ignition coil: electrical gurus needed!

I am attending Marine mechanics school down here in Orlando Florida, and today we did ignition coil testing. We did an ohms test across the primary and secondary side of ignition coils. I remember reading on here that doing ignition coil tests were inconclusive in that the windings can expand with heat and have an intermittent short. So if that's true, what's the point of doing an ohms test? Is there a better way to test a coil? I guess I should note that these are rising field coils, not collapsing like out tractors, but I figured there shouldn't be a difference...right?
Thanks in advance for any light you can shed!!!

Rob

The resistance readings that you can make on an ignition coil can diagnose opens and shorts in the coil. If either coil is open, either coil is shorted to ground, or either coil is far out of range, the coil is defective and needs replacement.

Resistance measurements alone will not detect all defects, however. Oftentimes the insulation between turns within a winding or between the primary and secondary windings will break down allowing the windings to arc internally. This condition usually cannot be diagnosed by taking resistance measurements. Indeed, such coils often function properly until heated to elevated temperatures where arching produces misfires.

Dean

If the readings, cold, are out of whack, you might as well toss the coil.

I posted some ideas the other day about coil testing. I have a SUN coil tester (which is actually ARMY SURPLUS). That tester "rings" the coil, and the though is that test PROBABLY weeds out some coils that would show OK with the simple OHMS tests.

Also, I have a distributor machine (synchrograph) that is used to test the vacuum and mechanical advance mechanisms on distributors by operating them at various speeds and vacuum levels (where applicable). You can operate the distributor with a coil, and my plan is to watch the high-tension waveform on an ignition 'scope to watch for changes as the coil heats up under load. I have been trying to collect a few coils of unknown condition, to test against each other.

I have to "fab" a mount for the oddball front-mount distributor, though, to clamp itin the machine.

The large number of PPL having problems with Condensers suggests that those coils (+ve & -Ve) have the secondary common connected to the +ve side of the primary. This places the condenser across the secondary if that connection is made to the dist, which most pll do if they have a positive ground.

I have tested (using Current measurements) all the "original" N type coils and found that the "Battery" connection is the common connected to the secondary (this is correct by theory) and places the condenser in the primary circuit of a positive ground system.

All of the automotive coils that I Tested have the +ve term connected to the secondary, which means they are meant to be used in a negative ground system.

I would be nice to have some scope readings of those coils. Theory suggests that the trailing edge will be most affected, if the secondary common is connected to the Dist side.

On the practical side if I was having condenser problems on a side mount dist, that had +ve & -ve marking, I would connect the +ve term of the coil to the battery and change the ground connection of the Battery to Negative (re-polarizing the Gen).

Regards,

For confusing, how about coils labelled "BATT", and "DIST".

Folks don't realize the implications of that... was the coil originally intended for a (-) ground machine, or a (+) ground machine?

I'm about 2 months late on this project. I am getting swamped at the shop, and it will be a while beore I have time to "play" with this project. I have a couple of offers of used coils, plus what I have lying around. Might as well shop ebay for a deal on a DSO for storing and comparing the high-tension waveforms!