When the Air Con' is switched ON and the compressor kicks in (whether at idle or on the move)....what sensor/switch or thingymebob causes the engine Rev's to rise?..

..is it the ECU that sets the rev's ?...i guess the TPS must receive a signal of some sort but from where!!....what actually takes place to cause the RPM to increase the instance the compressor starts to put load on the engine?


ps. there is a good reason for this question due to an odd erratic idle issue i have !....which appears to only happen once the A/C has been used

Thanks, in advance, Phil.