Thanks, Stan:

I'll try tightening it, but shouldn't the compressor kick back on after a bit? or does the rpm sensor keep it off line as soon at it senses a problem with the belt? By the way, the belt doesn't feel too loose to me. According to the records I have, the belt was replaced about three or four years ago at 119,000.

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Two hours later:

I tried tightening the tensioner. Tensioner tightens nicely but the AC still doesn't work. Chirped a couple of times and stopped blowing cool air. I'm still stumped.

This can't be that difficult!