Black wire disease

Black wire disease

I was trying to find why the rear marker lights were not working on the trailer and having checked the bulbs and then the terminals inside the lights, I moved to the trailer plug. All seemed well, but then I noticed a slight kink in the cable a little way back from the plug where it had been trapped at some point, probably in the trailer handbrake lever by someone careless (maybe me!). The outer insulation was intact but for a small hole. Fearing a break in a conductor, I cut the cable open and on one of the eight wires inside, the insulation was cracked and hard. It turned out it was the supply lead for the marker lights so I was pleased to find the likely culprit but when I felt the hard area I let out a little “oh, oh”. I cut into it low and behold - the dreaded black wire disease!

Where would we be without the copper that forms the conductor in classic vehicle wiring? Soft and pliable to form through bulkheads and inside enclosed spaces in the vehicle structure. It is resistant to corrosion without any coating required and it’s superb conductive qualities ensure almost no voltage drop between the battery, its charging circuit and the lamp or motor at the other end of the circuit. Despite this, copper wiring has a secret. It can be susceptible to its own little virus, which remains out of view but causes all sorts of electrical maladies. ‘Black Wire’ is where the copper becomes coated in a greenish/black furry coating that resists electricity. It can develop on the copper underneath the insulation, making it almost invisible to the vehicle owner until a light starts dimming, or not working at all. By that time the infection might be reaching a metre or more into the loom.


There seems to be a an area of mystery about its specific cause, with several viable theories but each can be undermined to a certain extent by real world examples. Certainly the blue/green furriness deposit you can find around your battery terminals is Copper Sulphate and the science is that this is generated by a reaction with acidic vapour from the battery and perpetuated by the electrical flow. This tends to be on the negative terminal but has been seen on the positive. The consensus is that if your alternator is over charging it will be on the +Positive terminal and if undercharging (or presumably simply battery drain during use) then it will appear on the -Negative terminal. Whilst this can interrupt the connection between the surfaces of the battery terminal and the cable, leading to poor starting, the next stage of corrosion is the truly destructive one, where it turns to Copper Oxide. This eats away at the copper strands from the outside in and is much more sneaky.

It can appear anywhere in your loom, but is usually near an end terminal and in our experience is most prevalent in the boot and close to the rear lights. It seems to be (though this is my speculation) where moisture can wick between the copper strands under the insulator and then, the current causes electrolysis that accelerates the corrosion of the copper.

It’s not confined to classic cars. Another hobby where it is more recognised and has a destructive reputation is in Radio Control Model Aircraft, where the problem if undiscovered can have a catastrophic outcome with loss of control. One common link seems to be that the model will have been stored in a garage or somewhere similar that isn’t heated and the battery pack was left in place…oh, just like classic cars…


If your car is showing any maladies such as dim lights at one location, thats the place to start. If no problems exist then have a scan of any terminals exposed to air. This might be inside the rear lights or near earth anchorages at locations around the loom. If the copper looks like its covered in a black soot then take a deep breath. To find hidden black wire syndrome run your fingers along the loom insulation and hunt for hard areas where it seems inflexible. This may just be a joint and is fine, but if the area is bulged (the corrosion furs and expands) and particularly if it creaks as you try and flex it, then its worth investigating.

If you find it, don’t leave it. It is not just a surface effect, it will eat into your cable and make it hard and brittle. Even just a surface coating over a long length of wire will reduce the conductivity of the wire as the strands can no longer work together. If the bulb or motor is trying to draw current but the electricity meets resistance it causes heating and your looms insulation will soon be melting together, or worse…


It is a b*******r to remove - abrasion doesn’t work unless you can clean each individual strand. You will struggle to solder it even if the area looks clean. If it is just at the end of the wire it is possible chemically and we’ll shortly be stocking a product to do just that. If it extends for any distance over a couple of inches it needs to be cut out and new cable spliced in to a high standard, not a ‘Scotchlock’ (a particular hate of ours) or a 13amp screw terminal wrapped in insulation tape!

Solder and modern heat-shrink is a wonderful thing and it doesn’t take too long if you need to pay someone competent to do it. Don’t forget to clean any excess flux off the joint before heat-shrinking it.

As for the trailer? I rewired the whole thing, but that’s pretty easy on a trailer. Good luck and be safe out there.

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