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Project

Onboard PLC Control System Troubleshooting for NIGATA 6L28 AHX Marine Engine

Table of Contents

Background

Industry
Marine
Solution
Niigata Engine
Troubleshooting
Services
Make & Model
Niigata
6L28 AHX
Asset
Offshore
Tugboat

Summary About The Project

The scope of work involved diagnosis of PLC start/stop logic and alarm indication issues. A structured troubleshooting approach was followed to identify power supply integrity, signal continuity, sensor health, and PLC CPU condition.

The investigation revealed internal damage to the PLC CPU power supply section and a defective MPU sensor. After sensor replacement and system trials, engine operation under local control was found satisfactory. However, due to internal PLC modification carried out later by a third party, the PLC CPU remained unreliable.

Final Outcome

  • Engine parameters and safety trips verified satisfactory during trials
  • PLC CPU identified as unreliable and replacement strongly recommended
  • Job completed up to troubleshooting and advisory level

Planning

  • Team Composition: Neptunus marine automation and engine control specialists
  • Duration: Onboard inspection, diagnostics, functional testing, and advisory
  • Work Scope:
    - PLC power supply verification
    - Sensor inspection and replacement
    - Start/stop logic testing
    - Alarm and safety trip validation

Procedure Carried Out

1. Initial Assessment & Toolbox Talk:
A detailed toolbox discussion was conducted with the Chief Engineer to understand the reported issues related to engine start/stop logic and alarm indication behavior.
2. PLC Power Supply & Wiring Inspection:
  • Power supply to the PLC CPU was found unavailable initially
  • All ECR panel wiring connections were inspected and confirmed satisfactory
  • 24 V DC input supply to the PLC CPU was measured and found within normal limits
  • Associated fuses and converters were checked and found operational
3. Engine Functional Testing:
  • Engine was started locally using the manual starter
  • LO safety checks were performed
  • During testing, the engine tripped without any alarm or indication appearing in the ECR
4. PLC CPU Inspection:
With customer approval:
  • PLC CPU was removed from its base
  • Burn marks were observed inside the CPU power supply section, indicating internal damage
5. Sensor Replacement & Trials:
  • Defective MPU sensor was replaced
  • Trials were conducted
  • All safety trips and alarms were checked and found satisfactory during operation
6. Parallel Third-Party Intervention:
  • Remote PLC repair assistance and a local vendor were engaged by the customer
  • The local vendor desoldered damaged components and applied a jumper inside the PLC CPU
  • Post-modification, the PLC CPU displayed continuous alarms and a red error indication
  • Alarms could not be reset, confirming unreliable PLC condition
7. Final Checks & Advisory:
  • Engine was operated locally as instructed by the customer
  • Start logic, engine parameters, LO safety trips, and alarms were verified satisfactory
  • Customer was strongly advised to replace the PLC CPU and reload the correct program

Remarks & Recommendations

  • The PLC CPU is not reliable due to internal damage and unauthorized modification
  • Continued operation poses a risk to engine safety and alarm integrity
  • Immediate replacement of ABB PLC CPU PM 573 with proper program loading is strongly recommended
This assignment reinforces Neptunus’ capability to deliver reliable Marine Engine Repair solutions, including control system diagnostics, safety logic verification, and technical advisory for critical onboard systems.

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