Thinking I'll check the 2 prong plug & wiring from the side of the alt. One is a sensing wire and the other gets an on signal from the ignition switch that travels thru a 10ohm resistance light.
From the factory on the sensing wire does the voltage travel into the interior of the car (from the alt) then back to the solenoid or does it go right there?
I'm going to try running a new sensing wire off my alt over to the solenoid to see if that helps.
If it's not a missing ground wire from the battery negative to the body (sometimes they break inside so I add extras) then the alt may be messing up. A bump on the alt with a rubber mallet will tell you if it's bad or brushes need replacing. If you have the stupid double belt system for the alt, the slack between them both can stop the alternator from turning!
Alt and belts are brand spanking new and tension-ed as tight as can be. Single alt belt driven from AC compressor on this car.
This is alt #3 on this car and 4th or 5th set of belts so I'm ruling those out.