1988 Mercury Marquis 1988 mercury a/c compressor wont turn

when you turn on the a/c the compressor wont engage. the solenoids open to allow refigerent to circulate, fuses good,. when I run power directly from battery to compressor it engages and it blows cold air so i know its charged. its electrical obviously but my repair manual is not very descriptive, i thing its the a/c cutout relay but dont know where it is any help would be greatly appreciated.