Will need a meter to check output. Make sure you have battery voltage to the regulator. If so then unplug the wires coming from alternator. Set meter at AC volts, insert one lead to each wire to alternator, and start engine, put to full throttle and AC output should be about 28 volts. If both test are within specification then replace regulator. If voltage from alternator is low, replace alternator. If you don't have battery voltage at the regulator then find out why.