Intermittent A/C (system pressure changes?)

[swensonator]

OK, so during the summer I had to replace the condensor on my AC system due to a leak. No problems with the replacement, went to the dealer and had them pull a vacuum and refill the system. AC worked great after that.

Just this week we finally kicked the heat wave we've been having all fall/winter and had a couple days with highs below freezing. I turned on the AC one morning to defog the window and the compressor didn't turn on (it acted just like it did when it was low on pressure). I started thinking that all the work I did during the summer was for naught and the compressor (or some other part) finally kicked the bucket.

One week later, the heat wave returns and the highs are in the mid 50s. While stopped at a red light I push the AC button, just out of curiosity. Miraculously the AC turns right on.

Now, I'm hoping someone on here is a chemist, a chemical engineer, or at least knows more about Tetrafluoroethane (R134a) than I do (I'm an astronomer and anything heavier than hydrogen and helium I simply call a 'metal' and couldn't care less about). I know there's going to be a vapor/liquid equilibrium in the AC system that keeps the system pressurized. But is is possible that the dealer underfilled my system, so that the system had enough pressure for temperatures >50 or so degrees, but once it got colder than that the pressure dropped too low for the compressor to engage? I tried to look up a pressure equilibrium chart for Tetrafluoroethane online, but didn't have any luck.

I'm hoping this is the case and all I need is a quick top-off, rather than some component being on its last legs and teasing me.

Thanks for your comments.

[Stillrunning]

See link for temp/pressure chart for 134a. There are static pressure and running pressures.

http://www.csgnetwork.com/r134apresstempconv.html

The ambient temperature would have to pretty low for the low pressure switch to not allow the compressor to turn on. There is also a temp sensor that prevents frost build up when evaporator outlet temp falls too low.

Just take the car back to where you had it serviced and get the pressures checked.

[swensonator]

Looking at the chart it appears that the pressure does change significantly with temperature. The ambient temperature was ~20F when I tried to defrost the windshield and the compressor did not turn on (the engine wasn't warm yet). Do you know at what pressure the low pressure switch prevents the compressor from engaging?

I'll take the car back to have the pressure checked, but it won't be for several weeks, that is the soonest they could get me in. (Either there is an extreme shortage of mechanics/technicians in this town or they're all just incredibly slow. I had my emission and inspection done this week at a local Ma and Pop place and I had to schedule it a month in advance. Drives me crazy. Where I used to live you could just drive up to practically anywhere and they had dedicated bays for inspections. Usually took less than an hour and you were on your way.)

[Stillrunning]

Originally Posted by swensonator

Do you know at what pressure the low pressure switch prevents the compressor from engaging?

My 95 specs are 28psi, yours probably the same. At -20F cold air going through the evaporator would freeze any moisture in the air and possibly blocking air flow. As I said there is also a temperature sensor on the outlet to cycle the A/C off/on to prevent freezing, even in hot weather. Your system may override at those cold temps until the engine heats up. I have another vehicle that I can by-pass the cold air off feature by turning the A/C control to Max A/C until the engine warms up. I'd also read your owners manual to see if it explains the operation in cold weather.

[swensonator]

Alright, I've gotten things figured out. After reading your last post, I realized what the problem was. In order to override the temperature sensor, you have to turn the vent all the way to 'defrost', which automatically turns on the AC as part of the defrost process. I had it set to 'defrost/feet' and when I push the AC button the compressor did not engage because it was prohibited by the temperature sensor.

We had another cold day today and I tried turning on the AC when set to the 'defrost/feet' setting and the compressor did not turn on. I then turned it to 'defrost' and the compressor automatically turned on. Everything worked as it should. I was just overly concerned because of all the effort I put into repairing my AC system this last summer and didn't think everything through. Thanks for helping me remember how my car is supposed to work.