Introduction

Allen Bradley 1606-XLE120EE troubleshooting cases are frequently misdiagnosed as internal failure. In reality, most issues are caused by load transients, wiring resistance, or improper DC grounding design.

In PLC systems, even short voltage dips can trigger CPU reset or I/O dropout.

Allen Bradley 1606-XLE120EE Fault Symptoms in Industrial Systems

Common symptoms include:

• PLC random restart or brownout
• DC OK relay flickering
• 24VDC unstable under switching load
• Output appears normal at idle but collapses under load
• Communication module resets intermittently

In one conveyor system, PLC resets occurred every 10–20 minutes despite stable idle voltage readings.

Allen Bradley 1606-XLE120EE Diagnostic Thinking Process

Field engineers typically follow a structured approach:

1. Does failure occur only during load switching?
2. Is voltage stable at PSU terminals but not at load?
3. Are multiple inductive loads switching simultaneously?
4. Is cable resistance causing DC drop?

Observation:
During solenoid activation, voltage dropped to 21.7V at PLC terminals, triggering CPU reset even though PSU output remained within spec.

Allen Bradley 1606-XLE120EE Case Study: PLC Random Restart

System Configuration:

• PLC: CompactLogix
• Loads: sensors + relays + communication module
• Power supply: 1606-XLE120EE

Symptoms:

• Random PLC restart every 15–30 minutes
• DC OK flickering during operation
• No internal PSU fault indication

Diagnostic Results:

• PSU output stable at terminals
• PLC input voltage fluctuating 22.0–24.2V
• Voltage drop increases during simultaneous relay switching

Root Cause:

• Long DC wiring run (~18 meters)
• Undersized cable cross-section
• All loads connected on a single DC branch
• High inrush from multiple relays

Correction:

• Split DC loads into two power branches
• Increased wire size
• Staggered relay activation sequence

Result:

• Voltage drop reduced to <0.3V
• PLC reset completely eliminated

Allen Bradley 1606-XLE120EE Common Fault Patterns

1. DC OK Relay Flicker

Caused by:

• transient voltage dips
• loose terminals
• high inrush switching loads

2. Output Voltage Collapse

Caused by:

• long cable resistance
• overloaded single DC rail
• simultaneous inductive switching

3. Thermal Stress Behavior

Occurs when:

• cabinet temperature >45°C
• continuous near-maximum load
• poor ventilation around DIN rail

Allen Bradley 1606-XLE120EE Repair Strategy

Step 1: Load Isolation Test

Disconnect external loads:

• Stable → external wiring/load issue
• Unstable → PSU or AC input issue

Step 2: Voltage Drop Measurement

Compare:

• PSU output terminals
• PLC input terminals
  If difference >1V → wiring problem confirmed

Step 3: Switching Behavior Test

Stagger relay/solenoid switching to reduce peak inrush current.

Engineering Insight

The 1606-XLE120EE is generally reliable in industrial environments. Most failures attributed to it are actually caused by DC system design errors rather than component degradation.

Final Conclusion

For 1606-XLE120EE-related issues, the correct troubleshooting priority is:

1. Check load distribution
2. Verify wiring resistance
3. Analyze switching transients
4. Inspect PSU only as last step

Most field cases are resolved without replacing the power supply.