Ignition Wiring Blurbs

Date: 24 Aug 1994 16:21:54 -0600
From: "Rob Reilly"
Subject: Re-coil wiring question

>Subject: Time:1:51 PM OFFICE MEMO Re:coil wiring question Date:8/24/94
>I am finishing up the wiring on the '63 B and have a question regarding the coil. In the Haynes manual it
> identifies the two plug connectors at the top of the coil as "sw" and "cb." When I look at the coil I see
> only a (+) and a (-). Keeping in mind that this is going to remain a (+) earth car, I suppose that the lead
> that goes from the coil to the plug on the body of the distributor is (-)? Yes/No?
>Will Zehring

The jag-lovers list talked about this on June 21-23. Weren't you paying attention, Will? ;-) Anyway, I saved a couple of gems: -

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From Ned in AU (sorry Ned your last name's not in the message):

I will quote from the book to clarify. In the chapter on ignition system service (note that this is a book published in USA, therefore most examples given are for the cars made in US). "The coil polarity must be connected into the primary circuit so the coil polarity (+ or -) marks correspond to those of the battery. If the battery negative post is grounded, which is common practice, the negative terminal on the coil must be connected to the distributor. This will ground it through the contact points. By connecting the coil in this fashion, the CENTER electrode of the spark plug will assume NEGATIVE polarity. The center electrode of the plug is always hotter than the side electrode. Since it takes less voltage to cause electrons to move from a hot to a cold surface, current flow must be from the hot center to the cooler side electrode. ... If the coil is connected so the plug center electrode is positive, up to 40% more voltage will be required to fire the plug."

... The text goes on to describe TWO ways of checking the polarity of the coil. The test should only tell you if the connection is OK. Since we are talking about 20,000V (coil output) I don't think that a simple voltmeter can measure this easily.
Auto Service and Repair by MW Stockel and MT Stockel
The Goodheart-Willcox Company, Inc, 1984 (looks like 4th Ed)
South Holland, Illinois ISBN 0-87006-466-5

Ned nbx@ansto.gov.au -
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Rob's interjection: This sounds to me like an argument for going to negative ground. -
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And from Ken Hampton (ken@sd.com):

The basic Kettering ignition circuit is thus:

   IGN ___/\/\/\/\/\__/\/\/\/\/\/\/\/\/\/\/\/\___>spark happens 
            primary |    secondary windings    | contact breakers

The fundamental idea is that when the contact breakers (points in American) are closed current flows through the primary of this transformer and builds up a magnetic field. This field is in common with the secondary because the secondary windings are would around the primary. The speed of current buildup, hence magnetic field strength is governed by the resistance and impedance (different from resistance but complex so I won't go into it here) in the primary circuit. When the contact breakers OPEN the magnetic field around the coil collapses very suddenly so the di/dt causes a large spike of energy in the secondary which is the spark. The ratio of turns in the coil in a non-electronic ignition is usually 250:1, that is for every one winding in the primary there are 250 in the secondary. 25,000 volts at the spark divided by 250 sounds like 250 volts in the primary when the coil is sparking. Can this be? Yes. This is why if you design an electronic ignition for your old Jag you need to use SCRs or Thyristors of at least 300 PRV rating. Remember, it is the relative rate of collapse of the magnetic field that causes this, not the 12 volts at some 10 amps you feed into the coil. It operates like a pump. When the contact breakers are closed it gets primed, when they open it surges.

But what does the condenser do? Two things. First, since the spike of energy can cause a fat spark at the plug end of the secondary it can also cause a fat spark across the contact breakers causing them to erode quickly. The condenser across the contact breakers. Second, the condenser along with the secondary forms a (sort of) tuned circuit which enhances the energy in the spark across the plugs. Now, how do we find out if the coil is wired correctly? Disconnect it completely then measure the resistance from the SPARK lead to each of the contacts (+/-, CB/IG, etc). The contact with the highest resistance reading (there actually won't be much difference, only an ohm or 2) goes to the ignition switch, the other goes to the contact breakers.
Ken -
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Sounds like the definitive answer to me. I think if I interpret this last paragraph correctly, then CB or - would go to the contact breakers regardless of whether the car was + or - ground, because of the way the windings are connected to the terminals. Do I have it right? Rob