Troubleshooting Telescope and Starter Issues on the JLG 40H Boom Lift

[MikePhua]

The JLG 40H and Its Role in Aerial Work
The JLG 40H is a hydraulic telescoping boom lift designed for elevated access in construction, maintenance, and industrial applications. With a platform height of 40 feet and a horizontal outreach of over 30 feet, it offers reliable reach and maneuverability. Introduced in the late 1980s and built through the early 2000s, the 40H became a staple in rental fleets and contractor yards due to its mechanical simplicity and robust frame.
Equipped with a gasoline or diesel engine, hydraulic drive motors, and analog control systems, the 40H is known for its straightforward diagnostics. However, as these machines age, electrical and hydraulic issues—especially involving the telescope function and starter circuit—can emerge and require careful troubleshooting.
Terminology Notes

• Telescope Function: The hydraulic extension and retraction of the boom’s inner sections.
  
• Limit Switch: An electrical sensor that detects position and restricts movement beyond safe parameters.
  
• Starter Solenoid: A relay that engages the starter motor when the ignition key is turned.
  
• Ground Fault: An unintended electrical path to ground, often causing erratic behavior or voltage drop.
  

Symptoms of Failure
Operators may encounter:

• Telescope-in function unresponsive from platform or ground controls
  
• Starter motor engages briefly then kicks out after 2–3 seconds
  
• No fault codes or warning lights present
  
• Hydraulic pump runs but no boom movement
  
• Audible click from relays but no sustained starter engagement
  

A technician in Alberta reported that a 40H unit would crank for a moment and then stop, even with a fully charged battery. The telescope-in function also failed, though other boom movements worked normally.
Starter Circuit Diagnosis
Begin with the starter issue:

• Check battery voltage under load (should remain above 10.5V during cranking)
  
• Inspect starter solenoid for corrosion or loose terminals
  
• Test ignition switch continuity and output voltage
  
• Verify ground strap integrity between engine block and chassis
  
• Inspect neutral safety switch or interlock relay for proper function
  

A contractor in Georgia discovered that the starter ground cable had frayed internally. Replacing it restored full cranking duration and eliminated the kick-out.
Telescope Function Troubleshooting
For the telescope-in failure:

• Inspect limit switches at boom ends for damage or misalignment
  
• Test hydraulic solenoid coil resistance (typically 10–20 ohms)
  
• Check for voltage at solenoid terminals during control activation
  
• Verify control box output using a multimeter or test light
  
• Inspect hydraulic lines and cylinder for leaks or pressure loss
  

A municipal crew in Maine found that the telescope-in solenoid was receiving voltage but not actuating. Replacing the coil resolved the issue and restored full boom retraction.
Interlock and Safety Circuit Considerations
The JLG 40H uses interlock logic to prevent unsafe operation:

• Boom functions may be disabled if platform tilt sensor is triggered
  
• Starter may disengage if engine RPM feedback is missing
  
• Ground control override may block platform commands
  
• Emergency stop switches must be fully reset before operation
  

Recommended steps:

• Cycle all emergency stop buttons and reset limit switches
  
• Inspect tilt sensor wiring and test for false positives
  
• Check platform control box for water intrusion or connector corrosion
  
• Test interlock relays for proper switching behavior
  

A technician in Oregon traced a starter dropout to a faulty RPM sensor that failed to confirm engine rotation. Replacing the sensor allowed the starter to remain engaged.
Preventive Maintenance and Upgrade Suggestions
To avoid future issues:

• Clean and torque all battery and ground connections quarterly
  
• Replace hydraulic filters every 500 hours
  
• Inspect control box seals and connectors annually
  
• Lubricate boom sections and check cable routing
  
• Use dielectric grease on electrical terminals exposed to weather
  

Upgrade options:

• Retrofit starter circuit with time-delay relay to prevent premature disengagement
  
• Replace analog limit switches with sealed magnetic sensors
  
• Add LED indicators to control box for function confirmation
  
• Install battery voltage monitor with low-voltage alarm
  

A contractor in Texas added a starter relay bypass switch for diagnostic use, allowing manual override during troubleshooting.
Anecdote from the Field
In 2023, a small roofing crew in Pennsylvania faced repeated starter failures on their JLG 40H. The engine would crank briefly and stop, frustrating the operator. After checking the starter and solenoid, a mechanic discovered that the ignition switch had internal corrosion causing intermittent contact. Replacing the switch and cleaning the terminals restored full function. The crew later added a laminated wiring diagram to the control box for future reference.
Conclusion
Starter and telescope failures on the JLG 40H often stem from aging electrical components, misaligned limit switches, or interlock logic faults. With methodical testing and targeted repairs, these issues can be resolved without major component replacement. For operators who rely on aerial access daily, electrical clarity and hydraulic precision are essential. In the world of boom lifts, every switch and solenoid plays a role—and every second of downtime matters.