A3 2.0 SE TDi 170 2008 model. Auto climate control. As soon as I switch on my AC unit The drag on the engine is enough to make it run roughly and appear to lack power. It certainly is nowhere near as smooth to drive with it switched on. It makes no difference what temperature is set, the drag is the same. With all other cars I have owned with AC fitted, there has been no noticeable change at all when AC is switched on. Is this common or normal, or do I have a fault somewhere? There are no fault codes showing.