If you're asking what to expect when you're watching it under the hood, you should see it click on and spin the compressor for typically 15 seconds to a minute, then click off. It will stay off for about the same amount of time if the system has a full charge of refrigerant. As the charge leaks out the compressor will run longer to pump the required amount of refrigerant, and it will stay off for a shorter period of time. Rapid cycling, like every one to five seconds is a sign of very low charge. Once the charge gets low enough, the low pressure cut-out switch will prevent the compressor from running because if there's a leak and the low side gets pumped into a vacuum, air could be sucked into the system. Air won't hurt anything but it's the humidity in it that causes trouble. Water and refrigerant R-12 combine to form hydrochloric acid which will eat away metal parts.
Some systems use a variable "wobble plate" in the compressor to adjust the flow of the refrigerant. Those compressors will run continuously when the AC or defroster are turned on.
Tuesday, November 13th, 2012 AT 9:45 AM