ac problem

I bought this tahoe in the winter so the ac never got checked so now that I want it, it doesn't work. I checked all the fuses, checked the pressure of the coolant, and the only thing I think it can be is the wiring. I know the condencing unit works cause I bypassed the rest of the system and gave it 12 volts to see if it would engage and it did but only if I do that. Any help would be appreciated. Jake