Likely that your system is low on freon. There is a low pressure switch on the receiver dryer, the large aluminum canister near the firewall. Remove the connector to this switch and jumper between the terminals on the wire harness with a piece of wire. It is a round connector and you need to squeeze the housing to remove the connector. It will be obvious which axis must be squeezed to remove. The wire jumper is replacing the closed low pressure switch. If the compressor clutch engages, this confirms that the freon charge is low. Do not run the AC continuously with the jumper in place, serious damage may result.

You then must decide to refil with r12 or convert to r134a. I did the conversion, but it was only a couple of weeks ago. Initially it is working fine. Don't know how it might hod up.

Dan