[AT] Questionable ignition coil

Wed Nov 6 11:44:09 PST 2019

I had an 80 model Suburban with 350 and GM HEI coil in the cap. I had 
completely tuned it up and took a trip to Denver from OKC. I noticed 
that at idle, there was always a sputter about every 10 seconds when 
warm.   I checked everything, changed plugs, still had sputter.  The 
coil was an O'reilly house brand.  I took it back, but no exchange on 
electrical parts.  I put the original GM coil back in and it ran fine 
for another 5 years and another engine until I retired the vehicle.
Cecil

On 11/6/2019 11:17 AM, Stephen Offiler 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 
> <mailto: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
>>     <mailto: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 <mailto: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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