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Mysterious short circuit

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It's too dark and cold in the garage to carry on experimenting.. but last time I used the prewar 16H I found a short circuit in the lighting. Ammeter popped across full! No fuse but no apparent damage in very short time.

But I can't find any chafed wires. I can't see how the live can short to earth inside the loom. Using a low power battery to test it. It happens on side and main headlamp. So cannot be dip switch nor head nor side. Now sitting here and writing, I'm thinking tail lamp might be faulty since that is the only thing live at both switch positions.

But can the switch do this? It's so simple I can't see how.

From Anna's post preceding this I think it might be U38. But that's a guess. It does not look like U39.

Help appreciated!

David Cooper 

 

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Tail lamp sounds logical. As you say probably not dip switch but not sure why you are able to eliminate the side lamp at this stage. Speedo illumination if fitted also a possibility. Not many circuits to test so starting within the headlamp shell identify the feeds to dipswitch, pilot lamp, speedo illumination and tail light. Disconnect one at a time and see if the short clears. Once you know which circuit is faulty work through that circuit until you can identify the cause. Fuse definitely recommended, stops the smoke escaping!

Good luck, Ian McD

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Thanks Ian. Too cold now... sleet lying on the ground and my fan heater has also packed in. Gremlins in the garage I suspect. I might pit  a bulb in adjacent to the battery to allow each circuit to be checked. I have LED bulbs so I think I must remove them to check resistances. Don't know how they behave with a digital multimeter across them!

 

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I wonder if LED bulbs can fail short circuit? Not necessarily the LED itself but more likely the components that regulate the voltage/current supplied to the LED. When you remove the bulbs re-check for short circuits.

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Took the bulbs out and no change. My bulb now in series with the battery lights up as soon as I turn the switch.  Then I detached the live feed to the headlamp, and touched it to earth, and the headlight came on!  So now as I write this I'm thinking it through, and my guess is that earth return from the headlight must be live when the light switch is turned.  The original diagram had no such earth return, so maybe I have put the headlight earth return onto a live terminal on the switch.    I can't think when!  But I've not done any night riding for months.

It's not helped by having the panel tank, and too many wires have non standard colours.  I think I'd better bite the bullet and rewire with the proper colours if I can find them.  Then I should be able to sort out any future issues a bit more easily.

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Sorted - it was the wiring error I described above.

Now to work out why the pilot light stays on with the headlamp.  Not correct, I'm sure.

Also - I see from reprint manuals from the 1930's that before the proper voltage regulator arrived, the switch had the pair of brass rockers at the top.  Just as it did in the 50s and 50s.  But mine does  not - I wonder if they just fell off?  They are not on the diagram for 1937 and 1937 but are clearly photographed for 1932 and 1933.  Another mystery.

I sell the proper colour cables, but the post 1963 colours ie the ones with a stripe down the side. 1950s cables and colours would be very hard to supply as things like two tones of red are no longer made, and so called cotton covered wire is often PVC inside with braided cotton on the out side. Also wired with 1963 colours which are standard across the British Car and Bike industry gives you a colour standard we ALL understand, ie Commandoes (crap spelling as no spell checker still)

aoservices.co.uk

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Thanks Al. Having cured the short (which was of course self-created) I now find my tail light does not work. Must be my fault as it was OK last year. But so many wires are black it's not simple to find the problem. In my defence it's a panel tank so although I think it looks nice it's horribly awkward to diagnose and easy to misconnect.

Tank has to come off... again. And of course it's only just been refilled...wait til it's warmer I think.

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When I rebuilt a 1931 Sunbeam a couple of years ago I used black (but not cotton covered) wires to keep it vaguely in keeping but found it a nightmare to keep track. So for the 1952 ES2 I've used thinwall modern colours (the colours are actually the same as the original wiring diagram) and found it much less stressful...

 

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