The capacitor serves to protect the electronic equipment of the car from voltage pulses in the ignition system, as well as to reduce radio interference.
Capacitor damaged or loose on alternator (poor contact with «weight») detected by an increase in radio interference when the engine is running.
Approximately the health of the capacitor can be checked with a megaohmmeter or tester (on a scale of 1-10 MΩ). If there is no break in the capacitor, then at the moment the probes of the device are connected to the terminals of the capacitor, the arrow should deviate in the direction of decreasing resistance, and then gradually return back.
The capacitance of the capacitor, measured with a special device, should be 2.2 microfarads±20%.