If I may share from the Keystone Region MG Club, Inc website an
excellent article:
http://www.keystonemg.com/tech/coil-polarity.html
Coil Polarity
When converting an older British car over from positive to negative
ground I came across the question of coil polarity. I discovered coil
polarity is very much misunderstood. In researching it, I was very
confused until I found out there are two definitions of coil
polarity. I talked to three or four knowledgeable people on the
subject and read several technical books and articles. Everything made
sense in itself but didn't jive together until I found out they were
talking apples and oranges.
Definition #l Coil Polarity (In relation to battery)
The polarity of the coil should match that of the battery by
connecting it so (+) goes to (+) and (-) connects to (-). But don't
worry about which way you install the battery (positive or negative
ground) or which way you install the coil (regardless of coil
markings) it will automatically adjust itself. The coil will work
efficiently and put out the same voltage either way it is hooked up,
but the spark plugs are more sensitive when it comes to polarity,
hence our second and more important definition.
Definition #2 Coil Polarity (In relation to spark plugs)
Coil polarity should be such so as to provide negative polarity to the
spark plugs center electrode.
It has been found that it takes approximately 15% less voltage to form
an arc at the plugs, if the hotter center electrode is negative, and
the cooler (by comparison) ground electrode is positive. The center
electrode is hotter since heat transfer from the tip must make its way
through the porcelain insulator past the sealing gaskets to the shell
block and then to the water jackets.
If your center electrode is positive, your car will probably still run
fine until, with its 15% handicap, it exceeds the coil output. If you
live where temperatures dip down to 00 you may not get your car
started. Driving with a full load and accelerating hard up a hill may
cause an ignition miss. If your ignition system is well worn to where
you have various voltage losses, you could get a miss.
Correct coil polarity won't eliminate these problems, just put them
off by 15%.
If your coil has - & + markings by the primary terminals, you will be
pretty safe by hooking it up by those marks, but test it for correct
polarity anyway, using one of the tests listed further on. If your
coil has CB & SW or BAT & DIST markings, there is no way of telling if
the coil was marked in relation to a positive or negative ground car,
and the only sure way to tell if the coil is installed right is to
test it out.
You test for correct polarity by hooking up a voltmeter with the
negative lead to the plug terminal (which should be of negative
polarity) and the positive lead to the block (which should be of
positive polarity). Set the meter on the highest volt range. These
connections remain the same whether you have a positive ground or
negative ground electrical system. The secondary winding's polarity
which we are testing is determined by the combined hookup of the
battery and primary windings, so it may or may not match the battery's
ground.
Cranking the engine over (you don't have to start it) should show an
upward swing of the voltmeter needle (don't be concerned with taking a
reading). If the needle swings down off the scale, your coil is hooked
up wrong. To correct, reverse coil primary leads. Do not worry about
the coil markings (refer to definition #1).
If you don't have a voltmeter, test by removing a plug wire from a
plug and hold a plain lead pencil point in the path of the arc. A
flair (hard to see) towards the plug shows correct polarity while a
flair towards the coil shows reversed polarity.
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