I just replaced the alternator with a mazda factory reman as well as a dealer belt. So here is the problem, during the day the car runs fine, gauge shows 14 volts but at night with the lights on it falls between about the 13 and 14 and then if I turn the rear defroster on it drops to 12 so olbviously I have a problem somewhere. The battery is about a year/year and half old. I did run only on the battery for a couple days while I was waiting for the alternator, I would take it out when I got home and put it on a charger. So my question is, is this an alternator or battery, or wiring somewhere problem? The battery never dies and my car still starts fine I'm just at a loss at the moment.

Thanks

 

If this happens while you're at idle, then this isn't anything to worry about. My guess is the pulley on the new alternator is somewhat larger in diameter than what was on there.

 

Yeah, it's at idle. It's an S4. Just caused from the large electrical load then?

 

Yup

 

The S4 alternator is terrible. IIRC it's rated at 70A. Which is **** poor compared to some other cars. It's made to BARELY run the factory electrical equipment.

 

True that.