Ok so I may have an issue. My a.c. clutch engages every 15 seconds and with it engaged am only seeing 25 lbs of pressure. It is in the 20s here so could that be playing a part? I added a small can of refrigerant with no change in pressure.

 

Your compressor should not run at all under about 35 F. The vapor pressure of refrigerant is too low and the low pressure switch keeps it off. If it does cut on, suction pressure will drop and immediately cycle it off.

This makes for foggy windhields from interior condensation in cold weather in all vehicles until warm air defrost is working. By design, AC doesn't dehumidy air, it would freeze up if it did.

Overcharging system is a $$$$ death sentence .
It will feed liq to compressor when it runs. AC systems have to be carefully charged by weight with working components to last.

If suction temp is below freezing, 32 f, evap core will quickly ice up. This is why AC systems don't run that low. This happens when systems are low on freon too.

At 32f, r134a vapor pressure is 27 psig. This is all you will measure in a no running system, regardless of how much you put in.

To assess performance of ac system with gauges, you need to be near design temps that benchmarks are at. The condenser air temp =90f, correct engine rpm, etc. You can't tell anything accurately at low air temps. System will appear to work ok with less freon than it needs at design conditions.

 

Thank you. I have a allow leak in my system that I thought had gotten much worse. I'll bleed off any excess pressure when temps get back above freezing. That was extremely informative.

 

By all means use a gauge set. It will pay you back by taking guess work out.

 

My Toyota pickups compressor runs well under 32 degrees. I was standing outside of it for over an hour the other day when it was 20 F. I could hear compressor cycling. I intentionally leave the AC on because the cab quickly fogs up without it, especially if the dog is in the cab, so I believe it has to be dehumidifying. I rarely ever cut the AC off because I want to keep the seals lubed and like the dehumidifier effect in the cab. It certainly hasn't hurt. The truck is 24 years old and still has the original AC, which has never needed service.

It is my understanding that many new cars automatically cycle the AC when the defrost is on....All of my stuff is old. I have to do it manually.

Back to the OP's topic. I have a car that had a leaky AC from lack of use. When it was low on refrigerant, it short cycled....I agree with others. You need to use a gauge to properly charge your AC.

 

Thanks y'all. I'll find a shop around here with real gauges and see where I'm at.

 

Once the engine compartment and the heater core is warm the refrigerant temperature can rise and get above the low pressure switch cut off and the AC can kick on. These can be way above ambient outside temperatures. This is why it can cycle when its cold after vehicle is warmed up. If you go out there to start the vehicle and it's 25 degrees it ain't coming on if the low pressure cutoff switch is working.

As you get near the low pressure, the evap core is cold enough to freeze water instead of condensing and ices up, blocking airflow. No airflow, no heat transfer. No heat transfer, no vaporized freon. I.e, compresor slugs liquid to it and makes chatter noise and dies eventually. Even without icing up as the ambient temperature approaches the temperature of the freon there's no temperature driving force for heat transfer and the same thing occurs. No freon vaporized, and liquid slugging.

Some newer vehicles have ambient air temperatures in the computer that prevent the compressor from running below 39 or 40 degrees F that have owners upset due to lack of dehumifification in winter.