Diagnosing and Repairing the JLG 40HA: A Deep Dive into Hydraulic and Electrical Troubles

[MikePhua]

Introduction to the JLG 40HA
The JLG 40HA is a 40-foot articulated boom lift, popular for maintenance, construction, and facility work. Designed with versatility and reach in mind, this machine combines hydraulic articulation and diesel or dual-fuel power to allow operators to access difficult vertical and horizontal positions. It features proportional controls, a swing-out engine tray, and a platform height of approximately 40 feet, with a horizontal outreach of about 20 feet.
While reliable under normal conditions, the 40HA, like many older aerial lifts, can develop frustrating electrical and hydraulic issues—especially when subjected to infrequent use, improper storage, or field-modified wiring.
Core Operating Systems in the JLG 40HA
To understand common failures, it’s helpful to identify the JLG 40HA’s major components:

• Hydraulic System
  Operates the boom lift, swing, platform rotation, and drive. Powered by a gear or vane-type hydraulic pump.
  
• Electrical Control System
  Includes a platform control box, lower control box, relays, limit switches, and a wire harness. JLG’s control logic often uses safety interlocks—circuits that must all signal “safe” before a motion is allowed.
  
• Power Source
  Typically a gas or diesel engine connected to an alternator, starter motor, and hydraulic pump.
  

Common Problems and Their Root Causes
Multiple users have reported issues with the JLG 40HA, particularly when it sits idle or has a complicated repair history. These issues tend to cluster in a few key areas:
1. Platform Controls Not Responding
Symptoms:

• Engine starts and runs
  
• Control panel lights up
  
• No function from platform joystick or switches
  

Possible Causes:

• Broken wires in the boom cable harness, especially near articulation joints where bending stress is greatest
  
• Corroded control box connectors due to exposure or moisture ingress
  
• Failed relay or diode board inside the platform control box
  
• Incorrect polarity or missing ground, often resulting from previous improper jump-starts
  

This problem often results in hours of unnecessary mechanical troubleshooting. Many times, it’s a simple broken wire—especially the common green ground wire in the cable track. A continuity test with a multimeter from the platform controls to the base is essential.
2. Boom Functions Working Intermittently
Symptoms:

• One function (e.g., boom lift or swing) fails while others work
  
• Only works from ground control, not from platform
  
• Audible clicking from relays but no hydraulic movement
  

Possible Causes:

• Micro-switch failure in the limit or control levers
  
• Relay sticking or contacts burnt inside the control box
  
• Hydraulic solenoid not engaging, due to coil failure or jammed valve spool
  

Each function on the boom is activated by a solenoid. If you hear a click but see no motion, you’re likely getting power to the relay but not enough current to energize the coil, or the valve spool is stuck due to debris or varnished fluid.
3. Engine Starts but Cuts Off During Boom Movement
Symptoms:

• Engine idles fine
  
• When attempting a boom or drive movement, engine dies
  

Possible Causes:

• Bad ground connection, causing voltage drop during current surge
  
• Faulty alternator, not maintaining battery charge during demand
  
• Hydraulic pump load too high, possibly due to seized pump or incorrect fluid viscosity
  

A real-world case involved an operator whose lift died every time he tried to elevate the boom. After days of checking sensors, he discovered the negative battery cable had rusted where it mounted to the chassis, causing a surge drop under load.
4. No Movement in Cold Weather
Symptoms:

• Engine starts
  
• Controls light up
  
• All boom functions sluggish or unresponsive
  

Possible Causes:

• Hydraulic fluid too thick due to cold temperatures
  
• Debris in filters or suction strainer causing pressure starvation
  
• Low-voltage issues—cold batteries don’t deliver full current, leading to weak relay activation
  

Older 40HA units often use 10W or 15W hydraulic oil, which thickens in freezing temperatures. Switching to synthetic hydraulic fluid rated for cold weather can greatly improve winter operation.
Electrical and Hydraulic Troubleshooting Techniques
To streamline diagnosis, consider this checklist approach:

• Step 1: Confirm Voltage
  • Battery: 12.4–12.7V (engine off), 13.6–14.2V (engine running)
    
  • Confirm ground continuity at platform and base
    
  • Load-test the battery; surface voltage can be misleading
    
• Step 2: Inspect Boom Harness
  • Look for pinched or stretched sections
    
  • Test continuity across all major wires, especially for common functions
    
• Step 3: Verify Hydraulic Pressure
  • Confirm pump engagement
    
  • Check relief valve pressures and flow to all actuators
    
  • Inspect hydraulic filters and fluid color (dark = oxidized, milky = water)
    
• Step 4: Test Control Logic
  • Use a multimeter at the platform control box
    
  • Confirm signal at solenoid coil for each function
    
  • Swap relays between working and non-working functions for quick diagnosis
    

Real-World Repair Stories
A contractor in Texas bought a used JLG 40HA for warehouse roof maintenance. After sitting idle for six months, the boom refused to lift. Mechanics replaced the hydraulic pump, spent hours on the wiring, and replaced the battery—still no lift. A retired mechanic finally discovered a single broken wire inside the articulated boom pivot that controlled the lift solenoid. A 10-minute soldering job restored full function.
Another owner in Ontario had a 40HA with random shutdowns when turning the turret. He eventually traced it to a worn turret slip-ring assembly—a component designed to transmit signals across the rotating joint. Replacing the worn brush contacts resolved the issue.
Tips for Preventative Maintenance

• Cycle all functions weekly, especially in winter storage
  
• Label and document all electrical work, particularly relay or diode board changes
  
• Use dielectric grease on all plug connectors to resist moisture
  
• Inspect boom harness annually for cracks or wear
  
• Flush and replace hydraulic fluid every 1,000 hours or every 2 years, whichever comes first
  

Conclusion
The JLG 40HA is a proven machine, but like all older aerial lifts, it demands respect for its electrical and hydraulic complexities. Most problems stem not from catastrophic component failures but from corrosion, wiring fatigue, or simple oversight. With careful inspection, methodical troubleshooting, and a good multimeter, most issues can be resolved without major expense. Machines like the 40HA still earn their keep—if operators are willing to learn their language.