[AJD] Another Plea For Education

[Bruce Mahr]

Terry and others: A word of caution on resistance testing. Resistance 
testing doesn't check the circuit under a load, therefore it can pass with 
the required resistance value but still be bad and/or fail after warm.

Bruce Mahr
----- Original Message ----- 
From: "Terry & Carolyn Welshans" <welshans2 at comcast.net>
To: <antique-johndeere at lists.antique-tractor.com>
Sent: Saturday, November 27, 2004 9:57 AM
Subject: Re: [AJD] Another Plea For Education


> 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)
>
> www.midwestoilburners.org
> www.fokisd.org
> www.terrystoy.com
>
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