Diagnosing Function Loss on a JLG 40H Boom Lift

[MikePhua]

The JLG 40H and Its Hydraulic-Electric Control System
The JLG 40H is a pre-1995 telescopic boom lift powered by a Ford gasoline engine and equipped with standard hydraulic controls. Designed for mid-range aerial work, the 40H features a telescoping boom, basket rotation, tilt, and steering—all controlled through a combination of hydraulic valves and electrically actuated solenoids. Unlike newer models with CAN bus diagnostics and multiplexed wiring, the 40H relies on discrete relays, dump valves, and direct wiring, making troubleshooting both accessible and nuanced.
Terminology annotation:
- Bang-bang dump valve: A solenoid-operated valve that either fully opens or fully closes hydraulic flow to the function manifold.
- Function manifold: A hydraulic block that distributes pressurized fluid to various actuators such as telescope, steer, and basket tilt.
- Relay: An electrically controlled switch that allows low-voltage signals to activate high-current circuits.
- Dump circuit: A safety feature that disables hydraulic functions unless specific conditions are met.
Symptoms and Initial Observations
Operators reported that several functions—including telescope in, steering, and basket rotation—ceased working simultaneously. The issue occurred both from the ground and platform controls, suggesting a shared fault in the hydraulic or electrical control path. A pressure gauge installed on the control valve assembly showed only 400 psi when attempting to telescope out, and no pressure increase when attempting to telescope in. Under normal operation, pressure should rise to approximately 2,000 psi during actuation.
This behavior pointed to a failure in either the dump valve activation or a shared electrical fault affecting multiple solenoids.
Electrical Testing and Relay Swapping
To isolate the issue, relays R16 through R21 were swapped to test for internal failure. Wire colors and terminal positions were noted, including:

• Brown wire to terminal 85
  
• Yellow to terminal 30
  
• Orange to terminal 87
  
• Orange-red to terminal 86
  

Interestingly, the wire connected to terminal 87 was not shown on the available wiring diagram, raising questions about undocumented modifications or schematic discrepancies.
Voltage at the dump valve solenoid was measured at 13.23 volts during boom retraction attempts—within acceptable range. Ground continuity on the brown wire was confirmed, ruling out a broken ground path.
Hydraulic Behavior and Valve Contamination
Two years prior, the same machine exhibited identical symptoms, which were resolved by cleaning the main relief valve cavity. Although new springs and internal parts were installed, the valve only began functioning after debris was flushed from the bore. This suggests that contamination—possibly metal shavings or hardened fluid residue—can intermittently block valve operation.
Recommendations:

• Remove and clean the main relief valve cavity thoroughly
  
• Inspect for burrs, sludge, or hardened seal fragments
  
• Flush the hydraulic system with filtered fluid
  
• Replace filters and check for signs of internal wear
  

Hydraulic valves on older machines are particularly vulnerable to contamination due to aging seals and fluid breakdown. Even a small particle can prevent proper seating of the relief valve, causing pressure loss across multiple functions.
Grounding Issues and Shared Circuit Vulnerability
A technician noted that losing ground to one side of the solenoid bank can disable multiple functions in one direction. This is especially relevant in machines where multiple solenoids share a common ground or power feed. If the ground path is compromised—due to corrosion, loose terminals, or broken wires—functions like telescope in, steer left, and basket tilt may all fail simultaneously.
Solutions:

• Inspect all ground connections at the solenoid bank
  
• Use a multimeter to verify resistance and continuity
  
• Clean terminals and apply dielectric grease
  
• Check for pinched or abraded wires near the boom pivot
  

One operator resolved a similar issue by replacing a corroded ground lug hidden beneath the control panel. Once restored, all functions returned to normal.
Preventive Measures and Long-Term Reliability
To maintain consistent performance on a JLG 40H:

• Replace hydraulic fluid every 1,000 hours or annually
  
• Clean and inspect the relief valve cavity during each service interval
  
• Test solenoid voltage and ground paths quarterly
  
• Label and document relay positions and wire colors
  
• Avoid high-pressure washing near electrical components
  

For machines stored outdoors, moisture intrusion can accelerate corrosion in connectors and relays. Installing sealed relay covers and weatherproof grommets can extend component life.
Conclusion
Loss of multiple hydraulic functions on a JLG 40H often stems from a combination of electrical grounding faults and hydraulic valve contamination. By methodically testing voltage, swapping relays, and cleaning valve cavities, operators can restore functionality without major component replacement. In legacy boom lifts, the interplay between fluid and electrons is delicate—and a single dirty valve or broken wire can silence the entire machine. With patience and precision, the 40H can be brought back to full reach and rotation.