How to test a coil

[NeilMcCubbin]

We have no spark at the plugs, and no spark from the end of the high tension wire that feeds the distributor when held close to the cylinder head while cranking.
To test the coil we disconnected the wire going to the contacts in the distributor and touched it on-and-off the coil terminal. Tiny spark at the terminal but no spark at the end of the high tension output wire from the coil. Not even a tingle to my finger.

The resistance of the low tension side of the coil is 0.3 ohms.

This suggests to me that the coil is due for a trip to the dump.

Any comments?
Am I missing anything?

[ndutton]

You're probably right about the coil but the testing description has me a little confused. The coil function test is done with a temporary wire installed on the small coil - post, the other end is touched and removed from the engine block (ground) with the secondary (large) coil wire held near the block to observe spark. I wasn't entirely clear that's what you had done.

The 0.3Ω coil resistance test indicates a dead coil provided your tester was on the correct scale. Having the measurement off by a factor of 10 merits a double check.

If the coil is dead, what killed it? The system needs the points to open and close for proper operation and if they have become loose on their mount causing them to move away from the distributor cam and therefore remain closed at all times, that would cook off a coil to certain destruction. A failed (shorted) condenser would do the same thing as would a pinched wire as it leaves the distributor housing. Point being, the entire system needs careful inspection to ascertain the cause and prevent a repeat performance.

[NeilMcCubbin]

Bad coil

Thanks for your comment
I ad tested essentially as you described, but different wiring

We replaced the coil with one from NAPA and engine started first bang, running more smoothly than before

I reckon the oints are OK, since she runs so well.

[ndutton]

Please tell us more about this NAPA coil. Model #? Internal resistance?

The question remains, what killed the old coil? They don't die on their own.

[NeilMcCubbin]

NAPA coil

I think C 14 in canada but am not sure
I will astkthe owner, my son, for info. He is away and has the receipt

It works fine and cost only $33 Cdn

[ndutton]

I'm pretty sure it's a NAPA Echlin IC14. It should work fine for your points ignition provided the gap/dwell is set to spec. If you ever upgrade to electronic ignition, BEWARE. There's a good chance the IC14 won't survive very long without adding a resistor in series. There are plenty of threads on the subject, this thread hits most of the salient points.

edit:
The question remains however, what killed the old coil? It's possible the old coil was not the right one (too low resistance), possible the dwell had risen (gap fallen) way out of spec, a failing alt. regulator could do it too. If it were me I'd sure like to know everything else in the ignition system is as it should be, that is, not detrimental to the new coil.

[NeilMcCubbin]

NAPA coil

You are correct