Diagnosing Hard Steering on the JLG 644E-42 Telehandler

[MikePhua]

The JLG 644E-42 and Its Hydraulic Steering System
The JLG 644E-42 telehandler was introduced in the early 2000s under the Lull brand, which JLG acquired to expand its reach in the material handling sector. With a rated lift capacity of 6,000 lbs and a maximum reach of 42 feet, the 644E-42 became a staple on framing and masonry sites across North America. Its standout feature was the horizontal boom shift system, allowing precise load placement without repositioning the machine.
Steering on the 644E-42 is hydraulically actuated, relying on a dedicated steering pump, orbital valve, and dual steering cylinders mounted on the axles. The system is designed for low-effort control even under full load, but over time, components can degrade, leading to stiff or unresponsive steering.
Symptoms of Steering Resistance
Operators experiencing hard steering on the 644E-42 often report:

• Excessive force required to turn the steering wheel
  
• Delayed response or jerky movement
  
• Steering improves slightly at high RPM but remains stiff
  
• No visible leaks or warning lights
  
• Tires and axles appear mechanically sound
  

These symptoms suggest hydraulic flow restriction, pump inefficiency, or valve malfunction. In one case from Minnesota, a framing crew found that steering required two hands and full body weight, even on level ground. The issue persisted despite fluid top-offs.
Hydraulic Fluid and Filter Condition
The first diagnostic step is checking hydraulic fluid level and condition. The 644E-42 uses a shared reservoir for boom, steering, and auxiliary functions. Contaminated or low-viscosity fluid can reduce steering performance.
Recommended checks:

• Inspect fluid level with boom lowered and engine off
  
• Examine fluid color and clarity—milky or dark fluid indicates contamination
  
• Replace hydraulic filters every 500 hours or annually
  
• Use ISO 46 hydraulic oil in moderate climates, ISO 68 in hot regions
  

In one case from Arizona, a contractor discovered that his fluid had thickened due to cold overnight temperatures. After switching to synthetic ISO 46 and warming the machine before use, steering returned to normal.
Steering Pump and Flow Output
The steering pump is gear-driven and mounted near the engine. It supplies pressurized fluid to the orbital valve, which then directs flow to the steering cylinders. A weak or worn pump may fail to deliver adequate pressure, especially at low RPM.
Diagnostic steps:

• Install a pressure gauge at the steering test port (typically 2,000–2,500 psi under load)
  
• Compare readings at idle and full throttle
  
• Listen for pump whine or cavitation
  
• Inspect drive belt and pulley for slippage
  

A technician in Ontario found that the pump shaft seal had failed, allowing internal leakage. Replacing the pump restored full steering function.
Orbital Valve and Internal Leakage
The orbital valve translates steering wheel input into hydraulic flow. If internal seals degrade or the valve body wears, flow may bypass or resist movement.
Symptoms of orbital valve issues:

• Steering wheel feels stiff or “dead”
  
• No change in response despite RPM increase
  
• Fluid bypass noise near the valve housing
  

Solutions:

• Remove and inspect orbital valve for scoring or seal damage
  
• Replace valve if internal leakage is confirmed
  
• Use OEM or high-quality aftermarket units rated for telehandler use
  

In a fleet in British Columbia, orbital valve replacement reduced steering effort by 60%, improving operator comfort and reducing fatigue.
Steering Cylinders and Mechanical Resistance
The 644E-42 uses dual hydraulic cylinders—one on each axle—to actuate steering. If seals fail or rods bind, movement becomes restricted.
Inspection tips:

• Check for external leaks at rod seals
  
• Extend and retract cylinders manually to test smoothness
  
• Inspect pins and bushings for wear or corrosion
  
• Grease pivot points and replace worn hardware
  

A framing crew in Texas discovered that one cylinder had seized due to internal rust. After replacement and system flush, steering returned to factory spec.
Cold Weather and Viscosity Effects
In temperatures below 20°F, hydraulic fluid thickens, reducing flow and increasing steering resistance. Preventive measures include:

• Using synthetic hydraulic oil rated for low temperatures
  
• Installing block heaters or hydraulic warmers
  
• Allowing machine to idle for 5–10 minutes before operation
  

In Alaska, a contractor added a 500-watt hydraulic tank heater to his 644E-42 and reported improved steering response during winter framing.
Field Modifications and Operator Ingenuity
In Pennsylvania, a builder added a pressure gauge panel near the operator seat to monitor steering pressure in real time. This helped detect early signs of pump wear and allowed proactive maintenance.
In another case from Georgia, a technician installed a flow restrictor on the boom circuit to prioritize steering during simultaneous operation. This reduced steering lag when lifting heavy loads.
Conclusion
Hard steering on the JLG 644E-42 telehandler is often a symptom of hydraulic inefficiency, component wear, or cold-weather viscosity issues. With methodical diagnostics—starting from fluid condition to pump output and valve integrity—operators can restore smooth control and extend machine life. Whether placing trusses or maneuvering in tight lots, responsive steering is essential to safe and productive operation.