.
Ed Bell

Lindsay,
Good point. How about a 35 amp fuse in the Brown/Blue wire next to the regulator and then a 15 amp line fuse in brown/blue wire between ignition switch and light switch? I think this would perhaps be a simpler job? The lighting circuit will effectively have a double fuse but my logic is that if there is a short in the lighting circuit, the 15 amp fuse will blow first, so the car should still get me home.
GrahamMV

In my teen years, my parents had a Datsun that would blow the light fuse when you turned on the windshield wipers.

Turned out the mechanical link between the wipers was rubbing on the wiring harness and would short the headlight wiring. It was an easy fix, but stumped us for quite a while. Some engineering design!

Larry
L Wheeler

Graham, if you've got to get behind the dash to put the lighting fuse in anyway, you might just as well fuse the ignition circuit separately there. Putting a 35A at the regulator terminal would mean that a short on the ignition or fuel pump circuit would take out the lights as well. Wet night, dark road, short circuit fuel pump no lights? No thanks Graham! My Mini 2 fuse panel is on its way, brand new, 4.90 off eBay. Unlike the standard MGA one, it has a cover so it won't get shorted if something like a control cable falls on it!
Lindsay Sampford

Lindsay
I like your thoughts on the fusing of the lighting circuit. My solution of just fusing at the regulator was done with the idea of protecting all the wiring in case of a fault- appreciate the problem that the car will be imobilised by a fault- will adapt my system to enable me to get home- many thanks for advice. My 6 way fuse box fixed next to the standard fuse box is a £5.99 box from Maplins with a clear cover that enables one to see a broken fuse it protects all the anciliary circuits ( wipers flashers stop lights int light etc ) looks quite neat and avoids that chunk of wiring going into the standard fuse box -just one wire from the new fuse box to the original fuse
Paul
P D Camp

Hi Paul, are you an early riser, or just got home after a wild, all-night party! The other place where there seems to be too much wire is the ignition switch, with that big fat blue/brown and its partner; just asking to fall out and cause havoc. My fuse box will eliminate that problem with only one wire to the live side of the switch.
Lindsay Sampford

Hi Lindsay
My problem last night was toothache which woke me up -just went online whilst the asprin went to work !!
I like your idea of the twin fuse box under the dash much better than my fusing of the A1 circuit at the regulator.I will look in our local Maplins as they do a range of fuse boxes with clear covers which use the ceramic type fuses - I get slightly paranoid re burning up the wiring or worse!due to a short which could happen on so many parts of the A
Regards
Paul
P D Camp

Linsay, Brilliant! I will certainly apply this theory when I install my new harness. Thank you for the advise. Stan
S. H. Shanks

Lindsay,
No one likes a know-all! But of course you're right, I'm just being lazy.
Be really interested to hear how you get on as I am not sure there's enough slack on mine to reach an accessible point so will probably need an extension with spade connectors.
GrahamMV

Yes I think all MGA owners (unless you are desperately trying to win concours and don't like to use your car) should try and individually fuse each circuit. This aid s fault finding and seriously adds to the safety of your car from the potential to go up in flames, as many have done over the years.
Here is mine.

2 fuses for always on and the rest on ignition control.


Robert (Bob) Midget Turbo

Right, I've done it! Took about an hour and a half. Not as sophisticated as your smart looking arrangement Bob, but I think it achieves the same amount of safety. I mounted my two fuse Mini (as in car) type fuse panel; it has a single screw fixing point in the middle, to one of the two centre dashboard steadying bars using an existing hole. On my wiring harness the two brown/blue wires were easily long enough to reach the fuse panel. The fat supply wire was fitted to one side of one of the fuses via a "Lucar" connector (a proper one and soldered!), and likewise the other brown/blue lighting wire went to the other side of the fuse. I made up another brown/blue wire to go from the other fuse to the ignition switch. Another very short wire with a "Lucar" on each end was used to bridge the "supply" side of both the fuses together. For now I have fitted two 35A fuses to the new fuse panel and put 15A fuses in the existing fuse panel, I will have a think about the ratings and may change them when I work out what the current draw on each circuit is. All in all, a simple job and everything on the car is now protected by a fuse.
Lindsay Sampford

Lindsay
Thanks for update. Impressed you had brown/blue wire to link to ignition switch.
GrahamMV

Old midget wiring harness (burnt out; they do it too!) under the workbench Graham. You never know when these things 'll come in handy!
Lindsay & Lynda Sampford

This thread was discussed between 10/08/2009 and 15/08/2009

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.