Wizard I think you are on the right track. The engine battery was so dead that it would not light the dash lights. So the solenoid must get power from engine battery to energize it. The solenoid is powered when you turn on the key. Seems a crazy way to have it hooked up for if the engine battery is completely dead you could not energize the solenoid. Now I believe if you just had a low engine battery it might work. I guess it is wired that way so you are charging coach batteries from engine. All the time I was thinking all the batteries was being charged from the converter.