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Coils positives and negatives
#11
And this is from Ian McGowan on the old Forum in 2013:

"Poring over some old Lucas publications recently, I came across a few things which I thought might interest some forumists.

1) Spark polarity.
Coils are normally wound to give a positive earth spark - that is the spark plug insulated electrode is negative with respect to the block; this usually refered to as a negative spark.
This has the advantage of the same sparking efficiency but at roughly a 10% lower HT voltage required to break down the plug gap, thereby reducing the "strain" on the HT lead insulation and that of the distributor cap and rotor arm.
A further advantage is little or no wear at the rotor arm, instead the wear is shared between the 4 brass electrodes in the distributor cap.
This is an excellent indicator of whether you have a negative spark(GOOD) or a positive spark (BAD). Clean the burn marks from rotor arm and distributor cap segments, then after running the engine for a hundred miles or so inspect both; if the burning is most evident on the rotor arm you have a positive spark.

How can this be? The coil "CB" terminal is connected to the distributor and the "SW" terminal goes to the ignition switch as it should be.

Up until mid 1935 all A 7s were wired with the battery (-)negative connected to the chassis. In mid 1935 the British motor industry generally, and Austin were no exception, changed over to the battery(+) positive being connected to the chassis. Consequentially, all coils made after this date (until, I think, sometime in the mid '60s when the industry reverted to negative earth) unless specially labeled "for negative earth only", were wound for positive earth installations. If an earlier, negative earth car has been fitted with a later coil designed for positive earth the spark polarity will be wrong. My 1929 Chummy was such a car having been fitted, at some stage of it's life, with a 1938 coil.
The solution is simple, just swap over the two wires on the coil such that the "CB" terminal is now connected to the ignition switch, and the "SW" terminal is connected to the distributor.
In theory there is a minor disadvantage in doing this as the additive auto transformer effect within the coil is lost but the advantages of negative spark easily outweigh this."

This is the method that alerted me that my coil was connected the 'bad' way round - the rotor arm was being greyed/eroded instead of the distributor cap contacts.

Regards,
Colin
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#12
Very useful and confirms that coils post-1935 were different, but is there an easy way of looking at a coil marked CB and SW and knowing which one it is?
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#13
(12-06-2018, 06:49 AM)Robin Oldfield Wrote: Very useful and confirms that coils post-1935 were different, but is there an easy way of looking at a coil marked CB and SW and knowing which one it is?

I would think not unless it was marked with a date.

Cheers, Tony.
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