JLG 600AJ Poor Drive Performance or Weak Torque

Symptoms

• Machine stalls or bogs down on grades of 10% or greater
• Drive functions are slow in both high-speed and high-torque (low-speed) modes
• Excessive noise/whining from the main hydrostatic piston pump or drive hubs
• Hydraulic oil temperature rises rapidly during basic travel operations

Possible Causes

• Internal bypass leakage within the active drive motors or piston pump
• Low pilot charge pressure (under 300 psi) failing to fully stroke the hydrostatic pump swashplate
• Incorrectly calibrated or malfunctioning Drive Proportional Valve on the main valve bank
• Mechanical binding or dry planetary drive gear hubs (Torq-Hubs)

Diagnostic Procedure

1. 1
   Perform Charge Pressure Verification

   Install a 1000 psi hydraulic pressure gauge to the charge pressure test port on the hydrostatic pump. Start engine and verify charge pressure is between 320 to 350 psi at low and high idle.
2. 2
   Measure Loop High-Pressure During Stall

   Safely tie/secure the machine or block the wheels. Connect 5000+ psi pressure gauges to both drive system loops (forward and reverse). Command a drive function and measure maximum system relief pressure, aiming for 4200 to 4500 psi.
3. 3
   Inspect Drive Motor Case Drain Bleed-by

   To isolate the drive motors, disconnect the case drain line of one motor at a time and direct it into a graduated container. Run drive function over relief. Flow exceeding 1.5 GPM indicates severe internal bypass and the motor must be rebuilt.
4. 4
   Check Electrical Current to Proportional Pump Solenoids

   Connect a JLG Analyzer and review the current (mA) delivered to the forward and reverse pump control solenoids. Ensure current sweeps smoothly from approximately 400mA to 1500mA.

Common Repairs

• Resealing or replacing worn hydraulic drive motor internal rotary groups
• Adjusting/replacing the hydrostatic pump charge pressure relief valve
• Draining, flushing, and replacing gear oil in the Torque-Hub planetary gearboxes

Safety Precautions

• Perform pressure tests in open, flat areas away from structures and other people
• Avoid injection injury hazards by using cardboard to check for high-pressure hydraulic leaks

Required Tools

• Hydraulic Pressure Gauge Kit (0-1000 psi and 0-5000 psi)
• JLG Handheld Analyzer
• Graduated Flow-Measuring Container
• Fitting Caps and Plugs Set

Related Fault Codes

People Also Ask

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Related Guides

Suggested Follow-Up Guides

• JLG 600AJ Hydrostatic Pump Calibration and Pressure Setup Guide
• Troubleshooting JLG Deutz D2.9 Engine Power Loss Under Load
• Step-by-Step JLG Drive Hub and Brake Chamber Overhaul
• Diagnosing JLG Fault Code 4-4: Load Limit and Governor Adjustments
• JLG 600AJ Hydraulic System Flush and Multi-Valve Manifold Diagnosis

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