Hello every one. had to recet my password in order to log bag on this forum. last time i was here was 4 years ago. lol
anyway, trying to get my mazda going. at idle when starting the car voltage is around 13.4v when something else (lights, a/c) is turn on it immediately drops to 12 or below volts. for this reason i put a different (used) alternator. same problem.
but when you rev engine up a little. voltage go up to 13.24v and stays there but as soon as engine returns back to idle the voltage drops to 12v or below. one thing i noticed is that my original pulley is slightly larger then the second alternator i got, and i had to install larger pulley, otherwise the belt will be lose with smaller pulley.
im coming to conclusion the voltage drops due to larger pulley on alternator.
have anyone had such problems? please help. and im sory if this have being posted before and i didn't bother/forgot how to look, its being 4 years since i being here.

just measured the pulley on the pulley that is on the car right now is 2.5'' in diameter. and the factory smaller one is 2''

 

Many under drive pulley sets run the alternator slowly at idle and will big time affect your idle voltage. Not just the alternator but the main pulley as well.

There are aftermarket pulley kits that are aluminum but stock or near-stock size.

Dale