When I had a suspected bad box, I carried my timing light that had a long enough cord that I could reach it while cranking the engine.

Hello again, Pete.
I mentioned that i thought the alternator was shot, in my first post. What I didn´t mention was that I also have had do charge the battery a couple of times lately. Maybe I should have mentioned that too, but I wouldn´t bring too many details in the post. When I startet the engine and measured the voltage 12,4V and 12,6V, the battery was just charged. It didn´t stop during the 20 mins run with the recent charged battery.

Now that we can confirm the alternator isn’t charging the battery, we can probably assume that a very likely cause is the CDI not getting enough power to do its job as the battery gets weaker. Both you and Bob have pointed in this direction, I believe. What do you think? If I connect a fully charged battery when the engine stops, I should get a quick answer as to whether low voltage is the problem. If it still doesn’t start, then I’ll move on and check for CDI overheating using all the tips I’ve received in this thread.

Good idea :)

Like Roy said earlier, best to check for spark during the time of the event, but not so easy unless you are prepared with a test sparkplug and wire to stick in the coil tower and an assistant to crank the engine while you watch. I suppose this could all just boil down to it being the CDI. A common situation when the discharge capacitor fails as it gets heated up. Happens all the time. The cap goes open (or sometimes becomes develops a shunt resistance or shorts) but 'recovers' once it cools off. You need a spare/test CDI unit to make the troublshooting easier.

Hi Bob,
I will do the test with a spark plug or a test tool for checking sparks that i hold close to the spark cable. Do you also know if I can use a CDI from an 83´SC to make the test, or are they different? Do you also think that low voltage from the battery (because of my bad alternator) can cause the problem as well as overheat?

The '83 SC CDI is different;
1. 6 pins
2. variable reluctance sensor in the distributor, not points

The CDI should function with a voltage as low as 8 volts.

Just use a heat gun on the CDI to quickly determine a temp problem.

You can easily test the 3-pin CDI by turning the crank until the points open, and then momentarily grounding the points terminal of the 3 pin connector.

It's not that complicated, requiring 20+ posts.

Thanks, Dave – that’s very helpful and much appreciated.

Good to know that the CDI should still work with as little as 8 volts. That probably means my low charging voltage alone (12.4V at idle, not rising with RPM) might not be the sole cause of the issue, though it could still be adding to the problem.

I like the idea of using a heat gun to see if it’s temperature related – that’s a quick way to confirm a heat soak failure without having to wait 30 minutes for the symptom to appear.

I’ll also try the 3-pin test you described: turning the crank until the points open, then momentarily grounding the points terminal. Simple and clear – and I see your point about not overcomplicating it with too many posts.

Between that, Bob’s suggestions on checking the points gap and power feed, and the heat test, I should be able to narrow this down pretty quickly. I’ll report back with what I find.

Yes, carry spare battery (fully charged) in place of the spare tire. When the car dies, connect the spare battery and see if it starts and runs for another half-hour. If so, then I think that replacing the alternator will solve your problem. If not, then we have ruled out that cause.

In my experience, a car can run for an hour or more on just the battery, if it's in good condition, and you don't have the headlights on or other accessories. But maybe 30 minutes is as far as your battery will go.