Magneto Issues

[Robert Clifford-Wing]

The push switches controlling the ML magnetos fitted to our 3 litre have started not isolating them when the engine is at normal temperatures; this is common to both mag switches. Oddly they work on and off soon after starting the engine.

Can members shed any light on this phenomenon?

[John Murch]

I've experienced similar in the past, it proved to be the little bakerlite cap on one mag was not seated properly causing a lack of earth when trying to switch off.
John

[Robert Clifford-Wing]

Thank you Jobn, i have had this happen to me too in the past. I removed both black caps and reseated them with the dashboard switches pulled out with but to no effect.

[Iain Warner]

If it is common to both magnetos I suggest checking the connections on both ends of the earth wire from switch rather than the individual wires to each magneto, (that is the cable from the centre connection on the back of the switch)
Good luck
Iain

[Robert Craven]

I've got Scintilla magnetos but the principle should be the same. The switches stop the magnetos by earthing them, don't they? So if temporarily you can earth the wires from the mags directly and that does or does not stop the mag in question you can tell whether the fault it is at the mag, or in the wire or at the switch. But of course that may mean cutting the wires which you may not want to do.
Instead, perhaps a start would be to check the earth at the switches. Presumably this is common to both switches, whereas it seems unlikely both mags would develop the same fault at the same time. Though a few years ago mine both suffered from oil leaking from the cross shaft, which sometimes made starting when hot difficult.

I now see that while I was writing this Iain has said much the same thing much more concisely.

[Robert Clifford-Wing]

[Dan Suskin]

I'm not sure you can say it's both magnetos. If one magneto does not get earthed, the car will keep running and the other magneto may be grounded and thus "off" - but the car still runs. You can check that first by checking for spark from each magneto. Then when you possibly find just one magneto at fault, it will be easier to find the fault.

[Robert Clifford-Wing]

[Dan Suskin]

I doubt that's the problem, as that would likely ground the magneto if it does anything at all - and grounding the magneto would shut it off.

Check for spark on each magneto when the switch is in the off position before doing anything else - I think you'll find that only magneto is still generating spark. Then track down the problem to that magneto or it's switch.

[Robert Craven]

Rob,
Just in case I'm misunderstanding you, what did you mean when you said in your original post that the switches are "not isolating" the magnetos? Did you mean that the switches will turn the magnetos off (i.e short them to earth) or that they will not turn them on, i.e. allow them to work?

[Robert Clifford-Wing]

[John Robins]

So you will now be presented with a further dilemma in the choice between modern cable insulation which would need to be hidden away in some sort of conduit, or go for originality. In view of the general lack of amps in a vintage car I think my approach would be to get as much copper as is feasible between battery, dynamo and load, and ensure that all the earth straps and connections are in good order.

[Robert Clifford-Wing]

[John Robins]

No doubt he will have all the best answers. Good luck and bright lights!

[Robert Clifford-Wing]

[Robert Clifford-Wing]