Diagnosing Milky Transmission Fluid in the Lull 644 Telehandler

[MikePhua]

The Lull 644 and Its Hydraulic Transmission System
The Lull 644 is a rough terrain telehandler originally manufactured by Lull Corporation, later acquired by JLG Industries. Known for its horizontal boom travel and precise load placement, the 644 model was widely used in construction and framing operations throughout the 1990s and early 2000s. With a lift capacity of 6,000 pounds and a reach of over 40 feet, the machine was designed for stability and maneuverability on uneven ground.
The transmission system in the Lull 644 is hydraulic, relying on fluid pressure to drive the wheels and operate auxiliary functions. Maintaining clean, properly rated hydraulic fluid is essential for performance and longevity. When the transmission fluid appears milky, it signals contamination—most commonly from water ingress.
Understanding Milky Fluid and Its Implications
Milky transmission fluid typically indicates the presence of water emulsified within the oil. This can occur due to:

• Condensation inside the reservoir, especially in humid climates or machines stored outdoors
  
• Seal failure allowing rainwater or wash water to enter the system
  
• Improper fluid type or mixing incompatible fluids
  
• Cooler leaks where transmission fluid and coolant cross paths
  

In the case of the Lull 644, the transmission is not directly connected to a radiator, ruling out coolant contamination. This narrows the likely cause to environmental moisture or seal failure.
Field Diagnosis and Fluid Separation Test
To confirm water contamination, technicians often perform a simple jar test:

• Drain a small sample of transmission fluid into a clear glass jar
  
• Let it sit undisturbed for several hours
  
• Observe separation: oil will float to the top, water settles at the bottom
  

This test provides visual confirmation and helps estimate the severity of contamination. If water content exceeds 5%, the fluid’s lubricating properties are compromised, increasing wear on pumps, valves, and bearings.
Recommended Remediation Steps
If milky fluid is detected:

• Drain the entire transmission system, including reservoirs and lines
  
• Flush with compatible hydraulic fluid to remove residual moisture
  
• Replace filters to prevent recontamination
  
• Inspect seals and breathers for cracks or improper sealing
  
• Store the machine under cover or use desiccant breathers to reduce humidity ingress
  

In one case, a contractor in New Jersey discovered milky fluid in his Lull 644 after a week of heavy rain. The machine had been parked without a tarp, and water entered through a cracked breather cap. After draining and flushing the system, he installed a new breather and began covering the machine during downtime—preventing future issues.
Preventive Maintenance Tips
To avoid transmission fluid contamination:

• Check fluid condition monthly, especially in wet seasons
  
• Use only manufacturer-recommended hydraulic fluid
  
• Replace breather caps annually
  
• Inspect for leaks around dipsticks, fill ports, and seals
  
• Avoid pressure washing near hydraulic components
  

Conclusion
Milky transmission fluid in the Lull 644 telehandler is a clear warning sign of water contamination. While the machine’s design isolates the transmission from coolant systems, environmental exposure remains a significant risk. Through careful diagnosis, fluid testing, and proactive maintenance, operators can restore system integrity and extend the life of their equipment. In rough terrain lifting, clean hydraulics are not just a preference—they’re a necessity.