low side—35 psi/high side—212 psi
Symptoms: The air coming from the vents is approximately 32-40 degrees. No bubbles are present in the sight glass.
Conclusion: The system is working correctly.
A good rule of thumb is that pressures should be in the region of 35 psi on the low side and 212 psi on the high side at idle. A reading of 35 psi is equivalent to evaporator temperature, which is what you feel coming out of the vents in the car; this should be close to the freezing point of water.
With systems using R134a, the high-side pressure usually will equate to 2.2-2.5 times ambient temperature. That means on an 85-degree (F) day, you should see high-pressure gauge readings of between 187 and 212 psi.
low side—2 psi (low)/high side—102 psi (low)
Symptoms: The air coming from the vents is only slightly cool or even warm. Bubbles or oily streaks are seen in the sight glass.
Conclusion: Insufficient refrigerant charge
Repair Procedure: When a low refrigerant charge is indicated, be sure to check for leaks. Leaks are the most common cause of automotive A/C problems. The vehicle's refrigerant will leak out and reduce the total pressure in the system, causing the low-pressure switch to disengage the compressor. This is a safety feature designed to protect the compressor from damage if it becomes low on refrigerant.
When looking for leaks, a good visual inspection of the entire system is a great first step. The system contains oil necessary to lubricate the compressor. The presence of an oily film around fittings, lines, the compressor or any components is a strong indication of a refrigerant leak.
If you don't find a leak with a visual inspection, your next step is to recover the remaining refrigerant from the system. Put the system into a vacuum for approximately 20 minutes and recharge it to the factory specifications.
There are several methods you can use to detect small leaks. The first is an electronic leak detector, which can be used with all types of refrigerants. The second method is to use a fluorescent-dye leak detector. To find leaks using the fluorescent system, the dye must be installed in the vehicle's A/C system.
If a component or line is leaking or defective, you'll once again need to recover the remaining refrigerant from the system. Replace the defective component and the accumulator/dryer, then put the system into a vacuum for approximately 20 minutes to boil out any moisture. Finally, recharge the system with the factory specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.