The Stovebolt Site Forums The Shop Area The Electrical Bay How do I Test an Ignition Coil?

I have an assortment of ignition coils. Some are 6v, some are 12v, some are who-knows-what?

How do I test these to determine whether to save them or discard them? Might as well test the installed ones, too.

Easy to check for definitely bad.

Using an ohmeter, check from + to - on the coil. Depending on the coil it should read between .5 ohms and 4 ohms. If not it is defective.

Then check from - to the metal in the bottom of the socket where the high voltage lead plugs in. Depending on the coil it should read between 5000 ohms and 30000 ohms. If not it is defective.

Anything else has at least a chance of being good and could only be tested by either trying it on the truck/car or by using expensive test equipment.

Otto Could just use your battery charger if it,s 6/12 volt especially,or jumper cables,I use battery charger to try HEI coils in the unit.

Thanks Bill.

I'm using a multimeter. I assume this is done with no battery power connected to the unit?

Also, is the .5 - 4 ohm applicable to both 6v and 12v coils?

Is there a way to tell the difference between a 6v and 12v coil if there are no markings on it? Does it matter?

Defective coils often ohm out good
The problem is the insulation breaks down internally so the spark is leaking inside the coil and not getting to the distributor.
A leakage test is much more involved because it’s voltage dependent and not linear and often temperature dependent too
Which is why a hot no start condition can be a bad coil sometimes.

A/B testing the coil on a working engine and getting it to temperature would tell you if it’s good or not but that’s a lot of work but maybe worth it if you want a known good spare coil under the seat for road-trips.
-s

Otto:

You are correct in assuming all wires disconnected from the coil.

No way to tell 6v vs. 12v coil that I know of on an unmarked coil. Not knowing if the coil under test is good or bad, 6v or 12v is why I gave such a large range of measurement.

Any coil failing the ohms testing I talked about is definitely bad. The "Anything else has at least a chance of being good and could only be tested by either trying it on the truck/car or by using expensive test equipment." is what 2ManyTrucks is talking about.

Thanks

Any time you're measuring resistance with a multimeter, you have to do that with the item being checked disconnected. With your meter in Ohms position, it uses the internal battery to run the meter, putting a little voltage through the item. Outside voltage (if connected) will throw the reading off completely.

You can do a cold check of a coil on the bench with a battery, a wire to a spark plug, some wiring to the battery. Connect it up, then disconnect power to the primary terminal and it should fire a spark (that's the function the points serve if installed with a distributor.) As stated above, that wouldn't tell you if the insulation breaks down as it heats up with use though.

It would be fairly simple to fabricate a test fixture using a spare distributor, a vise, and a variable speed drill motor. Set the drill up to spin the dizzy at a moderate speed, and hook up a battery (or a battery charger) and a spark plug to fire the plug for a while. If the plug is still firing after an hour or two, the coil is probably worth hanging onto.
Jerry

Jerry, that's way better than my idea.
Let that sucker get hot!

You can even use a hair dryer to speed up the process a little!
Jerry

... or the drill would be dead!

I've also used my lathe as a drive unit to test alternators, generators, distributors, etc. Unless the power company runs out of juice, the battery never runs down! If you really want to stress the coil, open up the plug gap to about .050"- - - - -That requires 20KV or so to fire the plug.
Jerry

I think I'm right in saying that, generally speaking, the primary winding resistance, between the + and - terminals of the coil would read higher (roughly double) on a 12 volt coil as compared to a 6 volt coil. I do not think that I would want to use that as an absolute test to determine if it is a 6V or a 12V. But, still, I think it's generally true.

There's an old saying in the engineering business- - - -one I learned back in the early 1960's when I thought I wanted to be a "train driver". "Constants aren't- - - - - -Variables won't!" Since the current flow in an ignition coil is being interrupted hundreds of times a second by the opening and closing of the points, the rules of Ohm's Law don't apply as they do in a constant current flow situation. There's a pesky little thing known as "inductive reactance" which usually applies to coils in an AC circuit, but it also rears its ugly head where pulsating DC is involved. To over-simplify things, the "resistance" of a winding in an induction coil or a transformer varies as a function of time- - - - -the current starts out at a high rate, and in a tiny fraction of a second, the effective resistance increases as the magnetism created by the current flow induces a "counter'-EMF" which opposes the flow of current. Then the points open and a spike of up to 100 volts or so gets induced into the primary winding. Then the condenser gets involved in the circuit to protect the points from flashing over due to the high induced voltage and the microscopically small gap between the points as they interrupt the current flow.

Simply measuring primary winding resistance might give a ballpark guesstimate about whether a coil is intended for 6V or 12V, but there's a LOT more involved in the calculations than in a simple DC resistive circuit.
Jerry

Like Jerry says, there's more than just resistance at work. Inductance in windings affects transient current. There's also capacitance in other types of circuits. If you had an oscilloscope, you could watch the current change (as a result of the inductance) as you apply voltage to a coil. It's probably too fast to see on an ohmmeter.
I'm not even very knowledgeable about circuits, as I'm just a Mechanical Engineer. I never could understand how the electrons get thru a wire when there's no hole in the middle for them.
The saying in Mechanical Engineering is "E=MCsquared and you can't push on a rope."

The conductor with a hole in it is called a "waveguide"- - - - -mostly used in radar systems! At certain frequencies in that world, a dead short is an "insulator"!
Jerry

In my world, it's called a pipe.

"Conductor with a hole in it" tried to stop the train robbers.

I really appreciate Jerry's (Hot Rod's) explanation of things...always learn something. Thank you Jerry for your posts/info!

It's a habit us old teachers have- - - -walk off and leave our mouth running! I hope someone can glean a little useful information from my ramblings!
Jerry