Water and Wiring: Diagnosing Post-Wash Electrical Failures in the JLG 400S

[MikePhua]

Introduction
The JLG 400S boom lift is a versatile machine used in construction, maintenance, and industrial applications. Its dual-fuel capability and robust hydraulic systems make it a favorite among operators. However, like many modern machines, it relies heavily on electronic control systems. When exposed to water—especially during pressure washing—these systems can become vulnerable to failure. This article explores a real-world case of post-wash malfunction, delving into electrical diagnostics, hydraulic behavior, and broader lessons from the field.
Symptoms After Washing
Following a routine wash, the 2007 JLG 400S exhibited several critical failures:

• Loss of steering and boom functions: The machine could drive forward and backward but could not steer or operate the boom.
  
• No response from platform or ground controls: Only basic drive functions remained operational.
  
• Error codes: Control cards flashed a 4-3 blink code, indicating a fault condition.
  
• Partial recovery via auxiliary power: When powered by battery alone, all functions worked—suggesting a power distribution or grounding issue.
  

These symptoms pointed to water intrusion affecting electrical components, particularly control circuits and solenoids.
Understanding the Control System
The JLG 400S uses a dual control architecture:

• Platform Control Station: Allows the operator to manage boom and drive functions from the basket.
  
• Ground Control Station: Offers redundant control for safety and diagnostics.
  
• Control Cards: Electronic modules that interpret input signals and activate hydraulic solenoids.
  
• Solenoids: Electromagnetic valves that direct hydraulic flow to actuators.
  

When water enters connectors or solenoids, it can cause short circuits, corrosion, or signal loss—leading to erratic or failed operation.
Electrical Diagnostics and Remedies
Technicians approached the issue methodically:

• Unplugging and drying connectors: All plugs were disconnected and air-dried to remove moisture.
  
• Silicone spray and WD-40: Applied to displace residual water and prevent corrosion.
  
• Swapping control cards: Verified that the cards themselves were functional by testing them in another machine.
  
• Error Code 4-3: Interpreted as a low voltage signal from the engine coolant temperature sensor, though this code can represent multiple faults depending on context.
  

Despite these efforts, the machine remained partially inoperative when running on engine power, suggesting deeper electrical or hydraulic interference.
Hydraulic Valve Investigation
A JLG technician suspected a faulty hydraulic valve. After swapping valves with a known-good machine, the issue persisted. This ruled out mechanical valve failure and pointed back to electrical control or power supply inconsistencies.

• Hydraulic Valve Function: These valves rely on solenoids to open and close pathways for fluid. If the solenoid doesn’t receive a proper signal, the valve remains inactive.
  
• Auxiliary Power Success: When powered by battery alone, all functions worked—indicating that the hydraulic system was intact and the issue lay in power distribution or grounding when the engine was running.
  

Environmental Vulnerabilities
Pressure washing, especially with high PSI, can force water into sealed connectors, solenoids, and control boxes. Even without detergent, water alone can cause:

• Capillary intrusion: Water wicks into wire insulation and connector pins.
  
• Corrosion: Moisture reacts with metal contacts, degrading signal quality.
  
• Grounding faults: Water can create unintended paths to ground, disrupting voltage levels.
  

One technician recalled a similar incident with a Genie lift where water entered the joystick controller, causing intermittent boom movement. The solution involved disassembling the controller and baking it in a low-temperature oven to evaporate moisture.
Lessons from the Field
In cold climates, machines washed and then parked outside can trap moisture that freezes overnight, expanding and damaging seals. During the 2019 Midwest freeze, several rental fleets reported widespread failures in aerial lifts due to post-wash icing in control boxes.
A seasoned mechanic shared a story of a scissor lift that refused to operate after a wash. After days of troubleshooting, the culprit was a single corroded pin in a multi-pin connector—barely visible but enough to break the signal chain.
Preventive Measures
To avoid similar failures, operators and technicians should consider:

• Low-pressure washing: Avoid high-pressure jets near electrical components.
  
• Protective covers: Use plastic bags or rubber shields over connectors during cleaning.
  
• Post-wash drying: Allow machines to air dry thoroughly before operation.
  
• Dielectric grease: Apply to connectors to repel moisture and improve conductivity.
  
• Routine connector inspection: Check for corrosion, wear, and seal integrity.
  

Conclusion
The JLG 400S case underscores the delicate balance between rugged machinery and sensitive electronics. While the machine’s hydraulic and mechanical systems are built for harsh environments, its control architecture demands careful handling—especially around water. Through methodical diagnostics, component swapping, and environmental awareness, technicians can restore functionality and prevent future failures. In the end, a clean machine is good—but a dry machine is better.