So please bear with me as I only have a basic grasp of electrical.

I have just completed a semi-rebuild of my 400 in my 78 f250. I am having a weird condition where the truck will not produce spark while it's warm. I have been eliminating a list of items but can not seem to find any source. The truck starts on a dime after it has been sitting for a while, but once it gets warm it shuts off just like i turned my key off (I didn't). I also will get a single spark when letting off of the key while cranking when it will not start. Here's what I know:

1. Coil is good starts and runs, but gets quite warm (happened to two coils)
2. Duraspark box is fine
3. Battery is fine.

Here is where its stumping me, maybe some one can piece it together.

If I put my multimeter across the battery i get 12.6 volts. But when I run my leads from the coil to ground I get 8V. If I disconnect the duraspark box from where it leads into the truck, I get 12 volts, but only when it is disconnected... It seems like high resistance would be the cause of this voltage drop but something has to be triggering it at a certain time because it happens the same way every time.

Any help would be appreciated, sorry if this was a little confusing.

 

Your coil while running should have 8 or 9v on it. There is a resistance wire in the harness that drops the voltage, it keeps the coil and the duraspark box from overheating. I was getting ready to say your problem may be a re-wire job that skipped the original resistance wire since you said the coil was getting hot. But it's normal for it to get warm, it has oil inside it to help cool the windings.

If you have swapped out the duraspark box for another one, I would try the pickup inside the dist. Don't believe you can take your duraspark box to the store and have them test it though, it needs to get hot before it will fail, and that test probably will not pick that heat failure up.