That depends on why you need to charge it, assuming the system had some type of repair done and working forward from there:
- Ensure the proper amount of oil is in the system, if you change an oil capacity component (evap, condenser, accumulator, compressor) you will need to add oil;
- Evacuate the system to -30psi, maintain that vacuum for at least 30 minutes (pump connected and functioning);
- Leak test the system, shut down the pump (close the valves) and watch for pressure change over 15 minutes;
- Charge the system with the required amount of refrigerant; monitor pressures (both low and high) for other system problems; system should be charged with engine running and cooling set to MAX. Make sure pressure/temperature does not go below 35F during rev; less WOT trigger.
Charging a system requires you to understand the system; the low side cannot go below 35 F or the evap will freeze water and the system will fail - this is 'at engine rev' not idle. The pressures invovled are relative to temperature, the high side should NEVER go above 400, and it is normally offset by ambient temperature plus the difference of evap cooling.
