A correct refrigerant charge is necessary to achieve optimum performance from an air conditioning system. This is very important with the vehicle A/C systems designed for R134a refrigerant. A total A/C system refrigerant charge has been determined for each chassis / Evans system.
When servicing the refrigerant system, the only way to be certain of an exact charge is to fill an empty system with the specified amount of R-134a refrigerant. If an A/C system is operating and the amount (charge) of refrigerant within the system is not known, some simple checks can be performed to determine if the operating charge is adequate:
1. Compressor clutch engaged, and compressor operating.
2. Suction hose fitting (at evaporator outlet) cold to the touch. This fitting may sweat or even frost lightly.
3. Chilled discharge air at the dash louvers, when the temperature control is set at the coolest setting (water valve completely closed).
4. CAUTION!!! Some systems may have a sight glass on the receiver drier. The sight glass is only useful when considered along with steps 1 through 3 above. R-134a compressor oil circulating in the system can make a sight glass appear foamy even with a proper system charge! Disregard a milky or foamy sight glass if the A/C system passes steps 1 through 3 above. If a clear sight glass can be seen, this simply confirms an adequate operating charge.
Dash Air Conditioner Refrigerant and Oil Capacity
HOW TO DETERMINE OPERATING PRESSURES
Every vehicle has its own operation parameters specification. Depending on whether or not the vehicle has a factory or an after market system, and whether it was retrofitted or is still original, among others. There is no established calculation to determine the exact operating pressures.
The low pressure (blue gauge) should be 35 or less at idle, regardless of the type of gas. Only if you're running a dual system, the low pressure may be between 45 and 50 at idle.
The high pressure, for starters, is directly related to ambient temperature: the higher the temperature, the higher the higher the pressure.
Bear in mind that dispensing a complete charge without lowering the pressures with water will result in higher readings. This is normal, and that is why you should spray water in the condenser at 2/3rds of the charge or once you have completed it. You'll note an immediate pressure drop when you spray water in the condenser.
To obtain a ballpark high side value, multiply ambient temperature by 2.2 if you have an R12 system. If you have a factory R134a system, use 2.3. To convert C to F, use this formula: C X 1.8 + 32.
Remember: this is only a ballpark. Factors like a an obstructed or very dirty radiator and condenser, weak or inoperative fan clutch, weak or inoperative radiator fan(s), either electric or mechanical, will make pressures go up and impair cooling, even in mild days.
Online Automotive A/C Reference Guide