GM-GMC: cooling fan slows down, engine vacuum, gmc safari

Question
Why would the cooling fan in a 2000 GMC Safari van slow down to almost a stop when accelerating. When acceleration stops the fan resumes. It doesn't matter what speed the fan is set on. I have also seen this in other GM vehicles.

Answer
Normal.  The cooling fan selectors use engine vacuum to move the mode doors etc.  During accelerating, engine vacuum falls and the mode doors will start to move, making it seem like no air is being delivered.  Once the engine vacuum returs, after acceleration, the mode doors will return and voila air again.  That is why they have gone to an electronic system.
Hope this helps