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The coil on my 1940 356 engine is the type that has the armored cable from the coil to the ignition key. What is the proper way to test this type of coil? Can get a small spark when grounding high voltage led. All of a sudden spark has gone from weak to none, (using an inline spark indicator). Spark plug wire tests no resistance and points are new and set properly. Do these coils generally slowly fail or do they do so all of a sudden.

The beginnings of coil failure usually show up when they are hot, still giving satisfactory performance at startup and before heating up.

With your particular setup, sudden failure is often not the coil itself but grounding of the hot wire inside the shielded cable due to failure of insulation. This is much more common than you might expect and if that internal wire hasn't been replaced, that's where I'd start. The action of the ammeter on turning on the key should give a clue.

replace the coil. Jacksons old time parts sells them for around $12

Dave is right. It's likely some other problem. I have 3 perfectly good new style coils, because the problem turned out to be elsewhere.

Here are a couple of links that may help.

http://classicmechanic.blogspot.com/2011/05/testing-and-ignition-coil.html

http://www.youtube.com/watch?v=rJYJ3KvPhhY

That sealed system to prevent hot-wiring the car is a bugger. My wire shorted in the center. The protective housing was cut to replace the wire and brazed up under the dash so it doesn't show.

Be careful opening the tabs that hold the lid on the coil. Take a look at the wire to be sure it isn't frayed. If it looks okay, do the You Tube test. If the coil is bad, you'll need a replacement. You can get a 6v that works at most auto supplies, but they won't accept the sealed cap. I ordered one from David Moe, Packard Seattle:http://companies.findthebest.com/l/27411051/Moe-Packard-David-Parts-in-Marysville-WA

Call on a Sunday to reach him.

Dave is right. It's likely some other problem. I have 3 perfectly good new style coils, because the problem turned out to be elsewhere.

Here are a couple of links that may help.

http://classicmechanic.blogspot.com/2011/05/testing-and-ignition-coil.html

http://www.youtube.com/watch?v=rJYJ3KvPhhY

That sealed system to prevent hot-wiring the car is a bugger. My wire shorted in the center. The protective housing was cut to replace the wire and brazed up under the dash so it doesn't show.

Be careful opening the tabs that hold the lid on the coil. Take a look at the wire to be sure it isn't frayed. If it looks okay, do the You Tube test. If the coil is bad, you'll need a replacement. You can get a 6v that works at most auto supplies, but they won't accept the sealed cap. I ordered one from David Moe, Packard Seattle:http://companies.findthebest.com/l/27411051/Moe-Packard-David-Parts-in-Marysville-WA

Call on a Sunday to reach him.

Thanks, I think the problem just got a lot more obvious. After ohm testing primary wires from coil to points and cleaning wire end terminals was just snugging down primary wire to coil post and the post pulled out of the coil! It appears the post was originally threaded into the coil but now was being held in place with solider only. I'm guessing the post had pulled out in the past and someone just poured hot soldier into the coil to connect the low voltage coil end wire to the post. This is something I definitely don't want to do. Will follow up as recommended on getting a new coil. This sounds like the time to also replace the hot wire from the ignition switch to the coil. Has anyone tried to disconnect the old wire from the coil base then tape the new wire to it and pull the new through the cable?