Cause of burned wiring

[Nick Salmon]

A pal of mine asked me to take a look at the wiring on his 1935 four-seat tourer this morning. He was concerned at some signs of heat damage to the loom - in particular the wire from the D terminal of the cutout to the ignition switch.

Sure enough, it transpired the entire length of that wire has burned - right through in places. He's a lucky chap his car didn't go up...

Having removed the dashboard I cannot see anything obvious by way of a short circuit. There are a couple of scorch marks on other wires but since the yellow wire from cutout to switch was sheathed in a piece of plastic sleeving I don't think yellow wire is the cause. 

Further, the black wire from ignition switch to ignition light is untouched. It connects with the burned yellow wire at the switch of course. Also, the wire from the dynamo to the D terminal is untouched.

My question is - what could have cause the short/burning? Is it possible for the switch to short internally and cause this?

Any clues will be gratefully received!


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[John Cornforth]

Hi Nick

That yellow wire appears to have got hot due to excessive current flow, possibly something that has been going on intermittently for a long time. It's also possible that the fault/short circuit that caused it has now gone away or been blown out. The lighter colours show overheating more readily of course. A poor connection will produce localised heat, which might discolour a wire at its end but not along its length as your pictures show.

That wire feeds current from the Dynamo output to the Field winding via the "Charge Rate" section of the PLC switch, in all settings other than Summer Half Charge when the Lights are off. Normally, you would expect no more than a couple of amps to be flowing through it which is well within its capabilities

PLC switches are simple and normally reliable, I have never come across one with an internal short.

At the PLC switch, it branches off to feed the Ignition Warning Lamp, via the black wire you mention. I'm wondering if there is or has been a short circuit between the warning lamp holder and the grounded dashboard metalwork. These lampholders have "cardboard" insulation which isn't that robust. The black wire won't show up so much if it is blackened !

The other possibility is that excess current is flowing in the green Field wire of the Dynamo, due to a short or intermittent short of the Field circuit to ground within the dynamo.

[Nick Salmon]

Many thanks John. Very helpful indeed.

[David Cochrane]

Yet another reminder to ensure that you have at least one fuse in the system, and preferably more. At the very minimum, fit an inline fuse of 20A in the wire from the battery to the ammeter. If a short-circuit occurs, then the fuse will blow rather than your rare and expensive ammeter!

[Nick Salmon]

Point well made David. If I cannot find an obvious cause ( the ignition switch tests OK) I will replace the burned wire and see what happens. Maybe put an inline fuse on it?

[David Cochrane]

First put an inline fuse between the battery and the ammeter. Then beg, buy or borrow an AVOmeter with a 10A range (there are loads on eBay, or you can borrow one of mine) and fit that at one end of the replacement wire. It will show you how much current is passing through that wire. If there is excessive current, then the AVOmeter's trip switch will pop up.

[Nick Salmon]

Thanks David. Happily, I have an AVO meter...

[Nick Salmon]

Little update - Switch back in and burned wire replaced with new, fused wire. Started up the engine and ignition light stayed on. Checked dynamo and found blown fuse. Shall replace tomorrow and have another go (and also put AVO on it).

Being ignorant of electrickery I'd like to understand the function of the resistance wires in the junction box on the dynamo - and if they are necessary when there is similar resistance wiring in the cutout. Could someone enlighten me please?

(Pic borrowed from another site).


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["Slack Alice" Simon]

Different models, different ways of doing the same thing.

Either the dynamo or the cut out are not original to the car.

You only need one resistance - it reduces the feed to the feed coil, so reduces the dynamo output to avoid bubbling the battery in the months when you are not using the lights.

[Pearls not a Singer]

Just for clarity I think Slack Alice's post should read feed to the field coils