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MG MGA - Coil question

I have a spare coil fitted to my car to make it easier to swap coils if it is ever necessary. Now I was thinking (I must stop doing that) if it would be possible to swap over to the spare coil just by flicking a switch. The low tension side of things seems reasonable. Connect the coil distributor wires from both coils to the distributor and connect the two coil power supply wires to the outside connections on a 3 position switch with the power supply being connected to the middle one. So switch in the up position powers coil 1 and in the down position powers coil 2 with the middle position being off. It is the high tension side of things where it might get tricky. The high tension lead would need to be split off in the middle so it goes to each coil, maybe with some kind of splitter as you have with a TV aerial cable when it goes to 2 different TV
p parmenter

I would think you would end up trying to drive the high tension side of the coil you are not using.

Although this would probably work, I think you would suffer from a significantly weakened spark overall and both coils will get hot together.

Also, if coil 1 failed then there is a fairly high probability it will be a HT winding short to ground. This would mean when you switch over you would just ground the No 2 coil HT through the failed coil HT.

The only realistic option I can see, would be to also switch the HT between the two coils. This needs quite a beefy switch with a very wide contact airgap (or one of the more exotic semiconductors). I'm thinking it would be possible using a 3 phase motor contactor but it will need a fairly high 12v current to hold it closed.

My 2p worth


P Reardon

If you put a relay of sufficent capability in the HT side of each coil, powered by low voltage, you could probably make it work. Coil 1 would have it's contacts closed and coil 2 relay would be open. When you fip the switch, low voltage to coil 1 and the relay on coil 1 is turned off causing coil 1 to turn off, the relay to open and appling low voltage to coil 2, closing the relay on coil 2. But, given the reliabilty of coils and the lack of reliabilty of relays, why bother?
G T Foster

Why go through all that trouble to install a spare coil. I have a spare coil mounted in the engine bay on the right hand side, just under the engine bay panel where the bonnet sits. All I did then is lengthen the wires to reach both coils. All is needed is to unplug the wires from one coil and plug into the other. The wires were only lengthened a few centimetres.

F. Camilleri

Yes Frank as I said I have a spare coil already in the engine bay (one in the 1500 position and the other in the 1600 position) and they can be swapped around quite easily. I was merely posing a question i.e. would it be possible to swap coils just by flicking a switch. It appears from the previous answers that it isn't possible, certainly not easily.

p parmenter

This thread was discussed between 27/10/2012 and 28/10/2012

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