'91 RS 305 TBI auto. A/C blows cold, then randomly starts blowing warm/hot for about 5 minutes, then goes back to ice cold again. This has happened twice in the last month, but never long enough for me to open the hood to look at anything.

Car still has the original compressor, converted it to R134a 3 years ago, vacuumed/recharged/added new oil/replaced the orifice tube and accumulator when all that was done. Compressor has always had a slight bearing noise since I've owned it, but has never gotten worse. A/C has been blowing cold 100% of the time until just recently.

 

Bad connection somewhere maybe. I would check wiring connections at the compressor and the cycling switch on passenger side.

 

Just tried wiggling the cycling switch connector while it was running, and the compressor cut out, so I would assume that's it. Seems to be on there good now.

One thing I thought was weird though is that my compressor never cycles on and off while the a/c is on, it just runs all the time. Aren't they supposed to cycle on and off every so often? I remember adjusting the cycling switch to blow colder in traffic when I did all the a/c work 3 years ago, but honestly it does a lot better while the car is moving. ​​​​​

 

Check your freon charge. I had a leak at a condenser connection and that's how mine acted as the freon was leaking.

 

It will cycle more frequently at lower ambient air temperature, and if it's hot outside it might run constantly with no cycling. It cycles off pressure, so lower temps result in lower pressures and subsequent cycling. The cycling keeps the system from freezing at colder temperatures.

Most of my a/c systems also work noticeably better at highway speeds. This is not unusual, even for new vehicles. If everything is ideal it should work just as good in traffic, but that can be hard to accomplish with a 30+ year old car.

 

Thanks guys, I guess I'll wait til it gets a little cooler again here, then see if it starts cycling before I do another evac and recharge to see if I've lost any refrigerant. I did do a quick visual inspection of all the lines and fittings to see if there was any built up oil from a leak, but everything looks good for now.