Does anyone know if the clutch on the a/c compressor would engage if the system is totally empty.
I replaced mine with another one and do not know if it is working.I am trying to determine if it is good before spending money on charging the system.
It would not engage if mounted in an empty system. However, applying power to a compressor standalone should cause the clutch to engage. You can also do a resistance test across the clutch windings to see if they are open or shorted. I'd expect several ohms on a good winding.
The clutch will not engage if the compressor is installed in the car and the freon is empty. The low-pressure cutoff switch will prevent the relay from closing. However, if the compressor is OFF the car, and you connect a 12V power supply directly to the electrical plug for the clutch, the clutch will engage if it is good.
You don't need to take the compressor off the car to test the clutch. Just unplug the clutch and use some jumper wires to apply 12 volts directly to the clutch. You'll hear the "click/clunk" as the coil draws the plate to the pulley. You don't even need to have the engine running.
It wouldn't hurt just to turn the clutch hub by hand to insure that the compressor is free to turn. With the engine off and the clutch disengaged you should be able to grab the compressor clutch center hub and be able to turn it. This will confirm that the compressor isn't seized.
You could test to see if there is still some refrigerant in the system by very briefly depressing the schrader valve tip on one of the presure/charging ports. If any refrigerant escapes at all that means your system does not need to be evacuated as it still is under some positive pressure from the refrigerant remaining in the system. You should then be able to add some referigerant with the system and engine off enough to close the low pressure switch. This will allow the compressor clutch to engage so you can charge the system.