JLG 40HA Not Moving

[MikePhua]

Introduction
The JLG 40HA is an aerial work platform (boom lift) designed to lift personnel and equipment to elevated heights. It uses hydraulic drive motors, a travel control valve (often part of a pump with a drive servo), interlocks, pedals or joystick controls, and safety switches. Over its production lifetime, it has been used on construction sites, in maintenance work, and in rental fleets. Because of its hydraulic-drive architecture and control electronics, one common issue is that the machine suddenly loses its ability to drive forward or reverse, while other functions like boom lift or steering may still work. Understanding what components are involved, what symptoms manifest, and how prior users resolved similar issues helps with diagnosing the fault.

Symptoms When Drive Fails
In failing drive situations, users typically observe:

• No movement in forward or reverse directions, even when controls are operated.
  
• Other hydraulic functions (lift, steering, basket functions) still operate normally.
  
• The engine does not labor (i.e. RPMs stay constant) when trying to command travel, suggesting drive function is not engaging.
  
• Manual lever on pump servo (if present) when moved doesn’t stall or load the engine, which normally indicates the drive pump swash plate isn’t being moved.
  
• On some units, funnily, drive function fades over time and then disappears completely.
  

Key Components & Technical Concepts
To diagnose this problem more effectively, these terms and parts are central:

• Drive Pump with Servo Motor: The hydraulic pump that provides the power for travel. The servo motor directs the swash plate angle (in variable displacement pumps) to control forward/reverse travel.
  
• Swash Plate: Mechanism that changes displacement of pistons inside the pump, allowing movement by shifting angle.
  
• Joystick / Travel Controls: The user input for forward/reverse travel. Sends an electrical signal (often via wires of certain color codes) to control the servo.
  
• Electrical Harness / Control Wires: Wires that carry signals (e.g. “forward” and “reverse”) and power to the drive pump servo. In many JLG 40HA machines, specific wires (e.g. brown/blue for forward, brown/red for reverse) are involved.
  
• Pilot Pressure / Charge Pressure: Auxiliary pressure that helps to operate the servo and move the swash plate; sometimes supplied via a smaller pump.
  
• Filter / Restrictor in Servo Circuit: There may be small filters or restrictors in the servo motor input lines which can get clogged, restricting control signal or hydraulic fluid necessary for drive engagement.
  
• Brake / Parking Brake / Brake Solenoid: The drive system often includes a brake that must be released (electrically or hydraulically) before travel is possible.
  

Possible Causes for No Drive
From user-shared experiences and technical troubleshooting, the causes fall into a few categories:

• Faulty or damaged drive pump, or internal wear in the pump preventing displacement change.
  
• Clogged filter or servo input restrictor in the pump/servo motor link causing insufficient hydraulic flow or signal to move the swash plate.
  
• Incorrect or too viscous hydraulic fluid: thick fluid can aggravate flow restrictions especially at lower temperature or when filter is partially plugged.
  
• Electrical signal problems: bad wiring, broken connectors, incorrect voltage to the servo motor or drive control signals.
  
• Brake solenoid or parking brake still engaged: preventing movement even if pump and servo operate.
  
• Joystick or control circuitry failure: joystick not sending proper signal, or the control module between joystick and pump not translating correctly.
  

Case Examples

• A user replaced a used drive pump in a JLG 40HA and initially restored drive, but after about six hours drive faded away in both directions until complete loss of travel. Pump was tested by a hydraulic shop and found functional, yet drive still did not return.
  
• Another user discovered that using hydraulic fluid which was too thick clogged a hidden filter in the servo circuit. The filter cleaned would allow drive temporarily, but would clog again after brief use. Removing the filter made drive return but created overly sensitive control.
  
• In other cases, replacing joystick or control switches solved the issue when diagnosis revealed faulty or intermittent signal from those components.
  

Diagnosis Steps
Here’s a checklist to systematically find the problem:

• Verify battery voltage and electrical system are healthy.
  
• Test joystick or drive control input: check whether voltage (or PWM signal) is present on forward and reverse control wires when joystick is moved. Use multimeter or diagnostic tools.
  
• Check continuity of wires from drive control through terminal strip to servo motor harness.
  
• Inspect the servo filter or restrictor (if equipped) for clogging. Remove, clean, or replace if needed.
  
• Inspect drive pump and servo activation: move manual lever on servo (if available) with engine running at idle; check if it loads engine or causes swash plate to shift.
  
• Check that the brake (parking / travel brake) is released; check brake solenoid operation.
  
• Evaluate hydraulic fluid viscosity and condition: is it correct spec? Is there contamination?
  
• Replace suspected components: joystick, switches, or pump / servo if confirmed faulty.
  

Solutions and Recommendations
Based on what people have done successfully:

• Clean or replace the hidden filter/restrictor in the servo motor input. This single component has been a culprit in multiple cases.
  
• Ensure hydraulic fluid meets manufacturer’s viscosity spec. Avoid using fluids that are “free” but too thick.
  
• Replace worn out or malfunctioning pump / servo unit. If pump is rebuilt or replaced, ensure servo motor is tested.
  
• Check and repair any wiring issues: corroded connectors, worn insulation, loose terminals.
  
• Ensure all safety interlocks are satisfied and brakes/parking brake are released.
  
• Use OEM-approved filters in the servo circuit if available, and include routine cleaning into maintenance schedule.
  

Conclusion
When a JLG 40HA refuses to move despite other functions working, the likely issues vary from hydraulic pump or servo failure, clogged filters/restrictors, voltage/signal problems, to braking or safety interlocks still engaged. Several users resolved the problem by cleaning filters, replacing too viscous fluid, or replacing sensors/joysticks. For reliable fix, it’s best to follow systematic diagnosis and not bypass safety components. Diligent maintenance of hydraulic fluid condition, servo filters, and electrical wiring tends to prevent this failure.