Troubleshooting Swing Function Failure on the JLG 120HX Boom Lift

[MikePhua]

The JLG 120HX and Its Hydraulic Control Architecture
The JLG 120HX is a telescopic boom lift introduced in the mid-1990s, designed for high-reach maintenance, construction, and industrial applications. With a working height of 120 feet and a horizontal outreach exceeding 75 feet, it was one of the tallest self-propelled lifts of its time. JLG Industries, founded in 1969 in Pennsylvania, became a global leader in aerial work platforms, and the 120HX was a flagship model in their high-reach lineup.
The machine uses a dual-bank solenoid valve system to control hydraulic functions, including boom elevation, extension, swing, and drive. Each function is activated via electrical signals sent to solenoids, which in turn direct hydraulic flow to the appropriate actuator. The swing function relies on a bi-directional hydraulic motor and a counterbalance valve assembly to ensure smooth and safe rotation.
Terminology Annotation
- Solenoid Coil: An electromagnetic component that actuates a hydraulic valve when energized.
- Counterbalance Valve: A hydraulic valve that prevents uncontrolled movement of a load by maintaining back pressure.
- Swing Lock Pin: A mechanical pin used to secure the turret during transport, preventing rotation.
- Shuttle Valve: A valve that directs hydraulic flow based on pressure differential, often used to release brakes or control flow paths.
Symptoms of Swing Failure and Initial Diagnosis
A common issue with aging 120HX units is unidirectional swing failure—where the boom rotates clockwise but not counterclockwise. In one case, the operator confirmed that electrical signals were reaching the solenoid bank, and voltage was present at the terminals responsible for swing control. This ruled out joystick or wiring faults and pointed toward a hydraulic or solenoid malfunction.
Upon further inspection, the solenoids were tested for resistance. Three measured approximately 5 ohms, while one—associated with the failed swing direction—measured 10 ohms. This discrepancy suggested a degraded coil or internal short. Swapping the solenoid coils between boom elevation and swing confirmed the diagnosis: the problem followed the coil, not the valve or actuator.
A Story from the Yard
In Edmonton, Alberta, a radio enthusiast purchased a used JLG 120HX to maintain his antenna towers. After discovering the swing issue, he performed bench tests on the solenoids using a regulated power supply. Both coils drew 2.5 amps and actuated smoothly, but only one direction worked reliably. After swapping coils and rechecking connections, the swing function returned to normal. The lift was then tested across the yard at low elevation, and no further issues were observed.
Component Replacement and Cost Considerations
The faulty solenoid was identified as part number 7012728, with prices ranging from $328 to $680 depending on the supplier. Unfortunately, the coil is not sold separately from the valve body, making replacement more expensive. However, installation is straightforward:

• Disconnect battery and depressurize hydraulic system
  
• Label and remove electrical connectors
  
• Unscrew solenoid from valve block using appropriate wrench
  
• Install new solenoid and torque to manufacturer spec
  
• Reconnect wiring and test function
  

For remote operators or those in northern climates, sourcing parts locally can reduce downtime. Mail order is viable but may be delayed by weather or customs.
Advanced Troubleshooting and Valve Behavior
If coil replacement does not resolve the issue, further steps include:

• Swapping hydraulic hoses at the swing motor to isolate valve vs. actuator faults
  
• Inspecting the counterbalance valve for debris or stuck shuttle
  
• Verifying that the swing brake releases in both directions
  
• Checking for internal leakage or spool binding in the valve body
  

If the problem follows the hoses, the valve is likely at fault. If it stays with the motor, the actuator or brake system may require service.
Preventative Maintenance and Best Practices
To ensure reliable swing operation:

• Test boom extension lockout system before each shift
  
• Inspect solenoid coils annually for resistance and corrosion
  
• Clean valve blocks and connectors during routine service
  
• Use dielectric grease on terminals to prevent moisture intrusion
  
• Monitor hydraulic fluid condition and replace every 1,000 hours or annually
  

Operators should also cycle all functions weekly, even if the lift is not in regular use, to prevent valve sticking and fluid stagnation.
Industry Trends and Legacy Support
As of 2025, JLG continues to support legacy models like the 120HX through its online documentation portal. Wiring diagrams, hydraulic schematics, and service manuals are freely available, making diagnostics more accessible. This transparency has influenced purchasing decisions, especially among private owners and small contractors.
Meanwhile, newer boom lifts feature CAN-bus communication, integrated diagnostics, and proportional controls. While more advanced, they also introduce complexity that can hinder field repairs. The simplicity of the 120HX remains a strength for those who value hands-on troubleshooting.
Conclusion
Swing failure on the JLG 120HX is often caused by solenoid coil degradation, especially in machines over 20 years old. With basic electrical testing and component swapping, the issue can be diagnosed and resolved without extensive disassembly. Understanding the interplay between electrical signals, hydraulic valves, and actuator behavior is key to maintaining this high-reach workhorse. For owners using the lift in personal or light-duty applications, proactive maintenance and access to technical documentation ensure continued reliability and safety.