[Kevin Sevcik]

Since we're 8 posts in and everyone has missed this obvious bit:

Have you tested your pressure switch and the cable coming from your pressure switch? The compressor won't run without a closed contact on the digital input for the pressure switch. So do this:
1. Make certain the cable for the pressure switch is plugged into the correct digital IO AND the correct way.
2. Short out the terminals on your pressure switch with a screwdriver or something. If the compressor runs, your switch is bad.
3. If 2 doesn't work, find a known good PWM cable, plug it into the pressure switch digital IO, and short the white and black pins together. If the compressor runs, either your pressure switch cable is bad, or the connection from the cable to the pressure switch terminals.