Troubleshooting a Miswiring Issue on an Older Commercial System

Eric’s first step is to investigate the controls, including the board, relays, pump-down switches, and more. Old McQuay units have separate compartments for fuses as well as controls and contactors for the compressor and condenser fan. In the case of this unit, Compressor 1 has been deactivated.

After reading the schematics, Eric realizes that the safeties were tied into relay R5, and there could have been an issue with the low-pressure switch. However, that is not the case. After the system pumps down, Eric reads infinite ohms on the wires extending from the low-pressure switch. The next potential issue is that the incorrect wiring was used; the circuit should be incomplete, so Eric turns the deactivated compressor back on to see if the unit doesn’t run (as it should) or if it starts and runs. When he turns the deactivated compressor back on, the unit starts and runs.

Further wiring investigation leads Eric to an issue on wire 129; it goes to terminal 2 on the Centronic, but it should actually be going to terminal 2 on the motor protector module. Wire 129 was swapped with 120. After Eric replaces the wires, he starts up the unit after a time delay, and the compressor pumps down and shuts off as it should.

Overall, the wiring was backfeeding power to the compressor all the time. Once the wiring was reconfigured correctly, the compressor stopped running and running into a vacuum.