samsonb

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ffas23

Based on what you describe it sounds to me like this compressor is on its way out also. Sometimes when someone replaces a compressor they forget to add oil to it or there may be debris in the refrigerate line from the old compressor going bad that wasn't cleaned out. Some of the later vehicles have filters called ortifice tubes in the line that get plugged up and will cause the compressor to go bad. I am not sure if your 81' has one though. What I would do in your case would be to go and pick up a can of pressurized refrigerant oil and shoot it into the system to see if that helps. You should be able to pick up a can for around $5. If debris was left in the line or if your compressor is already beyond any help this will do no good.

The A/C compressor runs even when the heater is on unless you unplug the plug from behind the compressor. I usually do this during the winter time on some of my GM cars because if you have refrigerant leak out during the winter you will ruin your compressor.

ICEMAN59

It could just be the clutch going out or the belt is not tight enough, mine was squealing, I tightenen the belt and all is well.

isosceles

About the belt tension. The A/C belt is pretty long wth a LOT of load on it when the clutch is engaged. If mine is not tight enough it vibrates like a huge rubber band and makes a grinding noise that seems to vibrate the whole car.Shifting into gear at ildle will drop the RPM's and make the grinding sound worse. It usually goes away with increased RPM though.

Can you hear your grinding noise throughout the RPM range? If so it is probably clutch or compressor

ffas23

Could also be a defective air pump making the noise if your car is equipped with one.