Hydraulic Troubleshooting on the 1999 Lull Telehandler

[MikePhua]

The Lull Legacy and the 644 Series
Lull telehandlers, originally developed by Lull Industries and later acquired by JLG, earned their reputation for precision load placement and rugged construction. The 1999 Lull 644 series was part of a generation that emphasized reach, stability, and operator control. With a rated lift capacity of 6,000 lbs and a maximum reach of 42 feet, the 644 was widely used in framing, masonry, and industrial maintenance. Its hallmark feature was the horizontal boom shift system, allowing operators to place loads with millimeter-level accuracy—especially useful for trusses and wall panels.
The hydraulic system powering this precision was built around a gear-driven pump, multiple control valves, and a series of pilot-operated solenoids. While robust, the system could develop faults over time, particularly in machines exceeding 5,000 hours or exposed to harsh environments.
Symptoms of Hydraulic Failure
Operators encountering hydraulic issues on the 1999 Lull often report:

• Boom functions slow or unresponsive
  
• Hydraulic whine or cavitation noise
  
• Jerky movement during lift or tilt
  
• Inability to build pressure at idle
  
• Functions improve slightly at high RPM but remain weak
  

These symptoms suggest either a loss of hydraulic pressure, internal leakage, or pump inefficiency. In one case from Arizona, a framing crew noticed that their Lull could barely lift a pallet of sheathing unless the engine was revved to maximum. The issue persisted even after replacing filters, pointing to deeper system degradation.
Pump Behavior and Pressure Loss
The main hydraulic pump on the 1999 Lull is gear-driven and mounted directly to the engine. It supplies fluid to the control valve bank, which then distributes flow to the boom, tilt, and auxiliary circuits. Over time, pump wear can reduce volumetric efficiency, especially if the oil is contaminated or the suction line is compromised.
Key failure points include:

• Worn pump gears or housing
  
• Cracked suction hose allowing air ingress
  
• Clogged inlet screen or filter
  
• Weak relief valve spring causing premature bypass
  

To diagnose:

• Install a pressure gauge at the test port near the valve bank
  
• Compare readings at idle and full throttle (should exceed 2,500 psi under load)
  
• Inspect suction hose for soft spots or collapse
  
• Check for foamy oil in the reservoir (a sign of air entry)
  

A technician in Wisconsin discovered that a collapsed suction hose was starving the pump. Replacing it restored full boom speed and eliminated the whining noise.
Control Valve and Solenoid Issues
The control valve assembly includes multiple solenoids that direct flow to specific functions. If a solenoid fails or receives weak voltage, the corresponding function may lag or fail entirely.
Common faults:

• Corroded solenoid connectors
  
• Weak coil resistance
  
• Stuck spool valve due to contamination
  
• Internal leakage bypassing flow
  

Solutions:

• Test solenoid voltage with key on (should read 12V)
  
• Measure coil resistance and compare to spec (typically 8–12 ohms)
  
• Remove and clean spool valves with solvent
  
• Replace damaged O-rings and seals
  

In one case from Georgia, a contractor traced intermittent boom movement to a solenoid with a cracked wire sheath. Moisture had corroded the terminal, causing erratic voltage. After replacing the connector and sealing it with dielectric grease, the issue was resolved.
Hydraulic Fluid and Filter Maintenance
Hydraulic fluid condition plays a central role in system performance. The 1999 Lull uses a reservoir with a spin-on filter and a suction screen. Neglecting fluid changes can lead to varnish buildup, seal degradation, and pump wear.
Best practices:

• Replace hydraulic fluid every 1,000 hours or annually
  
• Use ISO 46 or ISO 68 hydraulic oil depending on climate
  
• Replace filters every 250–500 hours
  
• Inspect fluid for discoloration or metallic particles
  
• Flush system after major component replacement
  

A framing crew in Oregon extended the life of their Lull’s hydraulic system by switching to synthetic hydraulic oil and installing a magnetic filter collar to trap fine particles.
Boom Function Prioritization and Load Sensing
The Lull’s hydraulic system includes load-sensing logic that prioritizes boom lift and tilt under certain conditions. If the system detects low pressure or high demand, it may divert flow away from secondary functions.
To optimize performance:

• Avoid simultaneous multi-function use under heavy load
  
• Operate at mid to high throttle during lift operations
  
• Inspect load-sensing lines and pilot valves for blockage
  
• Calibrate relief valves to factory spec
  

In one case from New Mexico, a telehandler failed to tilt under load. The technician discovered that the boom lift relief valve was set too low, causing premature bypass. After adjustment, both lift and tilt functions improved.
Field Anecdotes and Operator Ingenuity
In rural Pennsylvania, a builder retrofitted his 1999 Lull with a pressure gauge panel mounted near the operator seat. This allowed real-time monitoring of hydraulic pressure during lifts, helping him detect early signs of pump wear.
In another case from British Columbia, a contractor added a transparent return line to monitor fluid clarity. When the line turned dark, he scheduled a fluid change—preventing a costly valve rebuild.
Conclusion
Hydraulic problems on the 1999 Lull telehandler often stem from pump wear, suction issues, solenoid faults, or fluid degradation. With methodical diagnostics, proper maintenance, and attention to system behavior, operators can restore full function and extend machine life. Whether placing trusses on a windy ridge or lifting pallets in tight urban lots, the Lull’s hydraulic system remains its lifeblood—and deserves careful stewardship.