Under the Bonnet: Back to Basics – Your Ignition System

In the last issue, we discussed the basics of ignition timing. Since quite a few readers commented on this article. I would like to address some of their comments here, for all to share. I make no apology for trying to make this Under the Bonnet column as simple and as basic as possible. It is purposely designed (and hopefully written) to be such. The idea behind this is to provide the basic information which workshop manuals often assume the professional mechanic to know; information for the beginner rather than for those who could write their own workshop manuals.

The most common comment from readers was to point out that many workshop manuals do not specify dwell angles, but only point gaps. I do apologize for overlooking this fact. If the point gap is accurately set. the dwell angle will automatically be within acceptable operational limits. If the distributor is in good mechanical condition.

Generally, for point type distributors with no published dwell angle, look for a dwell angle of 60 degrees (plus or minus 3) on four cylinder engines, and 35 degrees (plus or minus 3) on six cylinder engines.

In this issue, we are reprinting two short articles which have appeared previously in Moss Motoring, which provide more interesting information related to ignition systems.

Coil Polarity

I was converting my older British car over from positive to negative ground when 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 #1 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)

Coll polarity should be such so as to provide negative polarity to the spark plug’s 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 elect rode 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. The ground electrode heat just has to get from the shell, to the block, then water jackets.

If your center electrodes 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 0° 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 a lot of 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 (-) and (-) 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 and SW, or BAT and DIST markings, there is no way of telling if the coil was marked in relation for 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 Irom 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.

(Our thanks to David Edgar of La Mesa, California for his 1984 contribution to Moss Motoring. —Ed)

Ballast Resistor Ignition Systems

Conventional ignition coils suffer the disadvantaged being designed to operate best at about 12 volts. Unfortunately, a 12 volt battery often produces as little as 7 volts when “run down” because of excess starter operation, especially in extreme cold. To produce optimum coil performance (and hence nice fat sparks at the spark plugs) under such adverse conditions, the “ballast resistor” or “ballasted coil” system was developed.

This system uses a coil which is designed to be most efficient at about 8 volts. For starting, full battery voltage is supplied! This makes this system as efficient at low battery voltage as a “conventional” coil is with the battery supplying a full 12 volts. (For any battery voltage above the coil’s design voltage, it’s even better – an “overboost” condition.)

However, an eight volt coil cannot be run continuously at 12 volts without over heating and failing. As soon as the starter switch is released, the coil no longer receives full battery voltage. It is then powered through the ballast resistor which reduces the 12 volts (the generating system is now working) to the coil’s design voltage.