300E A/C - please help diagnose

[300EE320]

Hi,

Yesterday the A/C in the 1990 300E abruptly quit. My wife used it on the way to the store and it worked fine. On the way home - nothing. The blower is fine, but the air that comes out of the vents is suddenly not cold.

It doesn't seem like it needs a charge because it was working very well. We had it charged about 2 years ago (with R12) when it slowly lost its effectiveness. The filter drier and high pressure switch were replaced at the time as well.

I checked the fuse (#7) for the compressor and it was OK. I changed it anyway for the heck of it. I also looked at the sight glass on the fliter drier. It looks clear. Not opaque and no bubbles. I also checked the electrical connections near the filter drier. All seemed okay.

Unfortunately, my manual does not cover the A/C system. I do have a generic automotive A/C book and I understand the theory of how it works. Can anyone give me some pointers on troubleshooting this or point out some of the common things that could cause it?

Thanks very much.