2001 saturn sc-a/c stopped blowing cold air. At first it needed the engine to be running, shortly after it stopped. Adding freon didn't work. Compressor seemed to turn loosly. Saw posts on blown fuse block (bad A/C diode located in the underhood fuse block), leak, low-pressure switch in the A/C systm. What should I check first?

Low freon will cause the compressor to short cycle, when the compressor runs the pressure in the low line is pulled down and the low pressure switch turns it off. Not running the pressure builds up and turns it back on. In the old days there was no low pressure switch, with no freon to carry the oil the compressor was destroyed.

with a/c you usually have to replace the part that is leaking.hose,condensor,evaporator.or if its a seal,seal replacement,schrader valve etc.this needs to be done at a shop that does a/c work as the system will need to be evacuated,repaired,vacumed,recharged,proper oil installed and rechecked for leaks,and checked for proper operation.

Hey M, the pressure could be nothing but "hot air" and not freon... (I had this same problem!!) I let off a bunch of pressure..(nothing but clear hot air!!) ..using a small flat punch..on the low pressure side... if freon on comes out right away..stop.. if nothing but hot air comes out.. keep letting it out until freon comes out,,, When mine started letting out freon,, I quit and tried the A/C again... and it has been working fine since.. Just my recent experience!!!!

The inside heat is provided by a small radiator running engine coolant. If you don't have heat and you don't have scolding water pouring on your feet  then the coolant isn't running - most likely the valve controlling the flow is broken, replace it.It's easy to find, its located on one of the pipes leading from the engine and going inside the dash, with a control cable attached to it.

check to see if compressor is running when it starts getting warm with car running ,if so, see if larger line is cold 

It sounds like by adding refrigerant to the system it may be over pressurizing. When this happens it will cycle on and off.

You will need to hook up a set of manifold gauges and monitor the high and low pressures to see what is happening.

check cooling fan operation while ac on.If fan is working proper,will probably have stuck exspantion valve.

you most likey have a empty system or low freon charge due to a small leak.

check coolant level in radiator with engine and cold,refill check for leaks

put a bigger fuse in...if that doesent work u have a short somewhere its a wire grounding itself out onto some other part of the car..its almost impossible to find the short tho..i would take it to a saturn dealer if the bigger fuse idea does not work..good luck