A/C? where for art thou?

[BBBurban at aol.com]

Bought my 1990 90q20v a few months ago and was concentration on fixing the 
absolutely necessary stuff.  Now I find the A/C is inop... I have checked 
both the relays and the both test good.  The fan comes on when the CC calls 
on the A/C but the clutch doesn't engage. Any ideas what I should check next? 
 is there some kinda low coolant pressure switch that prevents the compressor 
from engaging?  Any help is good help.. .TIA

Hank 90q20v