I suggest you don't start messing with relays until you've first checked the basics.
Open the hood and look to ensure that the compressor clutch is kicking in when a/c is turned on max cool and max fan. If it does, then look at the sight glass to see if there are bubbles. If there are no bubbles AND the clutch is turning the compressor, then it is low on refrigerant. Find and repair the leak and recharge with R12.
If it does not, hook up manifold gauges, or have them hooked up by a shop and check the charge. If it is low, find and repair the leak and recharge with R12.
If you determine that the system has adequate refrigerant first, THEN you can start troubleshooting the electric circuits, preferably not by just shotgun replacing relays. Start at the pressure sensor and electrically work back toward the relays.