Hi all:

This is regarding S4 NA to make things very clear.

S4 turbo's are a little bit different (air supply valve for PS) and S5's are quite different (load management module inside the ECU for BAC activation)

What does the ECU do when it senses an rpm that is lower that what it is supposed to be for a given throttle plate position (as indicated by TPS) ??

like, when you put some extra load on your engine.

I know that when you turn AC on or turn the wheels on a PS car, the BAC increases its duty cycle (I would assume at any rpm, not just idle). Note that on S5's, there are other loads like headlights that increase the duty cycle of the BAC.

but what happens when you just turn on the ventilation (on a S4), say at idle ? The rpm drops and throttle plate position stays unchanged, so the ECU should do something to correct, but what ?? Does it change the fuel injector cycles or change the BAC duty cycle ? Does it do anything at all ?

Can anybody explain how this thing works ?

thanks,
Hugues