What "tells" the A/C compressor to kick on/off?

[Jim_A_B]

My A/C compressor seemed to run constantly, but today it started to cycle off/on at varying times. What controls it?

thanksJim

[Silvia007]

There's a sensor inside to detect the amount of freon inside the compressor, when the compressor runs to compress the freon, the freon level drops to a point at which the sensor will cut the clutch to the compressor this is to prevent damage to the compressor as you know pumps that free pump are easily damaged. So this can only mean one thing, you're running a bit low on Freon. Hope this help.

[erich]

The compressor will turn off if the A/C heat exchanger (the radiator in front of your radiator) overheats. Have you changed the stock fans? When the A/C is on the stock electric fan should be on. If it isn't then the realy for the fan might be bad.

[Jim_A_B]

Well, I did notice the other day that the A/C was having a hard time keeping me cool, but then the next day it was ice cold. Weird! I certainly hope it is just low freon and not the A/C compressor about to go kaput!I recently replaced the mechanical fan clutch, so I checked the proper operation of the electric fan then. I will check it again though. So, the A/C compressor isn't supposed to cycle off/on under normal operating conditions? I always thought that they did. Thanks ya'll

[erich]

The compressor the Nissan used on the 240 is a variable displacement type. Instead of cycling on and off like older units it changes the displacement of the pump to set the output temperature.

[Jim_A_B]

So enrich, could it also be cycling off/on due to low freon, or will overheating be the only cause of the cycling?

thanks

[erich]

It will cycle on and off if the pressures are out of spec as well as overheating. There are over 50 pages on the A/C system in the FSM, it's frickin' complicated. There are several pressure sensors, a couple of temperature sensors, and I'm pretty sure the ECU is involved a little. If my A/C ever dies I'm taking it to a professional :-)