JLG 40F Manlift Control Delay and Brake Failure Diagnosis

[MikePhua]

A JLG 40F manlift from the 1980s exhibiting delayed control response, joystick reversal, and brake failure likely suffers from hydraulic viscosity mismatch, worn control valves, and degraded brake pads. These issues are common in aging aerial platforms and require targeted inspection and component replacement to restore safe operation.
JLG 40F Background and Control System Design
JLG Industries, founded in 1969 in Pennsylvania, pioneered the self-propelled boom lift market. The 40F model was part of their early lineup of diesel-powered articulating manlifts, offering 40 feet of platform height and robust steel construction. These units were widely deployed across North America for industrial maintenance, construction, and utility work.
The 40F uses two distinct hydraulic control systems:

• Bertae proportional valves for boom lift, swing, and drive functions.
  
• Racine bang-bang valves for basket tilt, rotate, and small boom articulation.
  

This dual-valve architecture creates different response profiles and warm-up behavior across functions.
Terminology Note

• Proportional Valve: A hydraulic valve that modulates flow based on input signal strength, allowing smooth control.
  
• Bang-Bang Valve: A binary valve that is either fully open or closed, offering rapid but less precise movement.
  
• Monotrol Joystick: A single-axis joystick controlling both forward and reverse travel.
  
• Brake Pad Relining: The process of replacing worn friction material on brake shoes or calipers.
  
• Hydraulic Viscosity: The thickness of hydraulic fluid, affecting flow and valve response.
  

Symptoms and Observations
The machine must warm up for 20 minutes before the main boom, turret, and drive functions respond. However, steering, platform tilt, and small boom operate immediately. Additionally:

• The drive joystick sends the machine in reverse regardless of position.
  
• Joystick replacement did not resolve the issue.
  
• Brakes fail to hold the machine on grade, raising safety concerns.
  

These symptoms suggest a combination of hydraulic fluid mismatch, valve wear, and brake pad degradation.
Root Causes and Technical Analysis

• Cold hydraulic fluid with high viscosity delays proportional valve response. In Canada’s climate, this is exacerbated by ambient temperatures below freezing.
  
• Joystick signal inversion may stem from wiring faults or control board misinterpretation. If both directions trigger reverse, the issue lies beyond the joystick.
  
• Brake failure is likely due to worn pads or contaminated friction surfaces. The 40F uses replaceable brake pads accessible beneath sheet metal covers near the drive wheels.
  

Recommended Solutions and Maintenance Strategy

• Switch to low-viscosity hydraulic fluid, such as ISO 32 or synthetic blends rated for cold weather. This improves valve response during startup.
  
• Trace joystick wiring from the control head to the valve solenoids. Look for pinched wires, corroded connectors, or swapped terminals.
  
• Test joystick output voltage with a multimeter to confirm signal polarity and range.
  
• Inspect brake pads by removing the top covers over the axle frame. Pads can be relined by a brake specialist or replaced with OEM parts.
  
• Clean brake surfaces with solvent and check for hydraulic leaks that may contaminate friction material.
  

Field Experience and Anecdotes
In Ontario, a technician servicing a JLG 40F noted that switching to synthetic hydraulic fluid reduced warm-up time by half. He also discovered that a joystick harness had been patched multiple times, causing signal inversion. After rewiring and calibrating the control board, drive direction returned to normal.
In British Columbia, a forestry contractor relined the brake pads using material from a skidder brake shop. The process involved removing the drive motor and brake assembly, then pressing out the worn pads. After reinstallation, the machine held reliably on slopes.
Conclusion
The JLG 40F manlift’s delayed control response and brake failure stem from aging components and environmental mismatch. By upgrading hydraulic fluid, inspecting joystick wiring, and relining brake pads, operators can restore functionality and safety. For legacy aerial platforms, proactive maintenance and system understanding are essential to avoid downtime and hazards.