Ive got a 2001 Jaguar S type, 3.0, Experiencing charging. System problems, symptoms are voltage draining from battery, and engine running, charging system stays steady on 12 volts!.. Was told that a main power relay could be the cause, need some advise before condemning the alternator!..

Have an alternator load test done, most auto parts stores can do it.

Could be the closed relay you were told about or the diodes in the Alternator shorted. Is the 12v reading from an external voltmeter or is the reading from the dash voltmeter? Use an external meter to get the true reading as it could also be a gauge.