Faulty low pressure switch?

[AlphaCrow]

Hi again, need some advice. My compressor keeps kicking on and off every 10-20 seconds when I'm at idle (only time I can hear it). Been doing it for a while. My father is an a/c guy forever and checked out the pressures and vacuum and said it looked within specs and had the prescribed amount of freon.

Was changing my shocks out today at the automotive center (so nice, full stall with lift and unlimited tools) and the mechanic said it may be my cycle switch. Said my blazer probably has a low pressure switch that may be faulty.

They can only advise, they don't work on vehicles there, so that's all he knew by looking at it.

Anyway to test or tell? Trying to google it, but hard to get any good info.

[old_master]

Rapid cycling usually indicates the system is low on refrigerant. The system must be fully charged to acurately perform diagnostics. With a clutch cycling system using R134a refrigerant, such as yours, it is impossible to tell how much refrigerant is in the system without draining, evacuating, and recharging the system with the specified amount. Older systems using R12 refrigerant, without clutch cycling, you could check high and low side pressures and get a pretty good idea of what's going on.

[AlphaCrow]

Is there any other test to be sure its the freon before having it emptied and refilled? I do have cold A/C (I realize that may not be a good indicator at all).

Just want to be sure before paying for the flush. Or is it possible to add freon to get it to a set psi or ounces?

[old_master]

If the refrigerant charge is low, there's a leak that must be repaired. Have the system tested for leaks. If there are no leaks, the system must be drained, evacuated, and recharged to diagnose any other problems. The clutch cycling switch should close at approximately 42psi which applies battery voltage to the clutch field and the compressor runs. When the pressure decreases to approximately 22psi, the switch should open and shut off the field and the compressor stops running. There are far too many variables involved with ambient temperature, humidity, cooling system performance, air flow across the radiator, etc etc that's why it must be fully charged to diagnose it. Too much refrigerant will cause poor performance and can also cause permanent damage to the system. Not enough refrigerant will cause poor AC performance, so just adding refrigerant is not the answer. Best to take it to a trained AC technician.

[AlphaCrow]

I had an A/C check it with a leak detector and he didn't find any. I see what you're saying how the PSI isn't a good test. I'm not quite clear, though, should I further investigate a faulty switch or is it impossible to tell without knowing the freon is correct.

Thanks

[old_master]

If the cycling switch cycles, as yours is: Set the HVAC control to MAX AC, high blower speed, and hold engine RPM around 2000. Watch low side pressure at the service port on the accumulator. Note what the pressure is when the compressor kicks on and off. That will tell you if the switch is working, (yours is) and if it's within range. Add the "ON" pressure and the "OFF" pressure together and divide by 2. Idealy you should have 30psi.