Diagnosing Voltage Drop Issues on the JLG 40H Aerial Lift Platform

[MikePhua]

The JLG 40H and Its Electrical Control System
The JLG 40H is a hydraulic drive boom lift introduced in the 1980s as part of JLG’s push toward reliable, mid-range aerial work platforms. With a platform height of 40 feet and a working height of approximately 46 feet, the 40H became a staple in facility maintenance, construction, and industrial access. It features a dual control system—ground and platform—with electrical switching that governs hydraulic valve actuation.
The electrical system is 12V DC, powered by a deep-cycle battery bank. Control signals are routed through relays, switches, and solenoids to activate hydraulic functions. When switching control from ground to platform, the system relies on clean voltage, solid connections, and responsive relays to maintain consistent operation.
Symptoms of Voltage Drop During Platform Activation
Operators may encounter the following issues:

• Voltage drops significantly when switching to platform control
  
• Platform functions become sluggish or fail to respond
  
• Audible clicking from relays without hydraulic movement
  
• Ground control remains functional while platform control fails
  
• Battery voltage reads normal until load is applied
  
• Platform joystick lights flicker or dim during use
  

In Alberta, a maintenance crew using a 40H for warehouse lighting upgrades noticed that the lift would hesitate when switching to platform mode. After testing, they found a 3V drop across the platform control relay, caused by corroded terminals.
Core Electrical Components Involved
The platform control system includes:

• Selector switch to toggle between ground and platform
  
• Platform joystick with integrated switches
  
• Control relay bank for directional valve actuation
  
• Wiring harness with multiple connectors and splices
  
• Battery bank and main power cables
  
• Safety interlocks and limit switches
  

Voltage drop typically occurs due to resistance in the circuit. This can be caused by:

• Corroded or loose connectors
  
• Undersized wiring for current draw
  
• Failing relays with high internal resistance
  
• Weak battery under load
  
• Ground faults or poor chassis bonding
  

Diagnostic Strategy and Inspection Checklist
To isolate the fault:

• Measure voltage at the battery terminals under load
  
• Check voltage at the platform control input and output
  
• Inspect relay terminals for heat discoloration or corrosion
  
• Test continuity of ground wires from platform to chassis
  
• Load test batteries individually and as a bank
  
• Wiggle connectors during operation to detect intermittent faults
  

Some technicians install temporary bypass wires to confirm circuit integrity. Others use clamp meters to measure current draw during joystick activation.
Preventive Maintenance and Solutions
To prevent future voltage drop issues:

• Clean and tighten all electrical terminals quarterly
  
• Replace aging relays with sealed, low-resistance units
  
• Upgrade wiring to heavier gauge if voltage drop exceeds 1V under load
  
• Use dielectric grease on connectors exposed to moisture
  
• Test battery health monthly and replace weak units
  
• Add supplemental ground straps between platform and frame
  

In Shanghai, a contractor retrofitted their 40H with a voltage monitor that alerts the operator when system voltage drops below 11.5V. This helped prevent mid-operation failures and reduced downtime.
Field Anecdote and Practical Insight
In Baghdad, a 40H used for stadium lighting maintenance began losing platform control during evening shifts. The crew discovered that the battery bank was undercharged due to a faulty charger. After replacing the charger and cleaning all terminals, the lift operated flawlessly. A simple oversight in charging had nearly grounded the entire operation.
Conclusion
Voltage drop during platform activation on the JLG 40H is often caused by electrical resistance, weak batteries, or compromised connections. With methodical diagnostics and preventive care, operators can restore full functionality and ensure safe, reliable lift performance. Whether changing filters at height or repairing overhead signage, a stable electrical system is essential to the 40H’s legacy of dependable access.