I installed a used A/C compressor on my 1991 Buick. Not sure if it was good to begin with. Anyway, I pulled a vacuum and it held very well. When I started to charge the system with R134A, it took the 1st can of charge and I noticed the pressure readings on the Low and High sides. It read 120PSI on both guages with engine on or off. The compressor clutch pulled in with no problem. I did previously changed the orifice tube, thinking it was stuck open, but that did not do the trick. The compressor is making a little noise. How can I confirm that it is bad/good? I did not put any more refrigerant in the system after I saw the high pressure on the low side. Any ideas? Thanks!

 

Couple of comments: First, converting R-12 systems to 134a is tricky at best and best left to professionals. For example, there are oil compatibility issues involved. If I remember right R 134a systems take either poly alkalene glycol (PAG) or Polyolesther oil (POE) (You need to know which one.) and a require (in terms of design) a slightly larger radiator, and thus have to be UNDERCHARGED. (R-12 /R-22 systems use mineral oil.)That is to say you cannot put in the Hood Label R-12 charge or they will run high head pressure. In fact books recommend putting a 300 or 400 psi head pressure switch in series with the compressor clutch to keep the system from self destructing. Remember, R-134a is not the factory designed/designated refrigerant on older cars!

Pressure readings: Suction side will be from 50 - 80 lbs High side 120 - 190 lbs. High suction - low head = bad valves If pressures are the same the compressor IS NOT pumping.
Low suction pressure: either restriction on low side or low refrigerant. High head pressure, either overcharge, or little air through condensor, possible restriction in condensor itself.

The biggest problem with amateurs is overcharging, and not doing the work right: replacing components, replacing oil correctly, and system driers....