I believe that the ECU-s vary the openings of the IAC-s so as to maintain 1050 rpm. When the AC compressor is switched on, its load first causes the idle rpm to go slightly down and it is quickly brought back to 1050 by increase of the IAC openings. I noticed that the same happens when the cooling fans are switched on causing additional load on the engine by the alternator.
Interesting even with the PWM controllers? I figured slow ramp would mask that at least the instant spike and draw. I know my lights or power no longer dims with 2 pwm's on the fan.
It does not matter how the additional load on the (idling) engine comes, gradually or suddenly. In any such cases, the ECU-s will increase the duty cycle (the width of the "on" pulse) of the PWM signals to the IAC-s to open them a bit more and compensate for the additional load. In the case of the AC compressor, the moment its clutch "clicks" (sudden load), the PWM "on" pulse on the IAC-s gets instantly wider. When my rad fans came gradually on (via my fan PWM-s), the width of the IAC PWM "on" pulse increased also gradually (not momentarily like in the case of the AC compressor).
