There is more than one type but pre engaged ones I have seen are like this: The solenoid windings are powered from the ign switch, and runs to earth via the brushes, not to the case directly. If you look carefully at the solenoid (certainly on older starters) you find that the 'earth' side of the solenoid connects to the power terminal that goes to inside the starter (to the brushes) If you can cross the big terminals on the solenoid you can prove whether it is a solenoid fault or a brush or winding fault.