Ive got an '89 convertible and I just recently converted the AC system from R-12 to 134a. After the conversion, the AC system blows cold air, but the AC compressor never turns off regardless of the temperature setting. It will turn off when I select economy, defrost, or other non-ac setting. When I was trying to figure out why the AC compressor runs all the time, I noticed that the low pressure side is about 80psi when engine is not on, and about 45psi when running and AC is on. I understand that this is pretty normal, but the low pressure side when the compressor is running is in the 25-35psi range (which is lower than the low side). Im getting cold air, and the system seems to be working fine otherwise, but Im afraid that this is a sign of something bad that might be made worse by using the system. Do I have something clogged, a part that is failing, etc?

Any help would be appreciated.

 

What you describe is how the A/C system works. As long as you have defrost, Max, Normal or bi-level selected the A/C compressor will run. The temp setting doesn't turn off the compressor, it mixes warm air in with the cold air to get the desired temp.
Low side at 80 psi with A/C off is good (as long as the high side is the same) With the compressor running your 25-35 psi is excellent, the lower the pressure on the low side (with a proper charge) the more efficiently the system will run (until you trip the low pressure switch).

 

Your compressor should not be continuously running, your instincts are right. There is a two pronged pressure switch located on one of the lines, many times when people are working on the A/C they run a bypass with a piece of wire so the connector makes contact continuously resulting in the compressor constantly staying on....check to see if they forgot to remove the bypass and if not, perhaps the switch is bad and needs to be replaced.

 

Could it be an overabundance of oil in the receiver/dryer or something else related to the conversion from R-12 to 134a? The fact that the high side pressure is actually lower than the low side seems to point to something being seriously wrong.

 

Pressure is a function of temp and humidity - they higher they are, the higher the pressures and if it's 70 or above, the compressor shouldn't cycle (which is good, because cycling creates the most wear and doesn't do a whole lot for idle quality either). The 45 psi on the low is a tad high - even if it's 90, so I'd say it might be overcharged (or it sucked in some air when you filled it or it's got a mix of R12 and R134 circulating around in it). I'd do this: Hookup your Gages, turn the a/c on max, open the windows, get the engine running at 1200 to 1500 rpms, disconnect the fan switch so that the radiator fan stays on and then disconnect the blower motor harness at the blower motor. The reduction in air flow across the Evaporator should cause the low side to drop and the compressor should cycle off at 25 psi (assuming you didn't adjust it for R134 - if you did adjust it, it should cycle off at 22.5 psi). If it won't pull down to pressure, hook the blower backup and note the high side and the temp of the air across the condensor (if you have electronic air, just use the temp readout since the sender is on the radiator shroud). Ideally, you want to see 2.2 to 2.5 times this temp on your high pressure gage. If it's 50 psi or greater than this temp, you probably need to get some gas out of it or the mix is contaminated. An a/c shop can check what's in it assuming you can't get the pressures down.