The air seems to be not as cold as I think it should be from my A/C. I was about to add some R134a yesterday afternoon to boost the cooling capacity. When I hooked up the gauge to the low-pressure A/C port with the engine off, I saw that the static pressure was about 90 psi. Turned on the engine and set the controls to full cooling with the doors open, and the low pressure dropped to about 46 psi with the engine idling.

That seemed to me as if adding R134a would overcharge the system, so I stopped.

My question is: how can I ensure that I have the system fully charged without overcharging it? I can't monitor the high-side pressure, as there is no port.

The conditions were: Ambient temperature: 99 degrees F; static pressure: about 90 psi; pressure with engine idling: about 46 psi and slowly rising over time. The compressor stayed engaged and did not cycle.

'95 945 NA 308K