How to measure AC pressure and charge in system...mini FAQ

[abscate]

Attempt to document AC charge measurements on Volvo ACs

1 putting a gauge on the fill port and measuring the pressure on n AC system with either the engine or the AC off is meaningless information -essentially you measure the vapour pressure of he R134A refrigerant. No pressure obviously means no refrigerant.

2 measuring pressure requires controlled conditions (thanks jimmy57)

1500 rpm
fan speed just below high
recirc on
windows up
allowed to operate 10 minutes with above settings

with engine idling the compressor is below its capacity and pressures are too variable.

Once it has run a for a while the low side may not be as high as 35 if the ambient temp is not mid 90's or above with lower humidity or upper 80's with high humidity.

If you turn down fan to lowest speed the pressure will go to 23-25 and compressor will cycle off, pressure will go to 43-45 and compressor comes back on and repeat the cycle off-on if it is charged within reason of correct amount. Too much will make it not want to cycle and too little will make it cycle but maybe too fast and the suction line (large line) from firewall that goes back to compressor will not be really chilly.

3. A well performing AC will give 40 degrees of temperature change between outside and inside

4 An AC compressor that runs for 10 minutes and then tops engaging probably has a clutch problem fixable with either reshimming inside or outside with the "bread clip" fix