How to diagnose a bad coil?

J

Jim Kroeger

Member
Is there a way to tell if the coil is on its way out on my 30 Model A? I know you can check for 6v at each terminal with a meter but will this diagnose if it is bad? Can they slowly start to falter therefore fooling you into thinking it is something else that is the problem on a rough running engine or once they go they go and the engine won't run anymore? Any adivice?
 
i have seen them test good but when hot act up. they can be intermittent and i would just plug another one on if you suspect a problem. its always good to carry a spare anyway
 
The only simple way to test a coil is with a coil tester.

Checking voltage and resistance is just not testing what fails.

A coil tester is just some kind of repeating make break device to make a spark across an adjustable gap. It should also allow current to flow for a little while to let it warm up. Most coils are only bad when they are warm.

You can buy coil testers or build one yourself.

How to build a coil tester.
 
HERE is what my coil tester looks like. It is a must because units that have tested good with driving or with VOM will show up as marginal once they have been heated & tested.

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here is my non scientific way of finding a bad coil, if you cant keep your hand on it after the engine dies because its to hot to handle, its bad
 
bhiland said:
Agreed.
Click to expand...

The coil I showed in that link above that I was testing is a coil however it won't get warm at all (much less hot). It even throws a blue spark ...just not very far, -and not very well under an extreme load causing the engine to skip and not produce all the power it was capable of.

So, why doesn't the "hand test" pick up on this substandard coil? One other question, if the hand test is/was accurate, then why were so many coil testers sold?? :o

What I learned is there are quite a few poor to "just OK" coils out there. Once you find a good one, they make a world of difference.

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Brent I couldn't get the link to open, so I don't know what your coil tester looks like. I assume it has a condenser built in to fire the coils, and it may be bad, especailly if the tester is quite old.
 
LOL brent, when your out on a cruise you dont have the benifit of a trunk full of shop test equiptment, so you have to rely on the old fashion way to try and find the problem, a very hot coil is one indicator its bad, another is when it cools down and engine restarts you can figure its bad
 
Brett's tester IMG_7191.jpg
 
Thanks Mike. I'm surprized Brent hasn't had better luck with the coils he's tested.
All the Model A coils I've picked up over the years have had excellent spark.
 
Tom Wesenberg said:
Thanks Mike. I'm surprized Brent hasn't had better luck with the coils he's tested.
All the Model A coils I've picked up over the years have had excellent spark.
Click to expand...

I agree. After buying that tester a few years ago, I went on a buying spree of coils. This was also during a time when Ahooga was not making the repro coils. The coils I had were all testing kinda weak and at that time I wondered if it was the norm. Even the "NOS coil" was not strong so I was questioning whether the tester was any good. After buying a brand new coil along with several other used ones, I quickly found the tester worked properly. Again, the units I did have would spark, just not at the full potential.
 
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