If the compressor clutch does not engage, the relay may be faulty. Remove the relay from the engine compartment fuse box and check it. There should be continuity between the terminals with power and ground connected to terminals and no continuity with power disconnected. If the relay is ok, check the voltage between the terminals of the relay socket and a good ground to make sure the relay is receiving power. If it isn't receiving power, there is an open circuit that must be repaired. After determining that the relay is receiving power, connect a jumper wire between certain terminals in the relay socket and listen for a clicking sound from the compressor clutch. If the compressor clutch doesn't click, remove the jumper wire, unplug the compressor clutch connector and check for continuity of the red wire that runs between the relay socket and the compressor clutch connector. If there is an open, repair it. If there is continuity, check the compressor clutch for damage. Unplug the air conditioning low pressure switch. Check for voltage between the pressure switch connector and a good ground with the ignition switch On. There should be battery voltage on certain wires depending on the model. If there is no voltage, there is an open. If there is voltage, turn off the ignition and check for continuity between the terminals of the pressure switch itself. If there is none, there is a problem with the switch or the system pressure.