Diagnosing LMI System Faults on the National Crane 800D

[MikePhua]

The 800D and Its Load Monitoring Architecture
The National Crane 800D is a hydraulic boom truck crane designed for lifting operations in utility, construction, and industrial sectors. With a maximum capacity of 23 tons and boom lengths reaching up to 100 feet with extensions, the 800D blends mobility with lifting power. Manufactured by National Crane, a division of Manitowoc, the 800D incorporates an LMI (Load Moment Indicator) system to ensure safe operation within rated load limits.
The LMI system monitors boom angle, extension, load weight, and outrigger status. It calculates the moment—the product of load and distance—and compares it to the crane’s rated capacity. If the moment exceeds safe thresholds, the system triggers warnings or disables further movement. This safeguard prevents tip-over, structural overload, and boom failure.
Common Symptoms of LMI Malfunction
Operators may encounter:

• Blank or frozen LMI display
  
• Incorrect boom angle or length readings
  
• Load weight not registering or fluctuating erratically
  
• Audible alarms without visible fault codes
  
• Crane functions locked out despite proper setup
  
• Error messages referencing angle sensor, CAN bus, or load pin
  

These symptoms often point to sensor failure, wiring issues, or software glitches. In some cases, environmental factors like moisture, vibration, or electromagnetic interference can disrupt signal integrity.
Key Components in the LMI System
The 800D’s LMI system includes:

• Angle sensor mounted on the boom base
  
• Length sensor integrated into the telescoping section
  
• Load pin or pressure transducer on the hoist line
  
• Display unit in the cab with override and setup functions
  
• CAN bus wiring harness connecting all modules
  
• Outrigger sensors for stability verification
  

Each component must communicate accurately for the system to function. A fault in any link can trigger a global error or disable lifting functions.
Diagnostic Strategy and Field Solutions
To isolate and resolve LMI faults:

• Cycle power to the crane and LMI system. Some faults clear after reboot.
  
• Inspect connectors for corrosion, loose pins, or water intrusion. Use dielectric grease to protect terminals.
  
• Check sensor voltages using a multimeter. Angle sensors typically output 0.5–4.5V depending on position.
  
• Verify CAN bus continuity and resistance. A short or open circuit can halt communication.
  
• Compare boom angle readings to physical measurements using an inclinometer.
  
• Test load pin calibration by lifting known weights and observing display response.
  
• Review setup parameters in the LMI menu. Incorrect configuration can cause false errors.
  

If the angle sensor is suspected, replacing it with a known-good unit and recalibrating may restore function. Some technicians carry spare sensors and bypass harnesses for field repairs.
Preventive Measures and Long-Term Reliability
To reduce LMI failures:

• Seal all connectors with waterproof boots and inspect quarterly
  
• Mount sensors with vibration-dampening pads
  
• Avoid routing CAN bus wires near high-voltage lines or hydraulic solenoids
  
• Update firmware annually if supported by manufacturer
  
• Train operators to recognize early signs of sensor drift or display lag
  
• Log fault codes and conditions for pattern analysis
  

Adding a diagnostic port or external CAN reader can simplify troubleshooting and reduce downtime.
A Story from the Yard
In 2022, a utility crew in Montana experienced repeated LMI lockouts on their 800D during pole setting. The display showed erratic boom angles and refused to lift despite proper setup. After inspecting the angle sensor, they found a cracked housing and moisture inside. Replacing the sensor and sealing the mount restored full function. The crew later added a rubber shroud and began logging sensor readings weekly.
Conclusion
The LMI system on the National Crane 800D is a vital safety feature, but its reliability depends on clean signals, stable sensors, and proper configuration. When faults arise, methodical diagnostics and environmental awareness can restore function and prevent costly delays. In lifting operations, precision is protection—and when the LMI reads true, the crane moves with confidence and control.