Does the clutch on the compressor turn on?
Turn the knob to defrost, the fan to #4, and the AC button on
The low pressure switch only needs 23 psi to remain closed
Pins 2 and 4 on the switch should have <2 ohms
Pin 2 on the harness is +10v
pin 4 is the return back to the CPU at pin #41 (grn/yell wire)