[AT] Questionable ignition coil

Wed Nov 6 13:04:07 PST 2019

Your right Steve, and all that was implied:   Ignition on and points closed.  I should have been more explicit.

I did run out to my tractor at lunch and my memory was faulty. There is no voltage drop on a good coil. 6.34V to the coil 6.34V coming out. Ignition on & points closed.

So my voltage drop test a few years ago that led me to the fact that the coil was bad was actually valuable. I remember I was getting ~6.3V to the coil but ~3.7V on the back side of the coil.   Like I said, nothing in the resistance readings would have indicated a problem.   My guess/working hypothesis at the time was something along the lines of a hairline fracture at a terminal/post or a bad solder that would heat up and exacerbate the problem with live current but not the trickle current of a meter.  Complete and total speculation of course.   

Of course, while it might be fairly definitive if you see a significant drop, a lack of a drop does not acquit the coil.  Also, after giving it some thought, I’ll bet this would only identify very specific failure modes.

Spencer

Sent from my iPhone

> On Nov 6, 2019, at 12:27 PM, Stephen Offiler <soffiler at gmail.com> wrote:
> 
> 
> 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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