realsync1

When hooking up the multimeter to my battery when the car is running without accessories on the voltage is 13 volts. When i turn on the air conditioner and the lights the voltage drops to 12.4 volts and the car starts to vibrate really bad. I have the aftermarket alternator from AutoZone. What voltage is good on lexus gs300 2003 oem alternator.

jettate

I am pretty sure I read 13.5 to 14.5 volts under load. AC, radio and headlights on.

Arussto

I believe ypu have to look at the readings when the alt is charging. At times, when the voltage is good say 13.5, the regulator shuts off the alt magnetic field until the batt volt is below say 12.5 .
SO You start and monitor. Batt volt should be going up to a peak say 14 at which point alt syops chrgng you put on lights etc to pull dowm batt volt amd at a min of 12, it starts to charge again.

The prob indicater may he idle control

I believe

volvodude

I had someone test my battery/alternator with a voltage meter, and it was bouncing around from about mid 13's to low 14's. At one time it did get low 13's for a moment. Is it normal for the voltage to not be steady?

TrueGS300

I've never seen 12. anything be an acceptable running reading. I'd say that is too low and to warranty it out. Autozone's Duralast line... no surprise there. Defective parts are common with that line.

Also, you should always test voltage at both the battery positive and negative as well as the alternator positive output and battery negative. The readings should be similar, but if there is a strong reading at the alternator say high 13's-14's, but only 12's at the bat, you'd have a cable/corrosion issue rather than a bad alternator. Worth a check.