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1957 Bel Air sport sedan
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I could have sworn there was a thread here that dealt with testing coils on the bench, but it’s not showing up in my search. Any easy method? I’m a materials/propulsion engineer…. I consider electrical stuff to be black magic…. hence my ignorance of the subject.

My coil.

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Well, bench testing is never a 100% guarantee that it works perfectly when hot / at high rpm, but there are a few things you can do. First is to check the resistance with a multimeter.

According to the manufacturer, the primary side is a 1.4 Ohm resistance, so set your meter to the resistance / Ohms / Ω setting and see what you get between the small positive and negative terminals, should be around 1.4 Ohms, doesn't have to be and is not going to be perfect dead-on, most common failure mode would result in a much higher resistance than factory spec. If resistance is zero or next to zero, that's a different failure mode and also a bad coil.

You can then check between either terminal (or both, go nuts!) of the primary side (the little terminals) and the secondary side (the big one that makes the neat sparks). I don't have an exact spec for this side, but I think 10,000 Ohms is a pretty common rule of thumb. A little less is fine, too much more and it's probably a bad coil.

If you're real ambitious, you can also hook the coil up and repeat the same tests after it's been running for a while and is good and hot. The numbers should be close to the same as mentioned above.
 

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Cocke County, TN. 55 Bel Air Sport Coupe
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Also you set your meter on it's highest ohm scale and see if there is any conductance between the terminals and the case. If so, toss it. Also if you already have a new battery in your pacemaker, or maybe not, you can run your coil wire from the coil to the negative terminal to see if it sparks when you apply 8 volts to it then break the connection. The spark happens when the connection is broken. You can buy a cheap spark gap voltage reader at most automotive parts stores, although the employees may not know they are hanging up in the tool isle. If you have a cheap, maybe 7 to 10 amp old style battery charger you can use that. Or even some little jumper wires from your car battery. Actually the voltage depends on if you use a ballast resistor or not. If so, use your resistor inline with the jumper wires. Or if your jumper wires are real skinny, you may get the voltage drop right there, but don't hold them on too long, just tap it, or you may melt your jumper wires. That should give you more info than you wanted!
 

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Have a coil tester it operates on battery voltage one side of the spark gap terminal towers is threaded the gap can be increased to determine the coil energy. It also has a heat function the coil is heated for 5 minutes then re tested.
Looked on Google, can't find one like it.
Fun tester makes buzzing / spark gap noise = Frankenstein movie stuff.
 

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Have a coil tester it operates on battery voltage one side of the spark gap terminal towers is threaded the gap can be increased to determine the coil energy. It also has a heat function the coil is heated for 5 minutes then re tested.
Looked on Google, can't find one like it.
Fun tester makes buzzing / spark gap noise = Frankenstein movie stuff.
Cool! Pic?
 

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This is a poor mans coil tester that I rigged up to my Sun machine. The wall switch runs 12V to a ballast resistor where it is reduced to 8V and on to the + side of the coil. The - side of the coil is connected to the points lead of a distributor mounted in the machine and opens and closes as the distributor is spun. The high voltage center post of the coil is then connected to the center post of the distributor cap and a bank of spark plugs is fed from the outer posts on the cap. When it is running I can see the plugs firing, not scientific but I can run up the RPM and see if all of the components are working.
I would think a simple battery test set-up would allow someone to see the coil 'fire' with a little work. As mentioned it is not a true test but would verify if it works or not... Al
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When testing coils you need to not only read resistance between + and -, but also from each terminal too the body of the coil. Coils can short to ground, and still give a good reading between terminals, but not work once powered.
I have an old megger I use to actually ring out coils, and electric motors, which is a better way to test coils. It will tell you if the coil breaks down when current flows through it, where a ohm meter wont. The megger I have is one handed down from my dad who was also an electrician. It's built from an antique telephone and has a crank handle on the side to crank up the voltage. Not much current flow, but enough to show if the coil breaks down, and enough to hurt if you hold the leads and crank it!
 

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Cocke County, TN. 55 Bel Air Sport Coupe
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When testing coils you need to not only read resistance between + and -, but also from each terminal too the body of the coil. Coils can short to ground, and still give a good reading between terminals, but not work once powered.
I have an old megger I use to actually ring out coils, and electric motors, which is a better way to test coils. It will tell you if the coil breaks down when current flows through it, where a ohm meter wont. The megger I have is one handed down from my dad who was also an electrician. It's built from an antique telephone and has a crank handle on the side to crank up the voltage. Not much current flow, but enough to show if the coil breaks down, and enough to hurt if you hold the leads and crank it!
I mentioned terminal to case testing in #3. I have an inductance tester in my electronics shop but don't really use it much. I'll have to check a spare coil just the the heck of it. I also have an antique phone, AND my brother gave me a phone tester that has I think a magneto in it and boy does that sucker shock you good! How do you test a coil with it?
 

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How do you test a coil with it?
I test a coil by going from positive to the case, and negative to the case. If the insulation is breaking down under load it will usually short to the case when using the old crank phone megger. Just using an ohm meter to test to the case wont usually tell anything, unless it's a dead short, and then it most likely would be blowing a fuse.
 

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I may be missing something here, what exactly happens while cranking the phone megger that tells you there is a short? I'm really interested in testing mine with the phone tester I have.
 

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I may be missing something here, what exactly happens while cranking the phone megger that tells you there is a short? I'm really interested in testing mine with the phone tester I have.
Nothing happens if the circuit is good, and you're reading it from either terminal to the case. If it's shorted, or insulation breaking down, it will arc to the case and complete the circuit.
I don't use it to read pos. to neg. as the ohm meter works fine for the normal resistance check.
My tester still has the two bells on it, so they only ring if there's continuity. No ringing means nothing is getting through.
 
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