I have an s5 NA.When I turn the ac on the idle starts bouncing, and recently it has started to do it when I turn the steering wheel at idle. Also when turning at low speed the power steering surges. I think that it might be the bac. Can anyone tell me if i am on the right path or what it might be.

 

To check the bac Youd need a digital multimeter. But Youre right sounds like a bac to me, it bumps up the idle when under electrical load to compensate for the loss in power.

U can get one pretty cheap at a junkyard

 

The S5 has an Air Supply Valve used to add extra air when power steering is used. Not sure if it is used w/the A/C. And make sure the Green/Red wire at the TPS measures 1 volt w/key to on and the engine as hot as it can possibly get.

 

You're probably right, I know more about s4 than s5.