Does anyone know what psi (a/c refrigerant pressure sensor) and/or temp (A/C evaporator temperature sensor) readings are needed to engage the ac compressor clutch?

 

These are not actual specifications.. rules of thumb, not from chevy books. The evap temp sensor should cut off the a/c at about 34* F and turn it back on at 36* to prevent freezing condensation on the coils. The low pressure cutoff is usually around 25-40 PSI (a fully charged system is 100+ (hi and low) with a/c off and 40-50 low / 200+ hi side with a/c on. Usually adding 1 can of 134 to an empty system will be enough to initially kick the compressor on then it may cut off as the low side pressure drops after running. Hope this helps..

 

Thank You