Ford Repair: air conditioning - 2003 ford expedition, mileage vehicle, air gap

Question
what could make the air conditioner cool at maximum sometimes and not cool at all other times? Is there a valve or switch i can replace?

Answer
is it actualy not cooling or is the air switching from vent to defrost-this vehicle probably has electric blend control actuators
first i would need to know if it is fully charged and not overcharged
even if it has had a leak and someone charged it if they do not remove (recover) system then air will be in it where freon should be and it won't cool properly-make sure condensor is clean-air gap between the front drive plate and pulley on compressor may be to wide if this is the case the clutch may not re engage after shutting off during a normal cycle when this happens you might be able to tap front plate to see if it will re engage if so you might have to remove a shim or replace clutch kit assembly not nec. recommended on high mileage vehicle best to replace compressor. you do have a low pressure cycle switch on the accumulator that could be bad you can take a test light and see if power is on both sides with it plugged in and engine running with a/c on. if power is only on one side it is low on freon or switch is bad.