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
