PLC fault repair steps - Solutions - Huaqiang Electronic Network

Probe domestic switch needle KG-300K, needle head diameter is 3.0mm, normally open switch needle

SMD aluminum electrolytic capacitor

L0504-Murata muRATA common mode inductor 90Ω 150mA

Special offer 0603 red light LED with quality assurance

1. General Troubleshooting Rules: When diagnosing a PLC system, the method includes: one touch, two look, three smells, four listens, five trace, and six replacement. First, touch the CPU to check if it's overheating—normal operating temperature should not exceed 60°C, as human touch can easily detect that. Second, visually inspect all board indicators for normal status. Third, smell for any burnt odors from components or cables. Fourth, listen for unusual sounds like loose connections or relay malfunctions. Fifth, trace the fault using schematics and process flow diagrams. Sixth, replace uncertain parts to isolate the issue. 2. Specific Steps: If the PLC software is malfunctioning, first confirm whether the CPU’s RUN status is normal. If not, reset the CPU and re-download the control program. For hardware issues, follow this sequence: - Check the power supply voltage (should be within ±5% of +24V). If the input is normal but output isn't, replace the power module. If no power is detected, use the schematic and wiring diagram to trace the issue. Check the air switch with a multimeter, and ensure the incoming and outgoing ends are powered. If external factors cause the problem, such as low voltage or overcurrent, resolve them before proceeding. If internal, replace the faulty component. - Understand the CPU’s operating mode (STOP, HOLDUP, STARTUP, RUN, etc.). If in STOP or HOLDUP, re-upload the program. If the CPU is faulty, replace it with an identical model. If a module is damaged, replace it carefully, ensuring compatibility with the original version to avoid configuration mismatches. - Check communication cables and LED indicators on each module. For digital outputs, each point acts like a switch. If online testing shows no signal, the point may be faulty. Replace the module if necessary. For digital inputs, when the coil is energized, the contact should close. If it doesn’t, check the wiring and update the software address accordingly. - Analog modules function similarly to digital ones. Each channel behaves like a closed switch. If the state is off, the channel is faulty. Replace the channel and adjust the control program address. Also, verify the range block settings and fuse status. Ensure the software is configured correctly for 1–5V or 4–20mA based on sensor type. - During normal production, avoid turning off power to output modules, as it may cause relays to malfunction. To test, coordinate with on-site staff to manually operate the equipment, then disconnect the power line for inspection. - Test various switches (relays, proximity switches, circuit breakers) by checking their open/closed states. Use a table to compare against known good devices. Replace any that show abnormal behavior. - For communication modules, use a new module to test if the current one is functioning properly. - Wire testing can be done by replacing an unknown wire with a known good one and checking the continuity. - Resistance detection is performed when the system is not powered, while voltage is checked when powered. Most faults can be resolved through these methods. Always keep safety in mind, especially when dealing with AC 220V and DC 24V systems. Follow proper electrical and gas safety procedures to ensure safe operation.