1998 a/c compressor problem

[Cappy]

This is a very valuable site and I appreciate any help or advice anyone has to offer.First post for me. I normally use the search to find what I need but not this time.
My truck in a 1998 w/t 6 cyl.
When I push my a/c button the light comes on but the compressor clutch doesn't engage. I have tried jumping the low pressure switch with no luck. When I jumped the relay under the hood the compressor did engage. I thought I found the problem. I bought a new relay and the compressor still wouldn't engage. Every time I jump the relay the compressor engages.
Earlier before jumping the relay I checked the compressor and low side pressure switch for power and found none.
I can't find a a/c wiring diagram for my truck anywhere. I also can't find a detailed description of how the a/c controls work. There is plenty of information for other years and models.
Why (or how) would the compressor only engage if I jump the relay?
Is there anything else I need to jump or test?

I did also notice that when the relay was jumped the air was not cold coming from the vents.