Troubleshooting the JLG 600S: Issues with High-Torque Drive

[MikePhua]

The JLG 600S is a popular model in the aerial lift and telehandler category, offering excellent maneuverability and versatility for a wide range of construction and maintenance tasks. However, like any piece of heavy equipment, it can occasionally run into problems. One issue that some operators have faced is when the JLG 600S fails to drive in both Rabbit and Turtle modes. Specifically, when the machine is shifted into high-torque mode, it begins to move but quickly comes to a stop after a few feet.
This article explores the potential causes of this problem, how to troubleshoot it, and provides advice on what to do next if your JLG 600S exhibits similar behavior.
Understanding the Problem
The JLG 600S has a dual-speed system, commonly referred to as the Rabbit and Turtle modes. These modes control the speed and torque of the machine’s drive system. Rabbit mode is for high-speed operation, while Turtle mode offers slower speeds but higher torque for difficult tasks such as rough terrain handling or when additional power is needed for climbing or pushing loads.
When the machine won’t drive in either of these modes and only moves a short distance before coming to a stop, it suggests an issue in the drive system or associated components. This can be particularly frustrating on a job site where time and reliability are crucial.
Potential Causes and Troubleshooting Steps

1. Hydraulic System Issues
   The drive system in the JLG 600S relies heavily on hydraulics. If there’s a problem with the hydraulic pump, valves, or fluid levels, the machine may not get the necessary power to drive effectively. The most common hydraulic issues include:
   • Low Hydraulic Fluid: If the fluid is low, the hydraulic pump will not be able to provide the necessary pressure to the drive motors, resulting in the machine stalling or stopping after a few feet of movement.
     
   • Contaminated Hydraulic Fluid: Dirty or contaminated hydraulic fluid can cause the hydraulic pump to malfunction or clog valves, limiting the flow of fluid and reducing power to the wheels.
     
   • Hydraulic Pump Failure: If the hydraulic pump itself is failing, it may still provide some movement but not enough pressure to keep the machine moving consistently. This could explain why the machine rolls a few feet before stopping.
     
   Solution: Inspect the hydraulic fluid levels, and check for any signs of contamination or leaks. If the fluid is clean and at the proper level but the problem persists, inspect the hydraulic pump for signs of wear or failure. You may need to replace or rebuild the pump.
   
2. Drive Motor or Motor Controller Issues
   The JLG 600S relies on drive motors that control wheel rotation. If a motor is malfunctioning or a controller is faulty, it can lead to the symptoms described. Issues with the motor can be caused by electrical problems, worn-out components, or even overheating.
   • Electrical Malfunctions: If there is an electrical issue such as a loose connection, blown fuse, or faulty wiring, it can prevent the motor from receiving the required signals to drive the wheels properly.
     
   • Worn-out Drive Motors: Drive motors wear out over time, especially in high-use environments. If the motors are damaged or worn, they might struggle to provide the necessary torque to maintain movement.
     
   Solution: Inspect the electrical connections to the drive motors, checking for loose wires, corrosion, or any visible damage. Test the motor controller and fuses to ensure they are functioning correctly. If everything looks fine but the issue persists, consider testing or replacing the drive motors.
   
3. Transmission or Gearbox Problems
   Another common cause of failure to drive, especially in high-torque mode, is an issue with the transmission or the gearbox. These components are essential for regulating the machine’s speed and torque, and if they are damaged or malfunctioning, the machine may not shift properly or provide adequate power.
   • Faulty Torque Converter: The torque converter regulates the amount of power sent to the wheels. If it fails, the machine might not operate properly in either high-torque or high-speed mode.
     
   • Transmission Fluid Issues: Low or contaminated transmission fluid can prevent the gearbox from shifting properly, leading to poor performance or failure to move.
     
   Solution: Inspect the transmission fluid and ensure it’s at the proper level and is clean. If there is any indication of contamination, drain and replace the fluid. Test the torque converter by checking if the machine responds differently in different modes.
   
4. Control Panel or Sensor Malfunction
   The JLG 600S uses a control panel and various sensors to manage the machine's functions. If any of these sensors are malfunctioning or out of calibration, it could lead to issues with mode switching, including not being able to engage Rabbit or Turtle mode correctly.
   • Faulty Mode Selector Switch: If the switch is not engaging properly, it could prevent the machine from entering the desired mode.
     
   • Sensor Calibration: Sensors that monitor torque, speed, and other critical parameters may be out of calibration or faulty, preventing the machine from functioning as intended.
     
   Solution: Check the mode selector switch and ensure it is functioning properly. If the switch is working but the machine still won’t engage the right mode, inspect the sensors and consider recalibrating or replacing them.
   
5. Braking System Interference
   While this may seem like a less obvious cause, interference from the braking system can sometimes result in the machine stalling after a few feet of movement. If the brakes are partially engaged or malfunctioning, it can prevent the machine from gaining full speed or even cause it to stop completely.
   Solution: Inspect the brake system to ensure that the brakes are not sticking or dragging. Check for any signs of brake fluid leaks or malfunctioning brake components. If the brakes are causing resistance, they may need adjustment or replacement.
   

Additional Tips and Advice

• Perform Regular Maintenance: Preventative maintenance is key to avoiding many of the problems that can lead to drive issues in the JLG 600S. Regularly inspect the hydraulic system, fluid levels, and drive motors to ensure everything is operating efficiently.
  
• Check for Software Updates: Modern machines like the JLG 600S are often equipped with diagnostic software. If you have access to a diagnostic tool, check for any stored error codes that could help pinpoint the issue.
  
• Consult the Operator Manual: Always refer to the operator’s manual for troubleshooting tips and recommended maintenance schedules. It can provide specific guidance for your model and help you avoid common pitfalls.
  

Conclusion
If your JLG 600S isn’t driving properly in Rabbit or Turtle mode, there could be a variety of factors at play, from hydraulic issues to drive motor problems. By following the troubleshooting steps outlined above and addressing each potential cause systematically, you can identify the root of the problem and restore your equipment to full working order. Keep in mind that regular maintenance and inspections can help you avoid these issues in the future and keep your machine running smoothly for years to come.