The situation
A chilled water solenoid was not running. The plant it sits on is controlled from an older switchboard with limited drawings available, which is a common position on equipment that has been in service a long time and modified along the way.
What we found
The control program was checked first using RSLogix 5000, and everything appeared to be operating correctly. The output was being commanded, so the fault was not in the control logic.
That put the problem between the controller and the field device. The Allen-Bradley PLC had been upgraded at some point in the plant's history, and the older output modules still in the rack carried individual fuses mounted behind them, out of sight and not shown on the drawings available.
Removing the module exposed the fuses, and one had blown.
What we did
Replaced the fuse and tested the circuit. The solenoid returned to normal operation.
Why it took finding
A blown fuse is a simple fault, but only once you know the fuse is there. With a control system that had been part-upgraded and drawings that no longer matched the installation, the useful work was ruling out the control side first, then working methodically down to the physical output rather than assuming the fault sat in the program.
