The Stovebolt Site Forums The Shop Area The Electrical Bay Testing a coil

How do you guys test ignition coils?

I have followed the internet instructions from a search, but am not sure they are correct.

I tested for resistance on the primary and secondary circuits. On 4 coils, there was no resistance on the primaries. School solution says this makes the coils bad. I am a little reluctant to trash all four, because at least two were working before I started tearing things down.

Sounds like you may need a tester tester for your tester. (No typos, BTW).

I don't know what you mean about no resistance on the primary but if you register an open circuit, ie. infinite resistance between the positive and negative terminals, it would be no good.

Your ohmmeter will have to be able to accurately measure resistances of 5 ohms or less. Most bargain basement digital meters simply aren't that accurate.

Since most 12 volt coils need to flow 5 amps or so through the points, a simple Ohm's Law calculation would involve dividing 12 volts by 5 to get 2.4 Ohms of primary resistance. So far, so good, but most GM "12 volt" coils are actually wound to flow that 5 amp maximum at about 9.5 volts, which will compute out to 1.9 Ohms. That's so an engine will start properly on a cold morning, when the load of cranking a cold engine puts a heavy load on the starter and drops the battery voltage to a little over 9 volts. Then GM puts a resistor in series with the coil to keep the points from burning when the charging system voltage goes up to 14.5 or so. A "good" external-resistor 12 volt GM coil will show less than 2 ohms of resistance. Good luck measuring that with a 5 dollar digital ohmmeter!

Secondary winding resistance will be 2,000 ohms or more, and even if a coil checks out "good" on both windings with an ohmmeter, there's no way to evaluate an internal insulation breakdown that will allow the secondary voltage to jump to the primary winding under load, causing an intermittent miss while passing, pulling a long hill, etc.
Jerry

Let me thicken the plot.

This is a 6 Volt system. I have a brand new 6V alternator, and a new 6V battery. I have a new and somewhat cheap digital multimeter and an old beater analog multimeter. I have since been told by another respected consultant that the digital does not do all things and an analog may be better. The digital will register Ohms on the secondary( Tower of the Coil) but I get no resistance on the primary, (positive post to negative post), even on lower settings. The analog does register, but I am unable to adjust to needle to see what the ohms are.

The ultimate problem I am trying to solve is getting a good spark. I had weak spark at the points when I tried to start the engine and nothing at the plugs. I wanted to see if the coil was good just to check everything in the system is in good shape.

A voltmeter will be a better diagnostic too than an ohmmeter. With the points open and the ignition switch on, you should be able to measure full battery voltage at both coil primary terminals. With the points closed, You'll get battery voltage at the upstream terminal (from the switch) and almost zero voltage on the negative terminal. More than a fraction of a volt at the points when they're closed indicates dirty points- - - - -clean or change them. DO NOT use a file on points- - - -I prefer to clean them with a piece of business card stock, etc.

A bad condenser can cause a no spark condition. Also, you'll need to test the coil output by holding the coil secondary wire close to a ground and have someone crank the engine. You should get a spark at least 1/4" long. If you get spark at the coil secondary and not at the plugs, check to see that the distributor cap and the rotor are the right size- - - -a tall cap and a short rotor will not get a spark to the plugs even if the coil is good.
Jerry

50 % of the time, I have had brand new points cause weak spark. The manufacturers apparently put something on them to protect against corrosion during the the slow boat ride in a container ship coming from China. Clean them as HRL describes.
I have many coils and they all read like you describe. It means nothing.
It is very easy to ID a bad coil. The juice is leaking out because someone "let all the smoke out of it". No one sells that smoke, and even if they did, you can't put it back in.

I had changed the condenser yesterday, so I changed the points today, the cap are both of the short variety and new. The points seem to have fixed it. I have an inline spark tester on the wire to #1 plug and it now lights. Engine still won't start, but at least there is some spark at this point.