JLG 40HT Auxiliary Hydraulic Pump Troubleshooting

[MikePhua]

The Rise of JLG and the 40HT Boom Lift
JLG Industries, founded in 1969 in Pennsylvania, revolutionized aerial work platforms with its focus on safety, reach, and reliability. Over the decades, JLG became a global leader in access equipment, producing boom lifts, scissor lifts, and telehandlers used in construction, maintenance, and industrial applications. The JLG 40HT, a high-capacity telescopic boom lift introduced in the late 1990s, remains a staple in mid-reach aerial operations. With a working height of 46 feet and a horizontal outreach of nearly 34 feet, the 40HT is designed for rugged outdoor use, offering a robust hydraulic system and simplified mechanical layout.
Sales of the 40HT and its variants have exceeded tens of thousands globally, particularly in North America and Europe, where rental fleets rely on its durability and ease of service. Its popularity stems from a straightforward design—no complex electronics, just reliable hydraulics and mechanical linkages.
Understanding the Auxiliary Hydraulic Pump System
The auxiliary hydraulic pump on the JLG 40HT serves as a backup or emergency system, allowing limited hydraulic functionality when the main engine-driven pump is unavailable. This is especially critical during engine failure, battery depletion, or when performing maintenance tasks that require boom retraction or lowering without full system power.
Key components of the auxiliary hydraulic system include:

• DC Electric Motor: Typically 12V or 24V, depending on the machine’s configuration.
  
• Gear Pump: A compact hydraulic pump driven by the motor, capable of low-flow output.
  
• Solenoid Valves: Electrically actuated valves that direct flow to specific cylinders.
  
• Manual Override Switches: Located near the base or control panel, allowing operator activation.
  
• Thermal Protection: Prevents motor overheating during extended use.
  

The auxiliary pump is not designed for full-speed operation but rather for slow, controlled movement—ideal for stowing the boom or retracting outriggers.
Common Failure Modes and Diagnostic Tips
Operators and technicians often encounter issues with the auxiliary pump system, especially in older units. Common symptoms include:

• No Response When Activated: Indicates electrical failure, such as blown fuses, bad relays, or corroded connectors.
  
• Motor Runs but No Movement: Suggests hydraulic flow blockage, low fluid level, or pump wear.
  
• Intermittent Operation: Points to loose wiring, failing solenoids, or thermal cut-off activation.
  

To diagnose these issues, follow a structured approach:

• Check Voltage at Motor Terminals: Ensure proper power supply when the switch is engaged.
  
• Inspect Ground Connections: A poor ground can prevent motor activation.
  
• Test Solenoids with Multimeter: Verify coil resistance and continuity.
  
• Manually Activate Valves: Use jumper wires to simulate switch input and observe response.
  
• Monitor Fluid Level and Condition: Low or contaminated hydraulic fluid can impair pump performance.
  

One technician recalled a case where the auxiliary pump failed to operate during a rooftop job. After hours of troubleshooting, the issue was traced to a corroded ground wire hidden beneath the chassis. Once cleaned and reconnected, the pump functioned perfectly—a reminder that electrical faults often masquerade as hydraulic problems.
Preventive Maintenance and Upgrade Suggestions
To ensure reliable auxiliary pump operation, consider the following practices:

• Regular Electrical Inspections: Check wiring harnesses, connectors, and switches for wear or corrosion.
  
• Replace Aging Motors: DC motors degrade over time, especially in harsh environments.
  
• Install LED Indicators: Visual feedback helps operators confirm activation.
  
• Use Weatherproof Connectors: Protect against moisture ingress and oxidation.
  
• Flush Hydraulic System Annually: Prevents buildup of sludge and contaminants.
  

In one fleet operation in Alberta, auxiliary pump failures were reduced by 60% after implementing quarterly electrical checks and switching to sealed connectors. The investment paid off in reduced downtime and safer operations.
Terminology Notes

• DC Motor: A direct current motor used to drive the auxiliary hydraulic pump.
  
• Solenoid Valve: An electrically controlled valve that directs hydraulic flow.
  
• Gear Pump: A type of hydraulic pump using meshing gears to move fluid.
  
• Override Switch: A manual switch allowing auxiliary system activation.
  
• Thermal Cut-Off: A safety device that disables the motor when it overheats.
  

Field Anecdotes and Lessons Learned
In a construction site in Nevada, a JLG 40HT was stranded with its boom extended after the main engine failed. The auxiliary pump was activated, but the boom refused to retract. After a tense hour of troubleshooting, the issue was traced to a faulty solenoid coil. A quick replacement restored function, allowing the crew to safely lower the boom and avoid costly crane intervention.
Such stories underscore the importance of understanding and maintaining auxiliary systems. While often overlooked, they serve as lifelines in critical moments.
Final Thoughts
The auxiliary hydraulic pump on the JLG 40HT is more than a backup—it’s a vital safety and service tool. With proper care, it can extend the life of the machine and prevent dangerous situations. Whether you're a technician, operator, or fleet manager, investing time in understanding and maintaining this system pays dividends in uptime, safety, and peace of mind. In the world of aerial work platforms, redundancy isn’t a luxury—it’s a necessity.