I am losing head pressure and subcooling when the sun goes down....then the a-coil freezes up..

Sounds like bad valves in the compressor.
A good indicator is how fast the pressure changes when you shut the compressor off. It should take several seconds for the suction pressure to rise. If it's immediate, then expect a bad valve.

I don't use design pressures due to many variables such as ductwork, humidity, blower speed, o.d. temp, I.d. temp ect.... Learn SUPER HEAT AND SUBCOOLING values. Most guages give a pressure/ temperature relationship. At a pressure there is a temp that lines up with that pressure. If its r22 the temp is in the green and pressure is in the white(normally). If r410a temps in the pink. R22- Most evaporator are good between 40-50°F. If your coil is operating at 45°f your superheat value should be 10°-18°. Take the actual temp of the suction line(large copper line that's cold) and subtract that # from the temp on your gauge. The value should be 10-15°. Same on the liquid line. You o.d. coil is usually 90-105°F. The liquid line temp(small copper tube that's warm) subtracted from the pressures temp gives you the SUBCOOL value.
R410a? Subccoling
R22? Superheat and subcooling
If it has an expansion valve? Sub cooling
This method is used to take all the guesswork out of the scenario. BUT! Most of the machines will have a data plate that gives great info for this method. But some only give it in the owners/installation manual. You won't get a good answer to your question but this is the correct way all HVAC technicians SHOULD check freon levels and is the only way if we are achieving designed efficiency. If you know its low on freon, again most coils are "happy" between 40-50°F(68-84psig). And at that temp your high side should be 90-105°F(168-210psig). And again, so many variables! I usually shoot for 95°f o.d. temp, 80low 210high psig and 10° SH 12-18°SC.

Low ice production has multiple causes. First check condensing coil to see if they are dirty and not passing air. This is the most probable cause for low ice production. Next, measure freeze cycle time and harvest time. Freeze time is typically 20 to 30 minutes and harvest time is normally 1.5 to 3 minutes. If freeze time is long (> 35 minutes), first check water valve shuts after reservoir is full. If it does not shut off the water solenoid operated valve needs replaced. Second, find the harvest solenoid operated valve which branches off the compressor discharge and port high temperature gas into evaporator during harvest and see that evaporator side of harvest valve cools down to surrounding temperature in a few minutes 8 - 10 after freeze cycle starts and harvest time stops. If it does not cool off the valve is leaking by its seat and will require a technician to replace. Lastly, if long freeze cycle could be due to low refrigerant and will require and tech to charge properly. Ice machine are critically charged and a tech will know how to charge the right amount. Again, it is possible the metering device is not passing enough refrigerant and a technician can determine this by checking the superheat and subcool. Low subcooling <3 and a high superheat >20 typically indicate an txv (metering devise) needs replaced and will require a tech to replace.

Compressor sounds out on thermals from being overcharged. Check your airflow across the coil- perhaps a faulty blower or control, restricted duct, or dirty filter could be the culprit to the initial icing. If not, repost with the refrigerant type and pressures.

Hello,
When there is water blowing off of the coil there is too much subcooled liquid in the coil. If the pressures are correct then the next culprit would be too high of a static pressure with the airflow. Adding in ductwork (both supply and returns) will help reduce the water coming off of the coil.
Hope this helps James

If the condesnser coils are clean and condenser fan blde as well and refrigerants not low or(non condensibles) air in the system (high head pressure) than the superheat to the compressor should be ok, not more than 20 deg super heat at the compressor generally, if thats not it the thermal overloads bad or compressors low on oil or tired, take amp deaw and see pressures to know for sure, check super heat and subcooling too.

It sounds like you are low on charge. I think you have a thermostatic expansion valve and you need to check the subcooling, you should have 12 to 15 degrees at 95. At 95 degrees your head should be about 280# for a 12 or 13 SEER unit or higher depending on the effeciency. At 78 to 80 degrees indoor depending on the humidity, that is normal. The valve will open to try to fully feed your indoor coil and it sounds like your head pressure is to low to force enough refrigerant thru the coil.

Hi, sounds as though for this to be its 2nd compressor for such a newer unit, the superheat was not right when the compressor was installed. I don't know what freon you are using the new R-410A or the regular R-22. There's a big difference in pressure, but now I see your pressure reading and its R-22 for these readings.For a 2 ton unit, you would use the superheat charging method for a unit that doesn't use a thermostatic expansion valve ( T X V ) and not the sub-cooling. A 23 degree super heat at that outdoor ambient temperature is way to high for that unit, you will loose the compressor again!!. Super heat killed the compressor is the saying and that's a fact. Suction pressure should pull down just as quick as the head pressure unless the valves are going and weak, or you have a scroll compressor that has a valve plate that is going, or you are low on charge. With a ambient of 95*, you should be at around 270p.s.i. on your head pressure as 210 to 200 is way low. Suction line temp should be around 68 to 70, with a suction line temperature of 50 to 51 degrees, which would give you a 10 to 12 degree F super heat. I don't know when the second compressor was installed, but it has to be low on charge to be such a low head, and high super heat and you will loose this compressor again, its only a matter of time before you have a burnout. It was either under charged when installed, or has a very small leak at one of the joints. I hope the liquid line drier was replaced also. It shows me you have some knowledge on a/c operation, so you need to get that superheat down to between 8 and 10 degrees for this unit to cool properly, and leak check it also. Did you buy a extended compressor warranty? Lets get the head pressure up and superheat down and you should be OK unless the valves or valve plate is weak. Once you loose the valves, you will have a lower than normal head pressure and a high suction pressure. Compressor just won't pull down anymore. I hope I have been of help to you and ask of you to be kind when rating me. I will be here for you if and when you need me for anything.
Sincerely,
Shastalaker7
A/C & Heating Contractor

Hi, normally the suction pressure on a refrigerant -22 system will be between 60 pounds gauge pressure (which keeps it above freezing point) and about 80 pounds. This depends on the temperature condition.Make sure that the superheat and subcooling are correct to see what is to be considered normal for the conditions you have present.

Take care

try cleaning a-coil