Does the warning light come on for the alternator when there is a diode gone bad?

 

Not necessarily. Better to rely on the charging voltage - if below 13, there is a problem with the alternator.

 

If the large alternator output terminal post does not read above 13 .5 volts which is above the fully charged battery voltage of 12.75 volts you have a bad voltage regulator mounted on the alternator . If you have a bad diode in the rectifier on the alternator you will notice it in the radio on a AM station .. The local auto parts store will test this for free as they are in the business of selling you a new alternator . You can clean the small wire connector on the alternator as it's in a bad environment . If the alternator completely fails there is a fuse the is required to provide power to the voltage regulator before the alternator can generate power . This one fuse is also tapped into other systems which is a fault of the circuit design .

Keep asking questions and someone will help .