The compressor can be thought of as the heart of a vehicle's air conditioning system. The compressor clutch is what enables a belt-driven compressor to do its job. When the A/C switch is turned on inside the vehicle, electrical current is available to be supplied to the clutch. When the clutch coil is energized, it locks the compressor drive plate to the clutch pulley, causing the compressor shaft to rotate. The rotating shaft moves the compressor pistons, which pressurize and circulate the needed refrigerant. Thus, if the compressor clutch does not engage, the rest of the system will not function. If your A/C is not cooling, one of the first things that needs to be checked (as well as one of the easiest) is whether the compressor clutch is engaging.
Open the vehicle's hood while the engine is off and locate the A/C compressor. Use a flashlight for a better look if it is difficult to see. The compressor is the only belt-driven component with two pipes coming out of it. One of them leads up front to the condenser, which is mounted to the radiator, and the other leads into the passenger compartment, through the firewall at the rear of the engine compartment.
Study the front of the A/C pulley and notice the drive plate on it. If you look at the other pulleys, they don't have one of these. This plate is what engages the compressor shaft at the center of the pulley. When the A/C is not on or the clutch is not engaged, the pulley turns loosely around the shaft. When the clutch is engaged, the plate locks into place and allows the shaft to turn with the pulley. If the engine is off, you should be able to rotate the plate by hand, unless it is seized. This will allow you to get an idea of how it moves.
Turn on the vehicle and the air conditioner, then go back and observe the operation of the compressor.
Listen for the sound of the engagement. You should hear a moderately loud clicking that will coincide with the engagement or disengagement of the clutch as you observe it. Keep in mind that several seconds may pass between cycles before it turns on or off. If you can hear and see it, then the compressor clutch is working properly. If it is not doing anything, then a more in-depth diagnosis will be required by a certified mechanic, who will have both the equipment necessary to properly evacuate the refrigerant according to federal environmental regulations as well as the specific knowledge to perform various electrical and mechanical tests.
