TWEAKED FRONT END

I have a 1993 GMC 2500 Truck and I have replace
new the following parts; compressor, dryer, 134a
coolant and the oil, 0 rings, condenser, but the compressor still won't engage, but the engine idle
up when you turn on the A/c switch like it would
when the compressor was to engage on normal
conditions, pressure switch was changed too !
I cannot find any fuses or relays that would operate
the system that could be bad?
When the A/C switch is on and I unplug the compressor or dryer, the engine idle does change from faster to slower? So why won't the compressor engage? It has me puzzled. I have been a shad tree mechanic on my own vehicles all my life
beside all the techinical stuff to which I don't have the equipment for, it's to expense to buy for just working on my on cars and truck only! Well what do you think?