When they are disengaged only the pully spins.
I on the the other hand know how to build an AC for a car that runs without drive power.
Just put another heat engine between the cooling system and ambiant. Use the power output from this to turn a regular AC compressor. Viola free AC.
Of course every auto maker know how to do this also. The costs for the heat engine are prohibitive. For those of you without an engineering backgroud. Heat engine == device using a working fluid used to eather make energy from a heat differential or pump heat using energy. Basically you build a 'steam' engine to run off the radiator heat.
i know. but it still a couple of pounds of weight that can be lost, and a few less inches on the belt, which will turn the other pulleys faster.