Diagnosing Sudden Braking in the JLG 600AJ: Hydraulic, Electrical, and Control System Insights

[MikePhua]

Overview of the Issue
The JLG 600AJ articulating boom lift is widely used in construction and maintenance for its reach and maneuverability. However, one recurring issue reported by operators involves sudden, harsh braking during forward or reverse travel. The machine may grind to a halt, accompanied by high-pressure noise from the hydraulic pump. Interestingly, the fault disappears at creep speed but reappears when the travel speed is slightly increased using the yellow speed control knob.
This intermittent behavior suggests a complex interaction between hydraulic pressure, electronic control systems, and mechanical components.
Terminology Explained
- Creep Speed: The lowest travel speed setting, used for precise maneuvering
- Footswitch: A safety interlock that must be engaged for drive and lift functions to operate
- Hydraulic Bypass: A condition where fluid flows around a motor or valve instead of through it, reducing performance
- JLG Analyzer: A diagnostic tool used to interface with the machine’s control system and retrieve fault codes
Possible Causes and Diagnostic Pathways
Several hypotheses were proposed by experienced technicians:

1. Footswitch Contact Loss
   The footswitch is a critical safety component. If its electrical contact is intermittent—especially under vibration—it can cause the machine to halt abruptly. Since drive functions cause platform bounce, this could momentarily disengage the switch.
   
   1. Recommendation: Inspect and test the footswitch for continuity under vibration. Replace if worn or corroded.
      
   2. Hydraulic Oil Condition and Temperature
      Old or overheated hydraulic oil can lose viscosity, leading to pressure drops. If the brakes rely on hydraulic pressure to remain disengaged, a drop could cause them to engage unexpectedly.
      
   3. Recommendation:
      • Replace hydraulic oil with manufacturer-recommended fluid
        
      • Inspect for contamination or water ingress
        
      • Check operating temperature under load
        
   4. Drive Motor Bypass or Internal Leakage
      Wheel motors may develop internal bypass due to worn seals or damaged components. This can cause uneven pressure distribution and erratic braking behavior.
      
   5. Recommendation:
      • Perform pressure tests on each wheel motor
        
      • Inspect for external leaks or heat buildup
        
      • Replace or rebuild faulty motors
        
   6. Control System Deceleration Settings
      The JLG Analyzer allows technicians to adjust drive parameters, including deceleration rates. If set too aggressively, the machine may interpret minor speed increases as a need to brake sharply.
      
2. Recommendation:
   

• Connect JLG Analyzer and review fault codes
  
• Adjust deceleration curve to reduce abrupt braking
  
• Monitor system response during test drives
  

Case Study: Platform Bounce and Footswitch Failure
In one documented case, a technician traced the issue to a worn footswitch that lost contact during platform bounce. The operator reported that releasing and reapplying the footswitch temporarily resolved the issue. After replacing the switch and securing its mounting, the problem disappeared entirely.
Additional Parameters and Suggestions

• Hydraulic system pressure: ~3,000 psi
  
• Brake release pressure: ~1,500 psi
  
• Recommended hydraulic fluid: ISO 46 or ISO 68 (depending on climate)
  
• Drive motor type: Orbital hydraulic motors with integrated brakes
  
• Control system: CAN bus-based with fault logging capability
  

Preventive Maintenance Tips

• Replace hydraulic fluid every 1,000 hours or annually
  
• Inspect footswitch and control wiring quarterly
  
• Use JLG Analyzer to log and clear faults regularly
  
• Avoid prolonged operation at high ambient temperatures without cooling checks
  
• Train operators to recognize early signs of hydraulic or control issues
  

Conclusion: A Multi-System Challenge
The JLG 600AJ’s sudden braking issue is a textbook example of how hydraulic, electrical, and control systems interact in modern aerial work platforms. Solving it requires not just mechanical skill but a systems-thinking approach. Whether it’s a faulty footswitch, degraded oil, or aggressive deceleration settings, each component plays a role in the machine’s behavior. With careful diagnostics and preventive care, these machines can continue to operate safely and reliably in demanding environments.