'99 saturn sl a/c only blows cold when idling high

we just replaced the compressor and recharged the a/c and now it only blows
cold air when i'm actually driving (particularly when i'm on the freeway).
when idling, it only blows hot air. we were told my many people that it's
a switch..but which switch? where do we start looking? i can't afford
the dealership or to pay a mechanic. my friend is very competent and just
needs a little more direction as to where to look.

 

I'd be looking for whatever controls the 'flapper' valve in the
heating/cooling ductwork. Sounds like it is switching over to "Heat" at
idle. Pretty sure all of this is vacuum-driven, so either a hose is leaking
or wasn't attached right after the replacement (?)

 

Did you evac the system before you recharged it? If there is any air in
the system from when you had the lines open, you won't get good
cooling. Eventually you will rust out the internal parts as well since
ambiant air contains water.

-Robert