[FrankJ]

Is your pressure switch wired to a digital input on the side car (correct) or in series with the command on the spike (wrong).

The + 12 should go through the spike to the compressor. The negative from the compressor should go directly to the PD board or to the spike. Since the spike connects both outputs to 0V (negative), it is possible to use it as a normally closed relay if it is not wired according to the rules.