If you think that you have an issue with your A/C system but you are not sure which component is causing the malfunction it is a good idea to hook your compressor up to a pressure gauge.
An A/C System that is working properly should have 150 PSI on the high side and 30 PSI on the low side.
Obviously the main issue that people deal with when they have a broken A/C system is that the air coming out the vents is not cold enough. Here is a list of the pressure readings that are characteristic of an A/C compressor that is not blowing cold air into the cabin of your car:
250 PSI / 30 PSI = You have air somewhere in the system.
250 PSI / 50 PSI = The system is overcharged and the condenser is not cooling. Condenser may be blocked.
225 PSI / 80 PSI = It is likely that the expansion valve is opened too wide (not the case if your A/C system has an orifice tube) Also, you could have too much refrigerant in the system.
200 PSI / 70 PSI = There is a blockage somewhere in your system either before or at the expansion device.
160 PSI / 10 PSI = It is possible that the evaporator is frosted, the low pressure piping is faulty, or the expansion valve might be clogged. Check the evaporator, piping, and expansion valve to locate the source of the issue.
150 PSI / 30 PSI = There is water in the system.
150 PSI / >10 PSI = Your expansion valve might be stuck open, or there could be a leak somewhere you in your system.
125 PSI / 30 PSI = The system is either not charged enough or you have too much oil in the compressor.
100 PSI /100 PSI = The compressor is not engaging because there is no power coming to it or you have a burnt coil.
50 PSI / 50 PSI = The clutch is not engaging.
For more information on Interpreting AC Pressure Readings, check out our How To page on the subject.