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MG Midget and Sprite Technical - RW wire in wire loom burnt

Hello Everyone, I have been away from the hobby for too long and I have a question for you. I have a US spec 74 midget just before the switch to all rubber bumpers.
I noticed yesterday that the dashboard lights wire that is Red wire with White stripe is cooked in a few places. What should I be looking for as a cause? Bad rheostat? Or is it possible something else cooked that line?
It has been 8 years since this car was on the road, I don't remember if the dash lights worked.....I didn't drive much at night.

Thanks
Steven Conant

Hi Steven.
I don't see a rheostat in my electrical drawings. Does your car have a panel lamp dimmer?
What's your chassis number? I assume it'll be GAN5 - something.
So far as I can see only the panel lamps use red/white wires. I think there are four, in the speedo, tacho, oil/water gauge and fuel gauge.
The wire being burnt suggests that it is carrying too much current, which in turn suggests that something is earthing when it shouldn't. I suggest a good looksee to see if you can find anywhere the insulation might be cut or worn through allowing an earth. For example if a wire is taped or cable tied to the speedo cable it can chafe through and short. I know because that's exactly what set my Sherpa van on fire one night.
The good news is that none of the red/whites should be live unless the panel lamps are actually switched on.
Greybeard

Grey

North American cars had all sorts of stuff that we didn't.
Dave O'Neill 2

Red/white wires go to the parklights as well but that won't be the cause if it's burnt up behind the dash
An inspection of the burnt area would tell you if it had been sizled from inside by a short as Grey suggested or if something else ,hot has melted it from outside
One area to look at if it looks as though it's been shorted is to have a look at the globeholders in the instruments
pull the globes out and check inside the holders to make sure the centre(live) post is insulated or spaced away from the outer housing
or a shorted out globe--rare but it happens
William Revit

Aye the globe holders is a good catch.

Dave I have the US/Canada spec drawings too, but as you say they had some curiosities that we never got here. It may be that my drawings are incomplete. It's only in the last couple of years that I found California spec diagrams for example. Hitherto I hadn't known they were different again.
As I understand it rheostats control current so I didn't see why it would pass a high current since the lamps would only draw what they needed. I think dimmers are more usually potentiometer based (?). So my gut feeling is still along the lines of an unplanned partial earth.
All of which could be complete nonsense of course lol!
Greybeard

Thanks for the responses everyone. Car is disassembled, when I get back to electrical, I will tel you what I find, I was putting the wire harness back into the car.....today was another day of getting the car ready to run.....I wish I took more photos than I did on disassembly.....

I am at the where does this go and or how is this routed stage.....

Steven Conant

Steven

Welcome back to your hobby!

Sounds like could be cable chafing causing an short circuiting earth (ground) as Grey and others say. If you have the loom out, why not put a new one in at this point?

I'd want to see how corroded the wire is inside the insulation, plus how brittle the insulation is as well as checking corrosion on every connector and security of every joint, plus using a bulb and low power battery to test every wire. Also checking for other damage, meltdowns, bad old repairs, extra bits added, Scotlock snap connectors etc. Looms get old and tired as well as butchered.

If you want a new loom, Autosparks sell ones to specs sold in USA, see: https://www.autosparks.co.uk/finder under MG Midget MkIII LHD USA and then go for the option against your chassis number. Note they can add in Xtrac if you really need them.

Could be worth the money and hassle to go with a new look to have a reliable car with reduced fire risk.

Thanks
Mike

PS I will be burnishing my reputation for always wanting to make simple jobs longer!
M Wood

Steven
Been thinking about this, If the wire is melted from the outside by something else, it might pay to check the condition/security of the earth strap from the engine to the body
I saw a choke cable glow red hot on a mates MGB once and it was because he hadn't got round to fitting the earth strap and the starter motor was using the choke cable as it's earth
William Revit

Similar situation. Chevy S10 used the E brake cable as a path to ground. Fire and thick black smoke. I thought the truck was toast. Had computers in the cab, rushing to get them out. Someone had a fire extingusher to put out the fire. After all was said and done the rubber shield on the cable was all that burned. Lesson learned.
J Bubela

Update, replaced 2 feet of the Red wire with White stripe.....looks like it was grounding out for sure. It was the only wire in the loom that was "cooked".

I have the earth strap fitted and checked to be sure i have continuity in a few spots.

Thank you all I will be going through the dash bulbs and sockets at a later date. Trying to get her running today so I can do final painting when the darn humidity drops.
Steven Conant

This thread was discussed between 31/07/2018 and 05/08/2018

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