A/C problems
 

Ever since I got my 87 GMC, 262 V-6, the a/c compressor will not come on when you turn on the switch. All the vents work, every thing but the compressor, because it had no R-12, it has a thing to make it not come on if its to low correct? Anyway I wanted to know what I need to do get it ready to be recharged. Thanks

you have a leak if you are low on refrigerant. Thats the first thing you need to fix if you know you are low.

But how do I find it?

You are correct. It has a pressure switch that will prevent the compressor from coming on if there is not enough freon. It could also just be a bad compressor, though. If you are sure there is no freon, that would explain why the compressor won't come on.

Look for an oily spot somewhere on the system. The oil leaks out along with the refrigerant, and will usually leave an oily spot at the point where it leaked out.

Check the front of the compressor. If it leaks, it will probably come out the seal at the front. Look at the hood and fender to see if it has been slinging oil onto them. Check the hoses, the condensor, and the joints where the hoses and hard lines connect together. It could also be leaking from the evaporator (inside the black box on the passenger side firewall), but that would be hard to spot unless it leaves an oil spot around the drain on the bottom.

If your whole engine compartment is an oily mess, it will need to be pretty obvious for you to find it. If the engine compartment has been cleaned thoroughly since the refrigerant leaked out, which the previous owner might have done when he sold it to you, that would also make it difficult to find. If that is the case, you might have to take t to someone so they can use some dye or a sniffer to find the leak.

Slonaker