Title says most of it. I noticed the windows fogging up during a drive the other day, so I turn on the automatic defrost, which activates the air conditioning, but nothing happened. The fan was blowing hard into the front window, but the air was noticeably damp. After about 10 minutes, the compressor suddenly decided to engage and the window was clear in 30 seconds. Later that day, the same thing again.

Now it appears the compressor doesn't want to come on at all. I sit at idle and toggle the air conditioning on and off and never hear it engage.

The question is what can I DIY in this situation? Could be as simple as a loss of R134, although when it did engage, it seemed to work wonderfully. Also, during the summer, the air conditioned air was absolutely ice cold, indicating no slow loss of refrigerant.

What else can I do to troubleshoot before I take it someone and undoubtedly spend $1000+?

I would also check the pressostat switch. Close to the firewall. Unplugg it and jump it with a single wire, the plug and not the switch. The AC should immediately kick in.

Then check the pressure, it could be low due to the temperature, the lower the temp the lower the pressure. A small amount of R134a, about 1/2 can should bring the pressure up nicely. Do not shake the can, but hold it upright so you don't force a bunch of not wanted PAG oil into the system. The compressor needs to be running to add.

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.

OK, checking the pressostat switch by shorting the plug appears to engage the compressor. Furthermore, after I was done checking, the compressor appeared to respond to the dash button once again, and did for the rest of the day. When it was running, I tested the air conditioning performance and measured 39 degrees F at the vent outlets! Cold!

So the issue appears to be intermittent. When it works, it works well, although I haven't checked the system pressure yet. I'm thinking a failing pressostat switch or the beginnings of a compressor clutch failure?

At those temps, you have plenty of r134a. The clutch gap could be checked by tapping the center of the clutch with a long stick (carefully) and if it then engages, the gap is too large.

The pressostat is an easy fix, the system does not need to be evacuated and repressured, IIRC.

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.

Pick up a bottle of refrigerant with a gauge on it, and hook it up to read the pressure. If the A/C is not engaging at ALL (it should cut in and out quickly even with low freon), it may be a bad pressure switch in the system.