A couple of tests you can try:
- Use a multimeter to test the Relay signal/ground on the Sidecar. It'll be a steady 4.6v on one of the two signal pins matching position with the green LED. The other pin will read 0v.
- Use a multimeter on the far end of the PWM cable to see if the voltage is getting there.
P.S.
If you are getting a green LED lit for the Relay, then the code and pneumatic setup has ordered the compressor to turn on.
Don't bother looking at the pressure switch or code at this point.
The next stop in the command stream is that the Spike LED color should be matching the green Relay LED color.
If the Spike LED doesn't match the Relay LED, then there is the failure is between the Relay output and the Spike - PWM cable, reversed plug, plug not making contact, Relay output under powered, Spike failure.