[AJD] Another Plea For Education

[Terry & Carolyn Welshans]

Cal said:

>Man, am I learning a lot about engines from this list.

>Here's my next question.  What would be the symptoms of a coil with
>incorrect resistance value?  This is regards a 12 volt conversion.
>
>Thanks.
>
>Cal Barton
>Nacogdoches, Texas


Terry's comments about diagnosing this problem are right on.

In regards to the question about resistance readings, there are two coil
windings  in an ignition coil, the primary winding that connects to the
points on one end and the ignition switch on the other, which has the fewest
windings and lower resistance; and the secondary winding, which connects to
the spark plugs via the high voltage wire to the distributor on one end and
to ground on the other, which has many more turns than the primary.

If the primary coil gets shorted, it has lower resistance and higher
current.  Higher current makes more heat, and the condition Terry describes
happens - a time delay while the coil gets hot.  If the resistance gets
higher, you get less spark from the secondary.

If the secondary coil gets shorted, you have a weaker spark, as the shorted
windings don't produce much of a spark.

On the other hand, if either one is open, with a broken wire in the
windings, the spark stops.  This can be from overheating, fatigue, or
mechanical damage.

So to test a coil with an ohm meter, measure from the ignition switch lug to
the lug that goes to the points (marked with a "+" and a "-" ) for the
primary side and from the case to the center high voltage wire for the
secondary side.  Compare to another coil for values.

Hope that helps.

Terry Welshans
Homer Glen, Illinois

"Always do right- this will gratify some and astonish the rest. "
Mark Twain (1835-1910)

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