when i turn on my ac to max my voltage drops to 13v (autometer gauge) when im idling or driving. even worse is when i come to a stop, my idle drops to about 690-700 and then will surge from there to about 800 and back and forth for as long as im idling. as soon as i begin moving again its fine. if while idling i turn the ac off, my idle jumps to 890-900rpms (thats what i set my idle to be). why does the ac cause my idle to drop, surge and voltage to drop to 13v?

i have a 150amp alt, new voltage regulator, and 2g cables running from the alt, for my main power and for my grounds so its not that. no electrical issues except with the ac on

 

How old is the IAC? That could be your problem they are suposta raise your idle when the engine is under load. As well go throught the procedure to reset your base idle and I bet everythiung will be perfect. When your raise the idle on the TB the TPS goes as well and if you break the 1.00 you will idle in closed loop mode which causes surging in any of my Mustangs

 

Fan motors put a load on the Alt; so does the AC clutch. It takes more power to turn them. If you have underdrives, the problem is worse. I'd turn the idle up to where it's comfortable powering everything at full blast.

 

the IAC is only a few months old so i doubt thats the issue. i dont have under drives on so thats not it. i have a a load read out on my pms display so i can easily see if the computer is in idle mode or if its under light load (i.e. tps voltage above 1v) and it always reads idle, when im idling so tps voltage is fine. the idle rpm is about 900 when the ac is off so rasing it even more is out of the question

 

My car is doing this also. I have a mechanic and mustang Guru as a neighbor and he says the altenator is the culprit. So I took mine to AutoZone and they said it is ok. But they cannot do a load test to make sure that it is producing the rated amps it should produce. Need to take it to someone who can load test it. most garages should be able to hook up to your car and perform this test. I'm replacing mine tomorrow!! let us know what fixed it.

 

Those re-man alternators and starter from , auto zone, advance auto, pepboys a junk before you get them . Id spend the exrta money and buy a brand new Motocraft one from you local Ford dealership