Hi Robfnk,
The 300/500 psi-g pressures you read on the unit are most likely the factory test pressures.
Operating pressures vary minute-by-minute depending on many factors.
Some factors that have an effect on pressures are:
Ambient temperature at the condensing unit, air temperature across the evaporator coil, cubic feet per minute across each coil, enthalpy, whether it uses a capillary tube or expansion valve, the superheat, the subcooling. There are many, many more factors. Even how much wax has built up inside the capillary tube, pressure drop across the filter-drier or how much debris is on the expansion valve screen can have a tremendous effect on pressures.
R-410a complicates diagnosis even further because it is a blend. All blend refrigerants act differently than single component refrigerants.
If you're inclined to diagnose further, download a temperature-pressure chart for 410-A. Study how to use the chart and you can get an idea of about what the high side pressure should be. This, coupled with the suction pressure & line temperatures can at least help narrow down the list of possible reasons for low or no cooling. Be mindful that if your system is a capillary tube system, the charge amount is critical. There is no room for error on a small unit.
You mentioned that the line coming off the compressor is "cold just not cold enough". Assuming you're talking about the suction line (the larger diameter one), again dozens of factors enter into the diagnosis. Sometimes barely below ambient is normal, sometimes condensation to almost the compressor (but never ice) is normal.
You said "capacitor not drawing full amps". One doesn't check amperage at the capacitor. It will always be low. Compressor terminal "R" amps can also read below the name plate specification depending on the same many factors mentioned in the second paragraph.
As you can see from above, diagnosis isn't an "over the phone" kind of project. You may enjoy learning more about how this all works and experimenting on your unit.
S.W.A.G:
If the unit is old and you meant compressor (not capacitor) amps are low, the coils are clean, the air flow is good and the suction line is barely cool- your compressor valves are worn. (Just an over-the-phone guess. I could be way wrong, but this is common.)
One Important Caution is to Always Wear Eye Protection when working with refrigerants. Gloves are a good idea to prevent frostbite because the R-410a boiling point is 55.3 degrees-F below zero.
I hope this very long post isn't too terribly boring. I wanted to give you an overall view of how many factors can cause the symptoms you're experiencing. It's like a symphony. If even one of the musicians plays flat, the whole concert is off kilter.
Paul
PS: Many utilities will pick up and pay you for your old unit- working or not. Our electric utility gave me $50.00 for an old window air. Many also have rebates if you buy a new air conditioner- window, through the wall, mini-split or central.
If you like DIY projects, there are many mini-split units that are do-it-yourself installations. Everything is pre-charged- even the lineset. You simply attach the lines at each component with a wrench and you're in business. Most likely, you'll need a 20 or 30 amp, single phase, 230 volt circuit, but you're an electrician so the expensive part is not so epensive!
Mini-Splits tend to be more energy efficient than through-the-wall or window machines. You can even get ones with multiple room capabilities. Inverter units, too. Supply House Com has several. Even grow house supply companies sell them.
