[AT] Questionable ignition coil

Wed Nov 6 10:06:07 PST 2019

Yup.  And you should have seen my original note before I finally hit send.
It had that info on the points, and a bunch more; 3 solid paragraphs.  Most
hit the cutting room floor when I came around to the conclusion that v-drop
isn't much of a test for the health of a coil.

SO

On Wed, Nov 6, 2019 at 12:57 PM Jim Becker <mr.jebecker at gmail.com> wrote:

> If you measure voltage from the distributor side of the coil to ground,
> you are measuring drop across the ignition points.  If that isn’t near
> zero, your points have a problem.
>
> Jim Becker
>
> *From:* Stephen Offiler
> *Sent:* Wednesday, November 06, 2019 11:17 AM
> *To:* Antique Tractor Email Discussion Group
> *Subject:* Re: [AT] Questionable ignition coil
>
> Voltage drop test only works with current flowing.  In the case of testing
> the ignition primary circuit, points must be closed.  Voltage drop across
> coil depends on whether there's an external ballast resistor in the
> circuit.  If no resistor, a good drop across coil primary terminals is
> slightly less than battery voltage.  But with a resistor, some of the
> voltage drops across the resistor and some drops across the coil.  I'm not
> sure I can see how this test will tell you much about the health of the
> coil, frankly.
>
> SO
>
>
> On Wed, Nov 6, 2019 at 11:16 AM Spencer Yost <spencer at rdfarms.com> wrote:
>
>> Sorry to reply to my own message, but I forgot something. Also check
>> voltage drop. Going into the coil from the ignition switch, you should see
>> battery voltage. For instance if your battery is putting out 6.2 you should
>> see roughly 6.2 going to the coil. On the other terminal end of the coil,
>> there will be a slight voltage drop.  However on my bad coil with the Pacer
>> voltage had dropped all the way to 3.7 from 6.3V.   Can not remember what a
>> good voltage drop is.   Surprising large if I remember right, but not 50%.
>> Maybe I’ll run to the Pacer a little later and see.
>>
>> I don’t know electricity or electronics well enough to know how or why
>> that occurred with reasonable resistance readings but that measurement was
>> the only out of range number I got.
>>
>> I got that little tip from Brice Adams and what was the final straw
>> causing me to get an exchange rather than just continuing to run on the old
>> one.
>>
>> Spencer
>>
>> Sent from my iPhone
>>
>> On Nov 6, 2019, at 11:03 AM, Spencer Yost <spencer at rdfarms.com> wrote:
>>
>> With an ohmmeter across the two screw terminals, test resistance.  Then
>> test across negative and the coil wire socket.  Make sure you don’t have
>> zero or an infinite reading on either. Any other reading is highly coil
>> specific and hard to generalize.   I usually see 1-9ohms depending on which
>> of the two tests, voltage, brand, age, etc.
>>
>> However, having said this, I find the tests generally useless as most
>> coils fail on load and heat and these tests do not rule out these issues.
>> For instance I once had a Farmall A coil that would die when hot but tested
>> fine on the bench.  A new coil and the tractor purred all day long for
>> another year or two till I sold it.
>>
>> Another note:  about two years ago I gave the Pacer a tuneup.  Would
>> hardly run.   I couldn’t figure it out until, out of frustration, I put the
>> old coil back on.   Ran great.   The new coil tested fine.   Fortunately I
>> had bought the coil from Napa and they exchanged it.  The second new one
>> worked fine and I am still running on it.  So you can get a bad coil out of
>> the box in these days of poor quality analog ignition components.
>>
>> I’m sure others more knowledgable will jump in with additional advice.
>>
>> Spencer
>>
>> Sent from my iPhone
>>
>> On Nov 5, 2019, at 8:19 PM, Gilbert Schwartz <vschwartz1 at comcast.net>
>> wrote:
>>
>> 
>> I would like to try and delve into the wide knowledge and expertise of
>> the members within ATIS.
>> Is there any acceptable method of determining if an ignition coil is good
>> short of installing it on a running engine. Our 1949 AC G would not start a
>> few days ago after being parked for a short period of time. A quick
>> observation determined the contact points were beyond use. I went to the
>> local auto parts store and was advised I probably should replace both the
>> points and of the condenser because they felt like that condenser went
>> first and it caused the points to burn up. With this information I also
>> purchased a new coil even though I wondered if the coil was a bad.
>> Bottom line; Is there any way to bench test a coil? We have several used
>> coils laying around here of questionable viability. I would like to pitch
>> them if they aren't any good.
>> I appreciate anything you can tell me.
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