My voltage drops to 12 volts, sometimes lower, everytime i turn on my headlights and heater fan. i just installed new alternator, new battery, new cables thinking this would fix this problem but it didn't. i don't have any accessories on my car that would pull power from the battery. however i do have a dual belt alternator pully. don't know if this would affect this, i don't think it would. any help would be awsome.

 

First get your ALT checked.
Then get a alrger battery.
FD alt
Grounding

 

The dual pulley probably has some underdrive to it. If it is doing this at idle only, then it's pretty much expected, given the accuracy of the voltmeter. Check the measurements using a multimeter. Check the condition of your battery terminals (tightness and corrosion) and your alternator connections. Make sure the two prong connector in the back of the alternator is plugged in.

 

where did the new alt come from???

If it is an Autozone rebuilt alt, I have seen about 50% bad ones out of the box even though the autozone "tester" says it is fine.

 

the alternator is an oreilly reman. everything on it is plugged in. battery is a brand new optima. everytime it gets to idle, the volts drop to 12 or lower and all lighting gets really dim and stereo kicks off which gets really annoying. i guess i could try putting in an s5 or fd alternator but i just thought the stock alternator would at least not be this bad.

 

What kind of stereo system do you have?
Disconnect it and see what happens.

 

Kinda off topic but not. . . Does the S5 alts have voltage regulators built into them? Mine just blew and i need to rebuild my alt.

 

have you checked your grounds

 

If there stereo is kicking off, the voltage is dropping under 9 volts. This should never be happening with a stock alt.

I would replace the alt first.

Anytime the alt is dropping under 13.5 volts it is bad and there is insufficent voltage to even charge the battery.

Again who checked the alt to confirm it is good, and is the plug correct on it? You do know that a S5 alt wires different than a S4 alt and the wire colors are not the same.

 

All S2 and up alts have internal voltage regulators.

 

cool, i'm gettin a lot of information i didn't know. my voltage is definitely dropping below 13.5 volts so i'm gonna take the alternator i bought and take it back to get tested cuz i has to be bad then.

 

Or you have it wired wrong... again S5 alts wire different in S4 cars, and you can only use one of the two small wires, the other you need to take to the fuse box.

 

i am using a stock s4 alternator, i didn't upgrade to an s5. i am still guessing the alternator i got from oreilly is bad and ill try another one.