92 Chevy c\k A.C. compressor won't engage.

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92 chevy truck. A.C. compressor won't engage. Hard jump at both low pressure switch and compressor engage the A.C. With this method I was able to verify that the charge levels both high and low are in the green. Relay and fuse both replaced and working. Green wire at the head unit tested both connected and disconnected. Still won't engage without 12v power directly applied. Any suggestions?