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The Lull 644B and Its Mechanical Legacy
The Lull 644B telehandler, manufactured in the mid-1990s by Lull Corporation before its acquisition by JLG Industries, was designed for rugged lifting tasks in construction and industrial settings. With a rated lift capacity of 6,000 pounds and a reach of up to 40 feet, the 644B became a staple on job sites across North America. Its Highlander variant, introduced around 1995, featured a robust drivetrain and a multi-speed transmission system that allowed operators to shift between low, medium, and high gears depending on terrain and load. By the early 2000s, Lull had sold thousands of these units, with many still in service today due to their mechanical simplicity and durable frame.
Symptoms of Gear Lock and Initial Observations
Operators encountering gear lock typically report the following:
Key Components and Terminology
In Florida, a technician working on a 1995 Highlander variant discovered that the gear selector worked intermittently after motor mount replacement. After tracing the wiring harness and testing the TCM, he confirmed that the module was failing to hold gear commands. Replacing the TCM resolved the issue immediately.
In contrast, a contractor in Colorado found that his gear lock problem stemmed from a loose ground wire near the firewall. Once tightened, the transmission held gear selection without issue—highlighting the importance of electrical integrity in older machines.
Preventive Measures and Solutions
Gear lock in the Lull 644B telehandler is often traced to electronic control issues, particularly the transmission control module. While mechanical repairs like motor mount replacement may trigger symptoms, the underlying fault typically lies in electrical components or wiring integrity. With careful inspection and targeted replacement, operators can restore full gear functionality and extend the service life of this dependable lifting machine. The 644B remains a testament to 1990s engineering—simple, strong, and still working hard decades later.
The Lull 644B telehandler, manufactured in the mid-1990s by Lull Corporation before its acquisition by JLG Industries, was designed for rugged lifting tasks in construction and industrial settings. With a rated lift capacity of 6,000 pounds and a reach of up to 40 feet, the 644B became a staple on job sites across North America. Its Highlander variant, introduced around 1995, featured a robust drivetrain and a multi-speed transmission system that allowed operators to shift between low, medium, and high gears depending on terrain and load. By the early 2000s, Lull had sold thousands of these units, with many still in service today due to their mechanical simplicity and durable frame.
Symptoms of Gear Lock and Initial Observations
Operators encountering gear lock typically report the following:
- The machine starts in second or third gear but quickly reverts to low gear.
- Gear selection appears functional in neutral but fails to hold once forward motion begins.
- The issue arises after drivetrain or engine mount repairs, suggesting a mechanical or electrical disturbance.
Key Components and Terminology
- Transmission Control Module (TCM): An electronic unit that manages gear selection and shift timing based on input from sensors and operator controls.
- Motor Mounts: Structural supports that secure the engine and transmission to the frame. Misalignment can affect drivetrain geometry and wiring harness tension.
- Gear Selector: A mechanical or electronic interface allowing the operator to choose transmission range.
- Wiring Harness: A bundled set of electrical wires connecting sensors, actuators, and control modules.
- TCM Failure: In the case examined, the transmission control module was identified as the root cause. The timing of the failure coincided with motor mount replacement, but the module itself was faulty—likely due to age or internal component degradation.
- Pinched or Stressed Wiring: The wiring harness near the driver’s seat and motor mount area is vulnerable during engine mount replacement. If wires are pinched or stretched, signal loss or erratic behavior can occur.
- Drivetrain Misalignment: A broken motor mount can shift the engine-transmission assembly, placing stress on connected components. Even after replacement, residual misalignment may affect gear engagement.
In Florida, a technician working on a 1995 Highlander variant discovered that the gear selector worked intermittently after motor mount replacement. After tracing the wiring harness and testing the TCM, he confirmed that the module was failing to hold gear commands. Replacing the TCM resolved the issue immediately.
In contrast, a contractor in Colorado found that his gear lock problem stemmed from a loose ground wire near the firewall. Once tightened, the transmission held gear selection without issue—highlighting the importance of electrical integrity in older machines.
Preventive Measures and Solutions
- Inspect Wiring Harness During Mount Replacement: Always check for pinched, frayed, or stretched wires near engine mounts and control modules.
- Test TCM Functionality with Diagnostic Tools: Use a multimeter or scan tool to verify voltage and signal continuity.
- Secure Ground Connections: Loose grounds can mimic module failure and cause erratic gear behavior.
- Replace TCM with OEM or Certified Rebuilt Units: Avoid generic replacements that may not match the original programming.
- Check Gear Selector Linkage: Ensure mechanical linkages are properly aligned and lubricated.
Gear lock in the Lull 644B telehandler is often traced to electronic control issues, particularly the transmission control module. While mechanical repairs like motor mount replacement may trigger symptoms, the underlying fault typically lies in electrical components or wiring integrity. With careful inspection and targeted replacement, operators can restore full gear functionality and extend the service life of this dependable lifting machine. The 644B remains a testament to 1990s engineering—simple, strong, and still working hard decades later.
