Lull Highlander Telehandler Drive Failure and Troubleshooting

[MikePhua]

The Highlander’s Role in Material Handling
The Lull Highlander series—644, 844, and 1044—was developed to meet the growing demand for high-reach telehandlers in construction, agriculture, and industrial logistics. Manufactured by Lull before its acquisition by JLG Industries, these machines were known for their unique horizontal boom pivot system, which allowed precise load placement without repositioning the chassis. With lift capacities ranging from 6,000 to 10,000 pounds and reach heights exceeding 40 feet, the Highlander series became a staple on job sites across North America.
JLG, founded in 1969, absorbed Lull’s product line in the early 2000s, continuing support for Highlander models while integrating their features into newer telehandlers. Despite their age, many Highlanders remain in service due to their robust frames, intuitive controls, and straightforward mechanical systems.
Understanding the Drive System Architecture
The Highlander’s drive system is hydrostatic, meaning it uses hydraulic pressure to power wheel motors. The engine drives a hydraulic pump, which sends fluid through a directional control valve to the drive motors mounted on each axle. The system includes safety interlocks, solenoids, and sensors that monitor operator input and machine status.
Terminology note:

• Hydrostatic Drive: A propulsion system using hydraulic fluid to transmit power from the engine to the wheels.
  
• Travel Interlock: A safety feature that prevents movement unless specific conditions are met, such as seat occupancy or parking brake release.
  
• Directional Control Valve: A hydraulic valve that determines fluid flow direction, enabling forward or reverse travel.
  

The system is designed to be responsive and safe, but any failure in the electrical or hydraulic components can result in a complete loss of movement.
Symptoms of Drive Failure
Operators may encounter several signs when a Highlander telehandler refuses to move:

• Engine runs normally but wheels do not respond to directional input
  
• No hydraulic noise or pressure change when engaging travel
  
• Parking brake remains engaged despite switch activation
  
• Warning lights or fault codes appear on the dash
  
• Movement resumes briefly after restart but fails again under load
  

In one case, a contractor in Ohio reported that his Lull 844 Highlander would not move despite all controls appearing functional. After a detailed inspection, the issue was traced to a failed travel solenoid that was not energizing, preventing hydraulic flow to the drive motors.
Diagnostic Steps and Inspection Tips
To troubleshoot a non-moving Highlander:

• Verify hydraulic fluid level and condition. Low or contaminated fluid can disable the system.
  
• Check the parking brake switch and solenoid for continuity and voltage.
  
• Inspect the travel solenoid and directional valve for signal and actuation.
  
• Test seat switch and other interlock sensors for proper function.
  
• Use a multimeter to check fuse panel and relays related to drive control.
  
• Manually override the travel solenoid to confirm motor response.
  

If the machine moves after bypassing the solenoid, the fault is likely electrical. If no movement occurs, the issue may be hydraulic—such as a failed pump or blocked valve.
Repair Procedures and Preventive Measures
Once the fault is identified:

• Replace faulty solenoids and connectors with OEM-rated components
  
• Clean directional valve spools and test for smooth movement
  
• Flush hydraulic lines and replace filters
  
• Inspect wiring harness for abrasion or corrosion
  
• Recalibrate interlock sensors if adjustable
  

Preventive tips:

• Inspect electrical connectors monthly, especially near hydraulic components
  
• Change hydraulic fluid every 1,000 hours or annually
  
• Test travel function under load during routine maintenance
  
• Keep the fuse panel dry and sealed from dust
  
• Label wires during repairs to avoid misrouting
  

One operator in Alberta added a diagnostic LED to his travel solenoid circuit, allowing quick visual confirmation of signal presence. This helped him catch a failing relay before it caused a full shutdown during a lift operation.
Design Evolution and Manufacturer Insights
JLG’s later telehandlers incorporated CAN-based communication and improved diagnostics, reducing the frequency of drive failures and simplifying troubleshooting. However, the Highlander’s mechanical layout remains popular among technicians who value accessibility and field-repairability.
Sales data from 2024 indicates that over 12,000 Highlander units are still operational globally, especially in rental fleets and rural construction markets. JLG continues to support the series with parts catalogs and retrofit kits.
Conclusion
Drive failure in the Lull Highlander telehandler is often rooted in electrical or hydraulic control faults. With methodical diagnostics and attention to interlock logic, most issues can be resolved without major component replacement. The Highlander’s enduring popularity is a testament to its design—and with proper care, its wheels will keep turning on job sites for years to come.