Joined: Aug 2017 Posts: 120 Threads: 20
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I ought to know the answer to this but car electrics have never been my forte. If your coil has + and - terminals then for a positive earth car + is earth and thus the wire goes to the distributor and - goes to the ignition feed, and vice versa. All the A7 coils I have seen simply show SW or CB, not + or -. But when Sevens changed from negative earth to positive earth in 1935 did the coils fitted have their terminals changed round? If so how do I know if I have a post-35 coil or a pre-35 coil? I know it's not a disaster to fit them the wrong way round but it does not produce the stronger spark if you get it wrong. Or am I talking rubbish?
Joined: Aug 2017 Posts: 172 Threads: 1
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Location: Richmond, Texas, USA
I understand, though am quite prepared to be proved wrong, that coils marked SW and CB are positive-earth coils only. Negative earth are marked + / - .
No, you are not talking rubbish.
Joined: Aug 2017 Posts: 2,748 Threads: 31
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Location: Auckland, NZ
That rule may apply for postwar coils but early neg earth show CB and SW.
If using a very old coil really only certain by checking the polarity. The pencil lead method OK but avoid drawing long sparks or gaps which will not jump as stress the car sytem and beware shock which stresses the human system.
Some use an analogue meter but not intended fro thousands of volts.
Joined: Aug 2017 Posts: 295 Threads: 11
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From what I have been led to believe, it doesn't matter. Or at least not that much. An ignition coil is basically a low voltage to high voltage transformer and because it is an alternating current device it is not affected by polarity. The spark plugs will give the same quality spark regardless of coil polarity but there is an interesting theory that says electrons prefer to flow from hot to cold so you would ideally have a potential negative voltage at the central (hot) electrode with a positive earth system. As you cannot be sure if your coil is designed for a positive or negative earth car you can determine for yourself which way the current is flowing. Detach a HT lead and place it near an earth point (or a plug connector end) with a graphite pencil tip in-between. Crank the engine and you should see a spark jump between wire and pencil, then to earth (or spark plug). There should be a flare between the pencil and earth (or plug). If the flare is going towards the coil the polarity should be corrected by swapping over the wires on the coil.
Joined: Aug 2017 Posts: 431 Threads: 92
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Location: CALGARY AB CANADA
On coils marked CB and SW: CB is for contact breaker(points) and SW is for Switch or the power to the coil.
Stephen
Joined: Mar 2018 Posts: 698 Threads: 33
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Location: Lot region FRANCE
I think that what Ray has said is correct. The down side of getting the polarity wrong is that the High voltage will pass through the Low voltage winding. In effect, a coil is like a transformer, but with only 3 connections. The low voltage winding have a + and - end (irrespective of earth polarity). However the high voltage coil has one end going to the plug and the other end connected to one end of the low voltage coil. If the wrong coil is used. then the return from the high voltage winding will as I say pass through the low voltage winding which is not designed for the elevated voltage (The difference between 6 volts and 10,000 volts or so). Some coils will not take this overload and fail at some point.
Correct me if I am wrong, but aren't coils pulsed DC, not AC ?
Joined: Aug 2017 Posts: 120 Threads: 20
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The Berry article confirms that the connections are different for pos and neg earth systems. Woodrow shows a 1933 wiring diagram (neg earth with coil marked SW going to ignition as we would expect) and 1936 (pos earth) showing just the same. So were coils produced after 1935 with the terminal labels switched round somehow, or was the same old coil used because nobody bothered as the system would still work, albeit not as well as it should?
