Troubleshooting Fork Curl Control on the SkyTrak 6000M Telehandler

[MikePhua]

The SkyTrak 6000M and Its Military-Grade Utility
The SkyTrak 6000M is a rugged telehandler originally designed for military and heavy-duty industrial use. Built by JLG under military specifications, the 6000M features a 6,000-pound lift capacity, four-wheel drive, and a robust boom system capable of reaching heights over 36 feet. Its design prioritizes durability, simplicity, and field serviceability, making it ideal for logistics, base operations, and remote construction.
Unlike commercial telehandlers, the 6000M often lacks detailed labeling on its electrical and hydraulic systems, especially in surplus or decommissioned units. This can complicate diagnostics when components like fork curl controls malfunction.
Terminology Annotation

• Fork Curl Function: The hydraulic action that tilts the forks upward or downward, allowing secure lifting and dumping of loads.
  
• Solenoid Valve: An electrically actuated valve that directs hydraulic flow to specific cylinders.
  
• Control Harness: A bundle of wires connecting switches, solenoids, and relays to manage hydraulic functions.
  
• Schematic Diagram: A technical drawing showing electrical or hydraulic connections and component relationships.
  

Symptoms of Fork Curl Malfunction
A common issue reported with the 6000M is that the forks will curl upward but fail to return to the neutral or downward position. This asymmetrical behavior suggests a fault in the control circuit, solenoid valve, or hydraulic flow path.
Typical symptoms include:

• Forks tilt up normally but do not respond to downward input
  
• Audible solenoid click only in one direction
  
• No hydraulic movement despite joystick activation
  
• All solenoids unmarked and wires predominantly white, complicating tracing
  

In one field case, a technician attempting to restore a surplus 6000M found that the fork curl solenoids were wired with unmarked white conductors, making it difficult to identify the faulty circuit without a schematic.
Electrical Diagnosis and Solenoid Mapping
To isolate the issue:

• Begin by identifying the solenoid bank responsible for fork functions, usually located near the hydraulic manifold.
  
• Use a multimeter to test voltage at each solenoid terminal during joystick activation.
  
• Listen for solenoid engagement clicks—absence may indicate coil failure or lack of signal.
  
• Trace wires from the joystick or switch panel to the solenoids, labeling each as you go.
  
• If all wires are white, use continuity testing to map each wire’s origin and destination.
  

In some units, black wires are reserved for ground, while white wires carry signal. However, this is not guaranteed and must be verified manually.
Hydraulic Flow and Valve Behavior
If the solenoids are functioning electrically, the issue may lie in the hydraulic circuit:

• Check for debris or contamination in the valve spool controlling fork curl
  
• Inspect hydraulic fluid level and condition
  
• Test pressure at the fork curl cylinder using a gauge
  
• Manually override the valve if possible to confirm mechanical movement
  

In one repair, a clogged orifice in the valve body prevented fluid from returning during downward curl, effectively locking the forks in the raised position.
Schematic Acquisition and Field Workarounds
Due to the military origin of the 6000M, repair manuals and schematics may be difficult to obtain. Technicians often rely on:

• Reverse-engineering the wiring harness
  
• Comparing with similar JLG commercial models
  
• Requesting documentation from surplus equipment dealers or military maintenance archives
  

As a workaround, some operators install toggle switches with direct wiring to solenoids, bypassing the original control logic. While functional, this approach requires careful attention to polarity and fuse protection.
Preventative Measures and Upgrade Suggestions
To improve reliability and ease of service:

• Label all solenoid wires during initial inspection
  
• Replace unmarked wires with color-coded marine-grade conductors
  
• Install inline fuses for each solenoid circuit
  
• Add a manual override valve for emergency fork positioning
  
• Use weatherproof connectors to prevent corrosion in outdoor environments
  

In one logistics yard, retrofitting the 6000M with a simplified control panel and labeled wiring reduced downtime by 60% over six months.
Conclusion
Fork curl control issues on the SkyTrak 6000M telehandler are often rooted in unlabeled wiring, solenoid faults, or hydraulic blockages. With methodical tracing, voltage testing, and valve inspection, technicians can restore full functionality even without factory schematics. The 6000M’s military-grade build offers durability, but its serviceability depends on clear documentation and thoughtful retrofits. For operators in remote or demanding environments, mastering these systems ensures that the machine remains a reliable asset rather than a mystery.