A/C Question

[Gene]

Just got my compressor back on after installing headers awhile back, and I had my sisters b/f come over to charge the system back with Freeze 12(R12 replacement). Anyways, he noticed with the system empty of freon that the compressor clutch stays engaged even though the system is bone dry. Now, I know absolutely nothing about A/C systems but he says is should be cycling on and off constantly with no freon in it. Now, I had to bypass the switch on the A/C door that sends power to the clutch because my switch was bad, all I did was put a toggle switch inline to send power to the clutch, my question is, what tells the compressor to cycle on and off? I mean I see no other switches that send power to the compressor other than this one and when it is set on A/C it is always sending power to the clutch? Thanks in advance guys, I know you can help me figgure it out