JLG 40H PQ Controller Box Insight

[MikePhua]

The PQ (Proportional Joystick) controller box aboard JLG 40H boom lifts serves as a crucial interface between the operator’s inputs and the hydraulic functions. Its role, construction, and fine-tuning determine how effectively lift, swing, and drive operations respond—making it a vital component in machine performance and user safety.
Controller Purpose and Wiring Connection
Within the platform console, the PQ controller includes a small capacitor designed to discharge residual voltage when the control foot switch is released. This ensures that lingering charge cannot inadvertently energize control circuits after operation. The capacitor is grounded via a brown wire that connects terminal 39 of the console's terminal strip to the main ground stud, which also mounts the circuit breaker and other grounding points .
Controller Construction and Adjustments
The controller dispatches voltage to proportional coils through outputs labeled "A" and "B", corresponding to handle movements in both directions. Maintenance professionals refer to the printed circuit board (PCB) terminals as follows:

• “(+)” for positive supply voltage (12V or 24V),
  
• “(–)” for ground,
  
• “(X)” for switched-on auxiliary output,
  
• “A” and “B” for directional coil outputs .
  

Tracking down control issues often involves swapping terminals A and B with a working controller to determine if the fault lies in the controller or downstream hydraulic valves .
Troubleshooting Steps via Manufacturer Guides
JLG’s Service Manual prescribes a systematic startup calibration:

• Adjust the ground control box's trim potentiometers for high engine speed, gain, and droop.
  
• Bring the engine to operating temperature, then calibrate the high and low engine RPM settings using the foot switch and high/low engine speed switch.
  
• Tuning the gain (P2) adjusts responsiveness, and the high-engine (P1) and low-engine screws set RPM thresholds.
  
• If engine surges persist, a surge screw on the actuator may need adjustment using a manual stroke test .
  

Common Issues and Diagnostic Tips

• If functions jump or lag abnormally, the “threshold” or ramp trim pots may need recalibration .
  
• A failure to switch between high and low range could indicate missing source voltage at the “R” terminal or misadjusted pot settings .
  
• If replacing the controller doesn't fix the fault, hydraulic components like valves should be examined next .
  

Key Identifiers and Spare Parts
JLG units may use PQ or OEM controllers; identifying the correct model number on the controller (found on white stickers) is essential before consulting part manuals or ordering replacements .
OEM controller boxes such as JLG part #1600141 are commonly available (priced around $900), and are compatible with several models including 40H, 45HA, 60H, and others .
** Summary Table**

• Role
  • Translates joystick input into proportional hydraulic control.
    
  • Equipped with a discharge capacitor to clear residual power.
    
• Electrical Connections
  • Brown wire: terminal 39 to ground stud for safe power discharging.
    
• PCB Terminals
  • “+” positive input
    
  • “–” ground
    
  • “X” auxiliary power output
    
  • “A” and “B” outputs for directional control coils
    
• Adjustment Process
  • Set engine RPM trims (High/Low)
    
  • Tune gain and droop for smooth response
    
  • Perform threshold/ramp adjustments for movement smoothness
    
• Common Faults
  • Malfunctions continue post-controller swap → likely valve or wiring issue.
    
  • Improper trimming leads to surging, lag, or failure to change speed ranges.
    
• Part Information
  • PQ vs OEM controllers distinguished by label; proper identification is important for support or replacements (e.g., part #1600141).
    

Practical Advice
When maintenance or troubleshooting is warranted, always begin by identifying whether your lift uses a PQ or OEM controller. Refer to the supplement manuals for specific adjustment values. If a function remains unresponsive or erratic after recalibration, swap the controller with a known-good unit, then inspect hydraulic valves if issues persist. Always ensure proper grounding and discharge circuitry are intact—for example, verify the brown capacitor connection is clean and secure.
By systematically ruling out wiring, electrical supply, and hydraulic path faults, you can address control box issues effectively and safely—and potentially save significant downtime and expense.