Ignition coil head scratcher

paul culver

Member III
I've been having engine (Atomic 4) cut-outs after about one hour of motoring. It "heals" itself after about 30 minutes of rest, then repeats this cycle. An in-line spark tester showed loss of spark when the engine died and return of spark when it recovered. Replacing the coil appears to have fixed the problem, at least up to two hours of running under load.

When I room temperature bench-tested the new coil with a VOM it showed resistances of 3.2 ohms across the primary and 8,790 ohms across the secondary. When I similarly tested the old suspect coil I got 3.0 ohms across the primary but couldn't get a reading across the secondary at any range setting on the VOM. Its as if the secondary was an open circuit.

So I wonder why did the old coil even work at all during start-up, and what happens when it craps out an hour later, presumably from being heated up. Thanks in advance for your answers.
 

Parrothead

Member III
Your symptom set is well presented and exactly fits a known side effect of an electronic ignition upgrade that was fully researched and resolved by members of the Moyer Marine forum 11 years ago.

Upgrading to electronic ignition brings twice the dwell of the original points which translates directly to internal coil heat. Once the coil has been heated beyond a certain threshold the insulation on the internal windings breaks down and coil failure results. Following a cool down the insulation properties are somewhat restored thus coil function returns but not completely. The coil is damaged internally and will never perform as before. The damage is cumulative so it may not start the first time electronic ignition is used but nevertheless you're already on the slippery slope.

The remedy: keep your ignition circuit current below 4 amps. This is done by using a proper internal resistance oil filled coil (see Moyer Marine's online catalog) AND calculating your system amperage, adding external coil resistance if indicated by the calculation. To do the calculation measure the internal resistance of your new coil primary winding. It should be around 3 ohms. Next, measure the coil input voltage with the engine running at a minimum of 1000 RPM. Depending on your alternator you can expect the voltage to be in the area of 14 volts. Divide the voltage by the coil resistance to get amperage. If it's 4 amps or greater, add a ballast resistor in series with the coil input post.

There is a slick ballast resistor calculator on the Moyer forum designed for this purpose. You input the voltage and coil resistance, it returns a resistor value if one is necessary.
 

paul culver

Member III
Thanks for the excellent feedback Parrothead. My replacement coil is oil ballasted and I have ordered a back-up to keep on board. I'll consider adding resistance if needed. I'm still wondering why an open circuit primary would allow the coil to work for awhile -- I might try to get a look inside of the thing.
 

Jerry VB

E32-3 / M-25XP
Thanks for the excellent feedback Parrothead. My replacement coil is oil ballasted and I have ordered a back-up to keep on board. I'll consider adding resistance if needed. I'm still wondering why an open circuit primary would allow the coil to work for awhile -- I might try to get a look inside of the thing.
Another failure mode is if the secondary coil wire winding breaks, which could happen due to heat or age. The secondary winding is creating the spark - while the gap is small, the coil can arc over the gap and still fire the plug. As the winding deteriorates, the gap increases and, at some point, the coil no longer has enough energy to jump both the wiring gap and the spark plug gap.

If you measure the resistance of the secondary, it will be "open" due to the broken secondary winding, which matches your measurements.
 

Parrothead

Member III
As you decide what to do, consider that since the research was done we have enjoyed a 100% success rate for those who faithfully followed the remedy while those who felt they knew better and freelanced continued to experience overheated coil shutdowns.
 

paul culver

Member III
Another failure mode is if the secondary coil wire winding breaks, which could happen due to heat or age. The secondary winding is creating the spark - while the gap is small, the coil can arc over the gap and still fire the plug. As the winding deteriorates, the gap increases and, at some point, the coil no longer has enough energy to jump both the wiring gap and the spark plug gap.

If you measure the resistance of the secondary, it will be "open" due to the broken secondary winding, which matches your measurements.
This makes sense to me now Jerry. Sorry my second post mentioned opened circuit in the primary when I meant secondary but you got it right. I can see how a gap in the secondary would increase or decrease with changes in heat but the current could spark over it when its small (colder).
 

paul culver

Member III
As you decide what to do, consider that since the research was done we have enjoyed a 100% success rate for those who faithfully followed the remedy while those who felt they knew better and freelanced continued to experience overheated coil shutdowns.
The only thing that would keep me from the remedy is adding one more point of potential malfunction to the system. For instance, upgrading to an electric fuel pump added an oil pressure sensor and an in-line fuse. Ethanol in the gasoline added a filter/separator system. Things are getting crowded in my engine compartment ! On the other hand, the ignition coil lasted at least 10 years after my change-over to electronic ignition and they aren't that expensive to replace or keep on hand for the next shutdown.
 

Alan Gomes

Contributing Partner
The only thing that would keep me from the remedy is adding one more point of potential malfunction to the system.
How would adding a ballast resistor (i.e., assuming the calculations show it is needed) be introducing a point of potential malfunction to the system? There's a strong body of evidence that adding one is necessary to *prevent* a common malfunction, i.e., when/if the current is at or above 4 amps. It seems like adding the resistor in such a case should be a no brainer. Or am I missing something here?
 
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