Diagnosing Electrical and Hydraulic Issues in the 1998 JLG 40H: A Practical Guide to Boom Lift Revival

[MikePhua]

Overview of the JLG 40H Boom Lift
The JLG 40H, particularly the 1998 model, is a hydraulic articulated boom lift powered by either gas, diesel, or dual-fuel (gas/propane) internal combustion engines. It features a simple mechanical-hydraulic design with electric-over-hydraulic controls, making it both durable and relatively straightforward to service. However, age-related failures—especially in wiring, solenoids, and control relays—are common in machines that have seen years of use without thorough electrical refurbishment.
Understanding the Dual Control System
This machine uses an electric-over-hydraulic (E/H) system, where joystick or toggle switch inputs trigger solenoid valves that direct hydraulic fluid to the appropriate cylinders or motors. Essentially, operator inputs are electrical, but the machine’s motion and power are hydraulic.
Key control components include:

• Limit switches: Define max reach or safe extension limits
  
• Start/run relays: Control ignition and auxiliary power
  
• Selector switch: Toggles control between ground station and platform
  
• Hydraulic solenoids: Engage specific valves for boom, swing, and drive
  
• Emergency lowering valve: Allows manual descent in the event of power failure
  

Aging Components and Common Failure Points
By 2025, a 1998 machine is over 25 years old. That means even with low hours, several components are prone to failure due to age rather than use:

• Brittle Wiring Insulation: Heat and UV exposure can cause the wiring harness to crack or short out.
  
• Corroded Connectors: Especially in wet or humid climates, oxidation inside multi-pin plugs can disrupt signal flow.
  
• Failed Solenoids: Hydraulic solenoids with weak coils or stuck plungers will fail to actuate, even when electrically energized.
  
• Weak Ground Paths: The 40H often suffers from high-resistance grounds, which causes inconsistent solenoid activation or relay chatter.
  

A common user-reported symptom is when the engine starts fine, but none of the boom or drive functions respond. In nearly every case, this leads back to either a failed function enable solenoid, a faulty ground selector switch, or corroded wiring at the ground control panel.
Case Story: A 40H Resurrected from a Junkyard
A construction equipment refurbisher in Missouri acquired a neglected JLG 40H from a county auction. The lift had sat idle for 8 years behind a salt shed. After replacing the battery and changing fluids, they encountered a no-movement issue—engine ran, but boom and drive systems were completely dead.
The mechanic traced the fault to:

1. A bad platform/ground selector switch—it failed to send voltage to the joystick harness.
   
2. A fried diode in the control box that prevented the main power relay from latching.
   
3. A missing function enable solenoid, which had been cannibalized and capped off.
   

After replacing the diode with a generic 3A/1000V unit and sourcing a used enable solenoid from a salvage yard, full operation was restored. The entire repair cost under $200, but required a solid understanding of E/H circuits and methodical testing with a voltmeter.
Practical Diagnostic Tips for the 40H
When diagnosing electrical issues, follow this sequence:

• Start with the Selector Switch: Confirm that voltage is getting to the platform or ground control, depending on switch position.
  
• Check for Voltage at Solenoids: Each function solenoid should get 12V only when the appropriate switch is engaged.
  
• Listen for Clicks: Working solenoids will audibly click when energized.
  
• Verify Grounds: Run a jumper from known battery ground to suspected solenoid or relay ground wires to bypass resistance.
  
• Test Joystick Outputs: Most joysticks on the 40H use simple momentary switches. With the joystick open, verify continuity when engaged.
  

Emergency Bypass and Field Fixes
In remote locations, creative fixes have kept these machines alive:

• Manual Valve Engagement: Using a screwdriver to manually shift solenoids at the hydraulic manifold in emergencies.
  
• Bypass Switch: Installing a temporary toggle directly to the function enable solenoid for testing.
  
• Jumpers on Relays: Bridging known-good power directly to solenoids to confirm whether wiring or the component is the issue.
  

However, safety should never be compromised. Always return to OEM wiring after diagnosis.
Key Replacement Parts to Have on Hand
For anyone operating a legacy 40H in a rental or contractor fleet, the following spares are essential:

• Function enable solenoid
  
• Joystick potentiometer or switch assembly
  
• Diodes and resistors from the control box
  
• Main power relay
  
• Ground selector switch
  
• Wire and Weatherpack connectors for field repairs
  

Maintaining the 40H in Modern Fleets
Despite its age, the JLG 40H remains a reliable and useful lift when properly maintained. Its simplicity makes it a candidate for refurbishing rather than retiring, especially when budgets are tight. Several contractors have built low-cost secondary fleets from surplus 40Hs, using them for indoor work, signage installation, or light exterior maintenance. Parts availability remains solid due to aftermarket support and shared components with other JLG models.
If you’re operating in colder climates, be aware that aged solenoids and wiring become more failure-prone during winter. Pre-heating the control boxes or applying dielectric grease to connectors can prevent downtime.
Conclusion
The 1998 JLG 40H is a testament to robust design. When electric-over-hydraulic systems start acting up, the solution is rarely complex—it’s usually a corroded contact, a dead relay, or an open circuit. With methodical diagnosis and a working knowledge of how these legacy machines operate, the 40H can continue serving reliably, decades past its original design life. Whether you’re in the field troubleshooting a dead boom or refurbishing a forgotten unit from a county lot, this lift has earned its place in the history of hard-working aerial equipment.