[AT] Questionable ignition coil

szabelski at wildblue.net szabelski at wildblue.net
Wed Nov 6 09:52:31 PST 2019


If you knew how munch wire was used to wind the coil, and the resistance per foot of the wire, you could determine what the resistance of the coil should be. Knowing the resistance you could calculate the expected voltage drop, but you would also need to know the current flowing through the coil at the applied voltage.

E = I/R

You would need a voltage source, an ammeter, and an volt meter. It wouldn’t matter what voltage you use since the current would vary with the voltage used, higher voltage higher current. The resistance would stay the same.

Once set everything up you could apply heat to the coil and see how much the resistance changes with temperature, or if the coil goes open circuit, or shorts out.

But keep in mind that if you determine what the resistance of the coil is, you still won’t know if the coil is good unless you know what the resistance is supposed to be. And that is the major issue with determining a good coil from a bad coil.

Also keep in mind that the coil resistance will vary some between two identical coils due to how well all the internal connections are, and slightly due to the fact that the length of wire used in each coil will vary slightly. That’s why you should check to make sure you’re within a specified range and not looking for an exact number.

Carl

----- Original Message -----
From: Stephen Offiler <soffiler at gmail.com>
To: Antique Tractor Email Discussion Group <at at lists.antique-tractor.com>
Sent: Wed, 06 Nov 2019 12:17:22 -0500 (EST)
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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