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
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