RPM drop when ac is turned on
 

It's just started while noticed it this morning it happend like twice.. the first time it was when I turn my ac my rpms dropped from about 750 to 400 or so but then went back up to normall.. the 2nd time was when I was at a red light same thing the rpms dropped to bout 400 or so then wen bout to normal.

I'M in las vegas super hot.. could it be my battery or alternator is going out or is it normal for this to happend.. thanks for he help

could be your high and low switch going out , most of them are on the passengers side of motor close to your ac's dyer canster,when it goes out your rpm's will go back and fourth up and down, it'll look like this >>
https://cimg6.ibsrv.net/gimg/mustang...00bfb74708.jpg

that switch will not make the car idle low, the pcm commands the compressor on and it will drive the IAC motor open a bit to keep idle up to where it belongs. you either have a compressor that is going bad and taking more power to turn than its supposed to, or your IAC (idle air control) motor is getting dirty and isnt opening up fast enough or is sticking. all that switch does is turn off the compressor if the pressure gets too high or too low

Do you hapoen to have aftermarket pulleys? They change the ratio of things and I think they make significant difference when ac is in or off. Not saying thats your problem but just a thought.

yes it will cause it to idle go up and down been there done that

that switch will not cause a low idle of 450 rpm. only a load or the IAC can do that.

i used to run underdrive pulleys before i went FI, and never noticed any difference in operation

i guess you have never ran into this issue but i have and learned from it,
just something to prove my point on the high and low pressure switch i said could be the problem >>

IF the refrigerant is low, then the pressure switch might turn the compressor on and off repeatedly, that may cause small surges in idle. But if your idle is dropping to 400 rpm when it kicks on then its because the iac is sticking or the compressor is getting bad and causing the engine to struggle momentarily.

finally, someone who knows how the system works ^^^^^^^^^^^^^