A carton diverter fails to actuate. Online logic shows the PLC output energized, and the output module LED is lit. The immediate temptation is to suspect a stuck pneumatic valve. Instead, the technician follows the chain. Voltage at the output terminal is correct with no load connected. With the valve coil connected and commanded, voltage collapses. The common return terminal for that group is discolored and measures a significant voltage drop under load.
The terminal is reworked using the approved repair method, the conductor and ferrule are inspected, and the group current is checked. The valve then operates normally. The important clue was that software state and module LED were only two links in the command path. Neither proved that the load received usable energy. A loaded measurement exposed the high-resistance connection immediately.
What Mattered
- An output LED does not prove voltage at the load.
- High-resistance faults often appear only under current.
- Measure across suspected connections as well as to common.
- Verify the whole output path from module to actuator.
Questions to Ask Yourself
- Which early clue had the highest information value?
- What tempting but unsupported action could have wasted time?
- Which measurement or observation moved the diagnosis across a system boundary?
- What should be changed so the same failure is detected earlier next time?