AC compressor clutch not getting power

[Ben Swann]

Two '91 200Q20V sedans with apparantly the same problem.  The AC system has
full charge -  approximately 90PSI static/35PSI low and 180 PSI high
pressure side during operation and outside temp around 80 deg. F.  Climate
control set to low and everything operates except for compressor clutch is
not engaging.  Applying power directly to compressor engages clutch and
system blows nice and cold.

I noticed on the one I was just working on that there was no relay in
position number 6 where the fuse box cover indicates AC relay should be.
Inserting relay makes no difference.

Any clues as to what is going on?  BTDT on common culprit(s)?

Ben