Hey
Recently we did a restoration of an old Volvo 242 DL from 1981. The project nearly came to an end after 1 year and endless problems but it now occurs that a ignition voltage loss happens right at/before the ignition coil.
The car completely starts normal when cold but after a few minutes you can clearly see the voltage dropping in idle with a multimeter until the moment the spark cannot be maintained to keep the engine running at 10 Volt.
The car had many problems overall and we already changed the whole ignition system components because of other problems.

The voltage drop clearly happens before the ignition coil and i guess it could be a corrododed wire (maybe the brown or blue ignition wire), accidental grounding or something like this. If you give it a little bit of gaspedal the engine keeps running by maintaining a voltage above 12 Volt.
Already replaced temporarily the brown cable from ballast resistor to ignition coil, voltage drop still happens, so i guess it might be the blue cable before the ballast resistor. Does it directly go to the fuses?

Theres one problem - the wiring diagrams from Volvo do not really represent reality. I guess the ignition voltage always comes through the ballast resistor, so theres no parallel wire the voltage might take, is this right?
Ive seen a lot of cares having a parallel wire to the ballast resistor one but the Volvo 240 does no seem to have this.
The only other problem im thinking about that this might be a faulty ignition switch - which i doubt - i still guess it might be a faulty/corroded wire which increases resistance over time until the spark cannnot be maintained.

Best regards