i have seen pump down systems that utilize a relay that is energized by out side of compressor contactor , this is done to prevent heaters from coming on during defrost , which creates high current draw and can cause breaker to trip on single units . as far as pump down goes . low psi control should be set at approx 5psi . As
others have mentioned use gauges to verify . the schematic supplied should help out ... if none available trace wires these systems are usually very simple .