I read several posts on here about clutch cycling and decided to check my a/c system pressures.  Today at 72 degrees my pressures were 225/30 on max a/c with blower motor wide open.  Air inside cab is good and cold and clutch stayed engaged. I thought I was in good shape, then I noticed as soon as I turn the blower down or change from max a/c my clutch starts rapid cycling and pressures change to 150/40 when clutch is engaged. Air is still blowing cold on the inside. Any ideas what is going on? Too low R134a maybe?

The compressor will cycle on the pressures, primarily low pressure. The reason for this is to keep from freezing up the evaporator (the coil inside the cab). The slower the fan goes the lower the load is on the coil so it will get colder and that causes a pressure drop and then the cycling of the compressor starts. As long as it stays cold in the cab I would not get to exited about the cycling of the compressor.

The only limit is you should see more than 3 cycles in a minute. (Sanden Compressor specs)

 

But if you compressor is oiled up and low side pressure is above 35 PSI then you should be fine. As the evaporator does cool it will drop the low side pressure and start cycling but low freon cycling is bad for the clutch. This is why I tend to slightly over-charge the system to prevent compressor cycling and prevent clutch damage. Solid lock compressor is more efficient than a cycling compressor. Remember when compressor cycles off the hot freon is rushing back to the evaporator and now it going to reduce cooling performance.

Thanks guys. That's the info I needed.