I don't understand why you think an alternator would need brushes or
slip rings. An alternator is an AC generator. The diode rectifies
its output (serving a function similar to what brushes would do in
a DC generator).


How is the field current controlled? Automotive alternators use a pair
of slip rings to power the rotor, regulation is achieved by adjusting
the current through the rotor as needed to keep the output voltage
constant. I've never seen one that was brushless.

BTW, in some sense all good capacitors are "open circuits." However,
an ohm-meter should initially show a low "resistance" (until the
capacitor charges).


I'm very familiar with testing capacitors, I've been repairing
electronics as a hobby for around 15 years now. I checked it with an ESR
meter as well as compared it to a similar motor run cap I had on hand.
The capacitor is bad, besides, the diode is good, there's no other
active components I can see and it stopped producing power suddenly
while under load so it isn't a matter of flashing the field.