the A/C shouldn't change where the engine idles, mines 800 + - 50 RPM w/ or w/o A/C on.
If your AC is functioning properly, then your car should idle UP when the AC is turned on. This is triggered in the ECM or ECU, whichever you prefer.
I on the other hand have a broken AC switch. By broken, I mean that it does not turn the AC compressor on. So what I did to fix it is run a wire from the blower relay (ground) to the magnetic clutch relay (ground), so whenever the blower is turned on, the AC kicks on via the magnetic clutch relay. Sucks cause whenever the blower is on, the AC has to be on, but it sure does beat paying Toyota to fix it.
Anyways, this is what I was trying to get at...
Since I bypasses the ECU (ECM), at idle when the AC is on, the car idles almost dangerously low. I'd say around 300-400 RPM's. It often sputters, but I have never had it stall out on me before.
The IDLE UP function is supposed to keep your car from idleing too low when the AC is on, hence, that's why your idle is supposed to increase when you turn the AC on.
Either way, your idle is either going to increase with the AC on (what it's supposed to do), or decrease like mine does (what it's NOT supposed to do).
But next time, check your RPM's at idle while toggling the AC on and off and see if you get a slight change in RPM's. If you don't, then it could be the fact that your IDLE UP function perfectly negates the parasitic effect that the AC Compressor puts on your engine when it turns on.
But for the most part, it should idle up at least 100 or so RPM's. Nothing too drastic.
Hope that makes at least a little sense.