The best and most accurate way to charge any refrigeration system is to measure the mass of refrigerant put into an evacuated system. Typically cars take anywhere from 21 to 23oz of r134a refrigerant. Always consult the service manual for the vehicle to be certain.
The cheaper way is to measure line pressure and blower air temperature while adding refrigerant. For cars I use a limit of 20psi on the low pressure line, measured with my digital manifold.
The danger of the quick method is the risk of over charge. Too much refrigerant in Oz, even by a small excess, will damage the compressor very quickly!
Read on for how I did a top up to improve my wife's car air conditioning. Note that in my country no license or permit is needed to handle or purchase refrigerants.
Step 1: The Tools.
-digital manifold gauge set.
-can of r134a.
Step 2: Checking for Leaks.
Step 3: Hooking Up the Manifold.
The red or high pressure valve remains closed for this job.
The r134a can must stay upright for the job.
The high pressure line is not used for Topping. It can be monitored for diagnosis purposes but never to enter refrigerant into the system.
Step 4: Adding Refrigerant.
I slowly opened the blue valve to allow r134a into the system. Keeping an eye on the low pressure gauge, I made sure to shut off the blue valve when the pressure climbed to 20psi. I left the engine on idle the entire time. I had to gently shake the can of r134a to get the gas into the system. Patience is needed here to add r134a and let the system equalize then add some more. This will take some time.
Step 5: The Desired Temperature.
When the system got close to the required charge, the pressure climbed quickly to 20psi when I opened the blue valve. This is a good indicator that no more refrigerant is to be added.
Beyond 20psi carries the higher risk of overcharging so that is why I don't go beyond that. If I'm not careful, even at 20psi the system can be overcharged but once the temperature at the vent is low enough, my job is done.
Step 6: Completion!
With only the front blower on, the temperature and pressure are shown in the last 2 pics. Inside at the rear of the car was quite cold.
In all this took half hour to ensure the pressures throughout the system were at steady state values.
I hope this instructable has been helpful to you my readers.