air conditioning causing overheating?

I have a slight problem....
Whenever I turn on my air conditioner, my truck (1988 C1500 5.7) starts to overheat slowly. After I turn it off, it cools down, but slowly. Can this be a sign of early compressor failure, or is it just the air condition compressor clutch, or do I need oil in the cooling system. I hear a low knocking sound coming from the compressor, but I am not sure if that is a problem. I took off the drive belt and engaged the clutch and found that the compressor was kinda hard to turn with it engaged. Can I solve this problem by just replacing the clutch?, I dont wanna evacuate the refrigerant. My guess is that when engaged the compressor slows the drive belt and slows the waterpump. Not sure if tis is right. Thanks...

Matthew

I would say on a '88, that you should check your fan clutch. Turning a compressor by hand with the clutch engaged is difficult, that sounds normal. If you think the belt is slipping, check your tension or try a new belt.

An engine will run hotter when using the A/C due to the fact that the condensor is mounted in front of the radiator. I am assuming that we are also talking about idlling and not driving.

Unless the compressor is really noisy, and unfortunately, I have to depend on your ears here, I would say to check the aforementioned items first.