1997 JLG 600S Fast Slow Drive Speed

[MikePhua]

The JLG 600S is a telescopic boom lift, part of JLG Industries’ mid‑to‑large scissor and boom lift lineup designed for construction, maintenance, and industrial applications. JLG Industries, founded in 1969 in the United States, has grown to be a leading global manufacturer of aerial work platforms. The 600S model, introduced in the 1990s, features a telescopic boom reaching approximately 60 feet and a dual-speed drive system that allows operators to select fast or slow drive modes depending on the terrain and precision needs. Issues with the fast/slow drive functionality often arise in hydraulic, electrical, or control circuits, and understanding the root causes is critical for safe and efficient operation.
Drive System Description
The JLG 600S uses a hydrostatic drive system where hydraulic motors power the wheels, allowing smooth speed variation and precise control. The machine is equipped with two drive modes:

• Slow Mode — Provides precise movement at low speeds for delicate positioning near work surfaces or obstacles.
  
• Fast Mode — Allows higher travel speed for moving across open job sites efficiently.
  

The dual-speed functionality is typically controlled via a drive selector switch in the operator’s control panel, which signals a valve or control module to adjust hydraulic flow to the drive motors.
Common Problems and Symptoms
Operators often report the following symptoms related to the fast/slow drive system:

• Machine moves slowly even in fast mode.
  
• The drive mode changes with delay or intermittently.
  
• Unusual noises or uneven acceleration when switching between speeds.
  

These symptoms can result from a variety of underlying issues:

• Hydraulic Flow Restrictions — Clogged filters or partially closed relief valves can reduce flow to the drive motors.
  
• Valve Malfunction — The solenoid or spool valves responsible for switching speeds may stick, wear, or fail electrically.
  
• Electrical Faults — Wiring or switch problems in the control circuit can prevent the module from signaling the valve correctly.
  
• Motor or Pump Wear — Over time, the hydraulic motors or main pump may lose efficiency, reducing speed output.
  

Inspection and Diagnosis Strategy

1. Check Hydraulic Fluid — Ensure fluid is at proper level and viscosity; contaminated or low fluid can affect speed selection.
   
2. Inspect Filters and Relief Valves — Replace clogged filters and verify that relief valves are set to the correct pressures.
   
3. Test Drive Selector Switch — Use a multimeter to ensure the switch correctly signals the control module.
   
4. Examine Hydraulic Valves — Remove and inspect solenoids and spools for sticking or wear.
   
5. Assess Motors and Pumps — Measure output pressure and flow to verify they meet manufacturer specifications.
   

Repair Solutions

• Replace clogged or dirty hydraulic filters.
  
• Clean or rebuild valve assemblies to restore proper spool movement.
  
• Repair or replace faulty solenoids or electrical wiring.
  
• Overhaul hydraulic motors or pumps if output is below specification.
  
• Confirm proper system bleeding after repairs to remove air that can affect speed selection.
  

Preventive Maintenance Tips

• Regular Fluid Changes — Use manufacturer-recommended hydraulic oil and replace at scheduled intervals.
  
• Filter Monitoring — Check and replace hydraulic filters frequently, particularly in dusty or harsh environments.
  
• Control Inspection — Periodically test switches, wiring, and solenoids for reliability.
  
• Monitor System Pressure — Keeping hydraulic pressure within specifications reduces wear on motors and valves.
  

Real-World Example
A rental company in Texas experienced intermittent slow speeds on a 1997 JLG 600S. Technicians discovered a partially clogged main hydraulic filter and a sticking speed selector valve. After cleaning the valve and replacing the filter, the machine operated correctly in both fast and slow modes. The repair extended the operational life of the lift by several years, avoiding the need for costly pump replacement.
Terminology Clarified

• Hydrostatic Drive — A system where hydraulic pressure powers motors directly to drive wheels or tracks.
  
• Solenoid Valve — Electrically controlled valve that directs hydraulic flow to actuate functions.
  
• Relief Valve — Hydraulic valve that limits system pressure to prevent component damage.
  
• Telescopic Boom — A boom that extends in sections to increase reach height.
  

Conclusion
The JLG 600S dual-speed drive system is a sophisticated yet serviceable design. Problems with fast/slow operation often stem from hydraulic contamination, valve malfunction, or electrical issues. Routine maintenance, inspection of the hydraulic and control systems, and prompt attention to symptoms like slow movement can prevent larger failures. Proper understanding of the hydrostatic system, including drive motors, valves, and selectors, ensures that the lift continues to operate safely and efficiently across varied job site conditions.