A.C. trouble PLEASE H E L P ! '95 Eldo ETC

I'm getting "bounced" all over on this one--A.C. compressor (new GM) won't power up. My non-dealer mechanic installed it, along with accumulator, rec/dryer, exp tube etc etc. Thought it might be relay, but no. Local expert says PCM (hasn't seen car). Mechanic has given up. I had an appointment at ($$)dealership-now cancelled 'til Tuesday morning. IN FORUMS I've been told so far .... low pressure or high pressure cut out switch may need replacing. AND/OR disconnect battery for 30 seconds because the flag will still say do not power comp unless this is done (I think it's safe to ASSUME he tried this, but I can't find out for 2-3 hours). IF, that's IF- he did this already and still no power, what do you think? IF he didn't, is he a BOZO?? THANKS!!