'01 Duramax A/C compressor no worky

[Tynee]

Anybody got a good link for trouble-shooting my buddies '01 duramax A/C compressor? He said he took it to get it charged, it was a little low, so they did indeed charge it, and checked for leaks, but found nothing. When it quit blowing cold again a couple days later, he took it back to them, they put the gauges back on, and the pressure was fine, but the compressor won't engage.

I'm headed out now to see if he's got voltage to the compressor clutch, and maybe jumper the low pressure switch to check that out. Any other suggestions? What else would make a compressor with a good charge not engage?