If your A/C won't blow cold at idle or low speed but does cool weakly at highway speed, don't let the Ford dealer convince you the compressor needs to be replaced. The symptoms can fool the tech into thinking the compressor is going bad and is weak. What they don't tell you is there policy is to replace not repair a compressor. The compressor is a variable capacity type that uses a scroll valve to increase/decrease the capacity based on the load. If the valve sticks you will get the symptoms I described. The scroll valve is a $40 part and can replaced in about an hour or less after the system is evacuated. Most A/C repair shops are not familiar with this repair either and recommend compressor replacement. After replacing the valve the system will need to recharged with R134a refrigerant.